Wireless Pers Commun DOI 10.1007/s11277-017-4058-x

Wireless Broadband Tools and Their Evolution Towards 5G Networks Ismat Aldmour1

 Springer Science+Business Media New York 2017

Abstract Telecom companies in the different generations resorted to different tools to conquer connectivity issues like coverage, throughput and quality of service. The success is driving new services of diversified requirements and more traffic which dictates a continuous increase in data rates and improved performance. This work reviews the different technology tools used to meet rates and traffic requirements of the past and the present with a look into their role in future networks. The coming fifth generation (5G), in particular, is confronted by a number of antonym challenges of capacity, spectrum, energy, connectivity, performance, complexity, and cost. Suggested tools proposed to meet these challenges and the way they act to do so are reviewed and compared to current and past solutions. The tools discussed are the use of millimetric waves, massive multiple input multiple output antenna systems, indoor–outdoor separation, ultra-dense cooperative networks, and the increased reliance on users’ terminals. The major constructional differences and tools differences between current and future 5G are finally emphasized. Keywords Wireless broadband tools  Broadband evolution  Broadband future  5G

1 Introduction Telecomm companies in the different generations resorted to different techniques, technologies, and methods (collectively will be referred to here as tools), to handle connectivity issues like coverage, throughput and Quality of Service (QoS). Examples of these tools are cell sectorization, cell division, the use of multiple input multiple output (MIMO) antenna systems and air interface updates.

& Ismat Aldmour [email protected] 1

Department of Computer Engineering and Science, AlBaha University, P.O. Box 1988, AlBaha, Saudi Arabia

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Tools are continuously evolving as the challenges themselves are also evolving. For example, second generation (2G) systems were confronted with coverage and capacity problems. On the other hand, third generation (3G) systems added to these challenges QoS issues as in these systems the more users within the coverage area the higher drop rate and lower QoS. Currently, the world is at the verge of deploying the fourth generation (4G) wireless communication systems. This is almost in accordance with the International Telecommunication Union (ITU) development framework in 2003 [1] in which the year 2015 is the planned year for the new technologies to be deployed in some countries (while the year 2010 is the planned year for finalizing the technologies). The ITU development framework [1] aimed at meeting the expected increase in users’ expectations and needs for new mobile services and applications by and beyond theses dates [1]. Nevertheless, it is expected that future networks beyond 4G, or 5G, will be more challenged. In particular, the challenge of improving data rates and satisfying traffic requirements will be much more stringent in near future. By 2020, a 1000 fold increase in traffic is predicted [2]. Also, mobile subscriptions shall grow from 6.8 billion in 2014 to 9.2 billion in 2019 with more than 50% of traffic coming from video [3]. Moreover, future 5G users’ will require fibrelike access rate, zero latency and consistent experience under diverse conditions [4]. The paper reviews how past technologies dealt with the data rate and traffic challenges that confronted them and how future generations will make use of the accumulated experience to further boost the data rates and fulfil the traffic explosion requirements. The paper discusses 5G challenges and enabling tools and emphasises the way and extent to which the tools of the past and present can be utilized in the future. 5G is not yet standardized. Detailed specification of 5G candidate technologies, committing to ITU IMT2020 are expected in 2020 [5]. Some recent papers tried to predict the 5G; what it will be [6], its cellular architecture and key technologies [7] [8] and its potential scenarios [9]. This paper is different as it concentrates on the tools and tries to relate the past and the present to the future. For that, the paper, after a brief refresh on Shannon’s theorem for reliable communication, classifies the different tools as either power or spectrum tools and tracks the progress of these tools throughout the different generations up to and including the future 5G. It surveys recent research in an attempt to predict how these tools may re-shape themselves, probably under new names and of different complexities, so as to satisfy the data rates and traffic volume requirements of future networks. The rest of the paper is organized as follows: Sect. 2 reviews the basics of data rates provisioning. Section 3 presents wireless broadband provisioning tools of the past and the present and tries to foretell their future. Section 4 reviews the different broadband technologies and tools evolution throughout them. Section 5 presents 5G challenges while 5G tools are in Sect. 6. Section 7 presents a comparison between future 5G and current networks. Finally, conclusions are in Sect. 8.

2 Data Rate Satisfaction The 1948 Shannon’s theorem [10] put the foundation for reliable communication over noisy channel. A brief chronicle of the development of the theory of reliable communication is given in [11]. Shannon’s formula specifies the maximum capacity C in bits/sec over a noisy medium (channel) below which bit error rate can be made arbitrary small using coding. The limiting capacity C is given by

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  C ¼ B  log2 1 þ S=N where B is the channel bandwidth in Hz and S/N is the signal to noise ratio. Hence, to attain higher data rates out of a communication link we either have to improve the signal conditions, i.e. increasing the S/N ratio, or to use more bandwidth, B. Bandwidth increase is more effective in increasing data rates than the signal to noise (S/N) ratio increase. This is because in the formula above data rate increases linearly with bandwidth increase while it increases logarithmically with S/N ratio increase. The capacity in Shannon’s is for a link, whether wireless or wired. Generally, the wired channel enjoys good signal conditions and the signal bandwidth can be doubled by laying wires in parallel. On the other hand, the wireless channel is a vagarious channel with much interference sources contributing to the noise with limited bandwidth. Therefore, the factors of bandwidth and signal to noise ratio are limited. However, many wireless advancements are now in place or being researched to overcome these limitations. Examples are: • First, regarding bandwidth tools: more spectrally efficient and improved sharing/ frequency re-use techniques, e.g. improved air interface technologies, spatial multiplexing using MIMO antennas and cognitive radio. Additionally, the wireless signals are, so far, utilizing but a small fraction of the wireless frequencies with much more unexploited spectrum opportunities in the upper bands. • Second, regarding S/N ratio: methods to improve link S/N ratio and to overcome channel vagaries due to multipath and interference, which will be referred to in this article as power tools, are continuously evolving. Examples are equalization, diversity techniques, interference cancellation and management and beamforming antennas. As a result, broadband over the wireless channel developed to provide higher data rates that are comparable to some of the wired technologies while at the same time the wireless link became more reliable. In the next section an overview of theses tools with a look into the future of each tool is given.

3 Tools of the Past and Present and Their Future Some tools can act as both power and bandwidth tools or at least have impact on the other side. For example, reducing cell size acts to reduce the necessary power emissions, while, at the same time, it results in more frequency re-use, hence increasing the total bandwidth available to the network on the account of more complexity and infrastructural costs. Extrapolating this fact, to satisfy future needs, more miniaturization can be utilized but more work has to be done on the side of complexity and cost. Hence, the objective of this section is to give a classification and an overview of past and present tools and the way they act in order to extrapolate their possible role in future networks. Again, these tools will be divided into tools for improved S/N ratio or power tools, and bandwidth and bandwidth efficiency tools.

3.1 Tools for Improved S/N Ratio The S/N ratio can be improved by either assuring higher received power, S (e.g. using repeaters) or reduced total noise, N (e.g. using interference cancellation receivers). Noise is

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the thermal noise and interference from others which is usually more decisive in wireless systems. That is why S/N ratio often referred to signal to interference plus noise (SINR) ratio. The following discusses some of these tools: • The cellular structure The cellular system divides the service areas into a number of regular pattern cells with each one of them served by its local base station with handover procedures at cell boundaries. This enables communicating using relatively low power signals while at the same time enables a re-use of the available frequencies geographically thereby resulting in capacity multiplication. The concept of frequency re-use in future networks will be well exploited; e.g. using small cells and 3Dsectorizatio. Heterogeneous multitier networks can be looked at as a perturbation to the well planned regular cellular structure. Additionally, future objectives of interconnecting all things and the nature of future services that will serve dedicated locations may render the regular cellular structure useless in many circumstances. Future resort to millimetric waves (mm-Waves) and other low range low power technologies will result in more miniaturization and more departure from the regular structure while at the same time more frequent handovers will have to be conducted both vertically between layers and horizontally between neighbouring cells.which can lead to handover failure problem [12]. • Power control Is a basic link conditioning technique used in most of the wireless systems. Power control ensures that the power emitted by mobile users is just enough to establish a reliable connection thereby reducing interference to others using the same frequencies in other cells. Power control in traditional one-to-many networks is mainly to protect the QoS of the connections. Power control is crucial in systems where the signals share common frequencies and transmits simultaneously such as the different code division multiple access (CDMA) technologies of 2G (CdmaOne) and 3G wideband CDMA (WCDMA). Power control in future heterogeneous wireless and ad hoc wireless can be used as a means for increased throughput whenever appropriate. Thus to guarantee service in future high densely 5G scenarios, power control can be used, together with sensing and coordination techniques to allow for increased opportunities for more power efficient higher rate communication. Hence, power control in future networks is a decentralized opportunistic power transmission of individual nodes rather than power limits on subordinates by main node or base station, e.g. [13]. • Equalizers Due to reflections, refractions and scattering in urban environments signal travels on different paths from the transmitter to the receiver. The multipath components arrive with different amplitudes, phases, and delays. The different paths combine constructively or destructively at the receiver resulting in a reduced received signal power level and inter-symbol interference (ISI). ISI is a self interference component in which the signal interferes with itself due to dispersion in time. Equalization is used to care for ISI created by multipath with time dispersive channels [14]; i.e. wideband channels with a bandwidth W greater than its coherence bandwidth. Equalizers are essential in TDMA wireless systems such as 2G-GSM. In the WCDMA rake receivers are suggested to account for the multipath components. More recent systems such as the LTE are designed with narrowband channels so that the ISI is combated using simpler equalization techniques. • Repeaters Are used to extend the range of base stations and to improve service to users close to cell edge, indoors, and in shadow areas. The intention is to reduce the transmitter-to-receiver distance, thereby allowing for higher data rates. Repeaters are

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used in 2G and 3G and its enhancements as well as they are suggested for LTE to handle coverage holes [15]. Signal relaying in repeaters can be an amplify and forward type of relaying, decode and forward or backhauling. In both amplify and forward and decode and forward the repeater is invisible to the terminal, but the decode and forward relaying regenerates noise free signal on the account of delay. The backhauling repeater assumes many of the functionalities of eNodeB such as scheduling, retransmission and mobility handling [15]. Improvement of data rates for cell edge users is one of the requirements of the ITU advanced international mobile telecommunications (IMT-Advanced) [16] set to arrive at radio interface technologies for 4G systems. Future networks will have to resort extensively to the use of repeaters of the different types in order to manage serving the different future scenarios. Repeaters can take the form of multi-hop communication utilizing the services of other peer nodes; i.e. a node can act as a transceiver for its own communication as well as a transceiver to relay the signals of the others; e.g. in high density mobile scenarios and in ad hoc and wireless sensors networks (WSNs) types of communications. • Interference avoidance techniques Interference has been dealt with differently in mobile communications. In analogue frequency division multiple access (FDMA) systems, the interference is avoided by segregation of signals in frequency. In 2G–GSM and its enhancements, time domain segregation is added. In WCDMA of the 3G, it was found that capacity can be gained by removing the boundaries and no more segregation in time or frequency. In orthogonal frequency division multiplexing (OFDM) systems such as LTE (and WIMAX long before LTE) segregation in frequency is back again whereby different narrowband carriers are used in the same cell/sector, thereby, the interference from neighbouring cells/sectors in which users use the same sub-channels becomes the main source of interference. However, with orthogonal carriers the spectrum is utilized more efficiently. It can be said that different systems are distinct in their way of handling the interference avoidance problem. As the future will have to support networks of higher densities, more creativity in avoiding the interference with less complexity is needed. New air interfaces technologies and updates to current air interfaces will be necessary. Having multiple air interface technologies in one device might provide one other way of avoiding the interference. • Interference management and coordination techniques A good example is the use of fractional frequency re-use (FFR) [17] with frequency division techniques such as the OFDM. In FFR all frequencies are used by the users close to the cell centre (close to the

Fig. 1 Fractional frequency reuse cellular layout

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base station) while users close to cell edge use only a partial set of the frequencies that are different to the partial set in the neighbouring cells (Fig. 1) thereby reducing interference for cell edge users. In LTE, techniques for inter-cell interference coordination (ICIC) are in place that use adaptive FFR with exchange of indicators on interference conditions of spectrum parts between neighbouring e-NodeB base stations to improve the scheduling of the users on the different subchannels [15]. In future networks with multi-tier high density networks more creative FFR methods are needed. For example, in macro-femto deployment scenarios, information exchange across tiers is used in [18] to dynamically adjust the frequency re-use set for femto users, while the mechanism in [19] is a combined frequency band and time slots allocation. Partitioning, either in time or frequency, between the various base stations avoids interference on the account of inefficiecy in spectrum usage. An alternative is to allow frequency sharing between the macro and small cells while optimizing some parameter of interest, e.g. guarratee minimum average throughput for all cell users. • Interference cancellation (IC) techniques In systems where signals can share a common bandwidth like cdma, multi-user detection (MUD) techniques are suggested in order to remove the interference effect of others sharing the channel. MUD techniques assume the knowledge of all users’ signatures and estimates of all users’ channel impulse responses in order to improve the detection of each individual user. The employment of this algorithm is more feasible for the uplink because all mobiles transmit to the base station and the base station has to detect all the users’ signals anyway [20]. The major drawback of joint detection lies in the receivers’ greater complexity. This complexity increases exponentially with the increase in the number of users to be demodulated simultaneously. On the other hand, successive interference cancellation (SIC) receivers attempts to successively detect interferers in order of strength and successively subtract them from the composite received signal to enhance the detection of lower strength interferers and the intended signal. Recent studies on successive interference cancelation within the context of 5G show that its advantage dwindles as the normalized target data rate, and the number of interferers increase [21]. This means that interference cancellation receivers have to be combined with other interference coordination and management techniques. Amongst these most researched techniques is to combine SIC receivers with cooperating base station schemes mechanisms e.g. [22, 23]. • Interference averaging techniques Interference averaging is obtained by signal spreading in time or frequency. Example is the frequency hopping spread spectrum (FHSS) used in Blutooth type of communication in which the channel of operation is hopping from one frequency to another providing interference averaging. In the direct sequence spread spectrum (DSSS) mode of WCDMA the signal spectrum is spread over the whole bandwidth as the tool for interference averaging. Similarly, in the orthogonal frequency division multiple access (OFDMA), all sub-channels, including the data and pilot subcarriers, can be hopping. This is called channel permutation which have a pre-specified hopping pattern for all the data and pilot subcarriers. Again, with high density networks of the future and the absence of centralized control in many scenarios, more resort to interference averaging techniques can solve many of the challenges. For example, OFDMA frequency hopping of the subcarriers can be made fully random to take into account the heterogeneity resulting from small cells as suggested in [24] in what is called randomized frequency hopping OFDM (RFH-OFDMA). Also, it is

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shown in [24] that this technique can achieve SINR gain of up to 5.3 dB and a reduction of the bit error rate (BER) by a factor of 8. Moreover, the low latency needs and short packet sizes in future networks may necessitate totally new air interfaces [25]. It is argued that advent of the internet of things (IoT) and the move to user-centric processing are rendering OFDM unfeasible [26]. The strict timing and synchronization requirements of OFDM add unnecessary complexity. Schemes like the fliter-bank based multicarrier (FBMC) [27], the universal filtered multicarrier (UFMC) [26] and other non orthogonal multiple access schemes may replace OFDM in the future. UFMC requires less signalling overhead compared to FMBC making it appropriate for short packets applications such as those in machine-to-machine type of communication of future networks [25]. • Diversity techniques When the same signal is sent over different frequencies, times, paths, etc., a diversity gain is obtained. Increased reliability is obtained by providing more signal paths that fade independently. Diversity can be intentionally designed or be a by product of multipath propagation. For example, in GSM time diversity is implemented by interleaving and coding over symbols across different coherent time periods. On the other hand, the use of equalizers in GSM to coherently combine the different signal components naturally dissected over multiple paths arriving at different times, phases and amplitudes results in diversity gain as well. Similarly, the use of rake receivers in WCDMA produces diversity gain. Diversity can be produced by utilizing multiple antennas at either the transmitter (transmit diversity) or the receiver (receive diversity) combined with proper selection or combining criteria. The use of Alamouti space–time codes [28] combines antenna and time diversity as a more spectrally efficient scheme. Diversity techniques will have its way into the tools of the 5G in newer shapes. The multiuser shadow-diversity technique in [29] is an example from recent literature. It suits densely device to device (D2D) networks. In this technique, a large number of UEs, including those in the sleep state, can potentially act as relays to enable communication between two nodes with weak cellular link between the two [29]. • Base station cooperation techniques The flat architecture of recent technologies like LTE lowers delays and improves scheduling efficiency. Additionally, flat architecture allows base to base cooperation and exchange of signals. In LTE-Advanced based on Rel-10 of the third generation partnership project (3GPP), coordinated multipoint transmission and reception (CoMP) is suggested. In CoMP more than one base station can cooperate on delivering the same data to cell edge users (Fig. 2). This results in improving the received signal quality and enhances data rates. This is similar to antenna diversity [15]. The opportunistic resort to such cooperation techniques might be a solution to users in future networks suffering high interference due to emerging scenarios (in addition to the traditional cell edge issue) in which a user suffers from a sudden increase of interferers or loss of signal strength. Generally, the high density of Fig. 2 Base station cooperation for cell edge users

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future networks will render the networks interference limited and a more versatile coordination techniques will be needed to mitigate the interference everywhere. It is shown in [30] that a smart combination of small cells, coordinated multipoint and massive MIMO can satisfy the 5G 1000 folds traffic increase goal. • Doppler mitigation techniques Supporting higher mobility scenarios is a requirement of future networks [31]. Broadband can easily be provided wirelessly at high data rates if we contend with fixed wireless with ensured LOS. On the other hand, mobility introduces frequency shift in the frequency domain and signal distortion due to time variations. In recent OFDM systems data chunks are carried using narrow band carriers of the order of 10–15 KHz or less in order to combat ISI without the need for complex equalization techniques. The mobile version of WIMAX uses a version of OFDMA called scalable OFDMA (SOFDMA). In this scheme, the number of carriers scales with bandwidth; doubling the bandwidth allotted doubles the number of subcarriers with no change in subchannels size. This keeps the inter-carrier spacing the same resulting in that Doppler effect on mobiles’ performance is kept the same and optimized for all bandwidths. New concepts are being researched to enable group connectivity to users on board of buses and trains such as the mobile relay [32] and the mobile femtocell [33] concepts. In both these two concepts, the users enjoy uninterrupted connection with an onboard central device that, at the same time, handles handover and backhaul connectivity.

3.2 Bandwidth and Bandwidth Efficiency (Spectrum) Techniques The bandwidth is a scarce resource. Generally, the bandwidth techniques can be classified as: • Direct bandwidth techniques to acquire more bandwidth in terms of more Hz. • Frequency re-use techniques in which the same frequency is re-used at different locations or at different times. • Indirect techniques in which the available spectrum is more efficiently utilized thereby increasing data rates without any extra Hz granted.

3.2.1 Direct Bandwidth Techniques These include, but not limited to: • New licence bands and the re-use of bands of abandoned technologies Examples are: the introduction of LTE 450 MHz band in Brazil in 3GPP Rel-12 [34] and the re-use of 2G bands (re-farming) for a more spectrally efficient technologies (e.g. LTE). The work in [35] reviews potential candidate spectrum bands in 5G both below and above 6 GHz. • Multicarrier systems It is also possible for an operator to use more than one carrier in a cell for the purpose of increased overall traffic in the cell or to pool the carrier frequencies for a single end user device thereby increasing its data rate. OFDM is a multicarrier system with higher granularity channels enabling better control for optimum signal reception. Hence, future networks will rely much on it or some variants of it that hold to this granularity • Carrier aggregation IMT-Advanced requested high data rates with large bandwidth of up to 40 MHz [16]. 3GPP targeted even larger bandwidth of 100 MHz in its LTEAdvanced specifications. Generally, it is not easy to get such large bandwidths in one

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piece, therefore, 4G technologies; WIMAX 2 and LTE-Advanced, resorted to combining chunks of available spectrum over different bands as a single wide channel thereby increasing the total capacity and data rates. In carrier aggregation of LTE five carrier components of 20 MHz each can sum up to the required 100 MHz. These can be contiguous (intra-band CA) or non-contiguous (inter-band CA). Inter-band CA for mobile nodes is generally a challenging practical problem compared to inter-band CA. Beside enabling higher data rates, CA can have the advantages of inter-cell interference mitigation, handover improvement, energy savings and load-balancing [32], Future networks will have to resort to more carrier aggregation especially in the traditional low bands [36]. • Resort to new and higher bands Most of the mobile communications concentrated in the lower bands of the spectrum around 1, 2 and up to 5 GHz. Congestion in spectrum at bands below 5 GHz makes the resort to higher GHz bands a feasible solution. After all higher bit rates can be obtained if we have larger bandwidths. At the same time, higher frequency bands allow for more frequency re-use because the signals attenuate more with distance at these frequencies. Therefore, higher frequency bands are suitable for small cells. Some systems like the local multipoint distribution service (LMDS) (1998–2000), a last mile metropolitan area network, headed to 28 GHz frequencies which gives them plenty of bandwidth and high throughputs. Unfortunately, such systems failed. Working at such bands requires a line of sight (LOS) operation and the signals attenuate quickly with distance, rain, and other weather factors. Nevertheless, much recent research interest is currently being conducted so that future networks head back to the mm-Waves frequencies (both indoor and outdoor). This will be later discussed in this paper in the context of 5G technologies and tools.

3.2.2 Frequency Re-use Techniques These include, but not limited to: • The use of the cellular system enabled re-using the same frequencies again after some re-use distance, that is, frequencies are subdivided between a number of neighbouring cells called a cluster and re-used in other clusters. The smaller the cluster size (number of cells in a cluster) the more bandwidth efficient the network is. Sectorization, in which the cell is subdivided into typically 3 sectors, is used as well as to efficiently improve spectral efficiency through more frequency re-use. Heterogeneity and multitier networks combined with the different interference handling techniques will enhance more frequency re-use in future networks resulting in higher density networks. 3Dbeamforming is suggested for future networks in which the vertical domain is utilized by vertical sectorization thereby producing capacity improvement over the traditional sectorization solution [37]. The work in [38] describes investigations of the potential of 3D beamforming with lab and field trial setups. It shows the capability of 3D beamforming to separate two signals sharing the same radio resources by taking advantage of reflections from building, walls and other strong reflectors. Consequently, the improved SINR values allow higher throughput for the addressed users and an overall increased system capacity [38]. • Multitier networks and small cells Small cells of the order of 1 km (Microcell), 200 m (Picocell) and 50 m (Femtocell) allow for higher S/N ratios. Therefore, they can be used to fill coverage holes and offloading high data rates traffic from the macro base station thereby increasing capacity. Small cells can be looked at as a perturbation of the

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well planned cellular structure, as generally they are randomly distributed, or even a total cellular paradigm shift. Nevertheless, operators are looking to small cells as solution to growing demand for wireless data of higher data rates and better quality. In future networks, macro-cell will continue to exist as the upper layer in a multi-tier network shadowing underneath it a large number of randomly distributed micro, pico, and femto cells. Small cells can achieve throughputs/capacity gains using deployments in the same frequency bands as an the existing macrocell network [39]. Other deployments use frequency separation between macro and small cells with the higher frequency bands used in the small cells (e.g. 3.5 GHz). The resulting heterogeneous structure needs to support high mobility and more frequent handovers between the different cell types. It is also necessary that efficient inter cell interference coordination (ICIC) techniques be implemented [40–42]. All these management issues need to be self organized. These networks are therefore called self organizing networks (SON) [43]. Other challenges to femtocell deployments are cell association of users, resource allocation and optimization [44], Uplink-downlink relationship [45], backhaul bottleneck [45], and the economic and regulatory matters [46]. • Multicode systems Users in spreaded systems like the CDMA with orthogonal codes can re-use the same frequencies within the same cell/sector. Users can be granted more than one code to increase link rate. To improve data rates for WCDMA, a special high data rate downlink shared channel in the high speed downlink packet access (HSDPA) of Rel-5 in 2002 is added. The shared channel is used whereby users in favourable conditions can be scheduled to utilize more than one spreading code to increase their throughputs.

3.2.3 Indirect Methods Those methods act to increase the data rates and the total offered traffic either by improving the efficiency of using the link (i.e. to make use of the link more) or improving the utilization of the total resources in the whole cell/network. Tools of such kind include, but not limited to, are the following: • User diversity One motive for spread spectrum 3G-WCDMA is the increased gain because silence detectors can be used to detect silence periods in order not to transmit harmful interference signals during such intervals thereby gaining capacity [47]. Similarly, in all IP systems like LTE, small bursts of data are sent during inactivity periods of either voice (VoIP) or data allowing others to utilize the bandwidth during these periods. In packetized voice services capacity can be gained more by sending totally blanked silence period frames [48]. User diversity can also be the result of that different users are located in different places in the cell with their signals fade differently. This can be utilized to design efficient algorithms to allocate resources, such as allocating the subchannels in OFDM [49, 50]. • Higher order modulations and coding In the same bandwidth, data rates can be multiplied using higher order modulations, e.g. QPSK symbols doubles the bit rates of BPSK symbols but transmits less bits per Hz than QAM16. This necessitates higher S/N ratios for the higher modulation orders. Similarly, forward error correction coding is used to ensure given BER at given S/N. Coding reduces the useful throughput. Therefore, improved S/N ratios can as well be utilized by using higher order coding (less redundant bits) in order to increase the useful throughput. The use of adaptive modulation and coding have been suggested to adapt throughput to link conditions to

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improve bandwidth efficiency (in bps/Hz) [51]. This is used in 3G ? systems of HSDPA and HSUPA (Rel-6 in 2005) as well as in HSPA? (evolved HSPA), LTE and WIMAX and their 4G versions. • Spatial multiplexing and MIMO techniques Multiplexing can be done over the wireless channel using MIMO techniques. Data rates can be increased in proportion to MIMO order. MIMO technique allows communicating at data rates that exceed the Shannon’s. This can be understood if we looked at the MIMO as multiple links rather than a single link. Hence the capacity of a MIMO increases linearly, not logarithmically, with the number of links (the minimum of the number of elements on both sides) resulting in linear increase of capacity with power increase (assuming each extra link necessitates extra power of the same amount) [52]. Spatial multiplexing techniques enable additional frequency re-use within cell sectors. A 4 9 4 MIMO system will roughly provide double the data rates offered by a 2 9 2 MIMO. This is similar to the case of wired systems in which the B in Shannon’s theorem can be doubled by running parallel wires. This is what can be called ‘‘wiring the wireless channel’’. Of course, there are no wires (Fig. 3). • Scheduling and overhead Scheduling can be designed to satisfy the QoS requirements of all connections while total throughput is maximized. Scheduler’s job is different in different systems. In current OFDMA systems, the scheduler assigns resources in both time and frequency domains and determines the coding and modulation based on channels state information (CSI) obtained using signaling from feedback channels. Overhead signals are lost bandwidth needed to send CSI and to request/grant connections. Distributed resource allocation and persistent or semi-persistent scheduling are methods to reduce the overhead of small packets that occur regularly like voice packets. Radio access network sharing and dynamic co-operation among operators (e.g. LTE in 3GPP Rel-13) are cost-effective ways to meet increasing traffic demand with less capital and operation expenditure. • TDD techniques The time division duplexing (TDD) systems were considered for 3G systems like TD-CDMA of china. WIMAX headed for TDD from the beginning. TDD provides a flexible way of more efficiently sharing the spectrum between the uplink and downlink while in the frequency division duplexing (FDD) the uplink can be lightly loaded. Video is growing fast nowadays and it is an asymmetrical traffic. LTE TDD mode (also called TD-LTE) allows for asymmetric downlink-uplink allocations by providing seven different semi-statically configured uplink-downlink configurations. These allocations can provide between 40 and 90% DL sub-frames [53]. The semistatic allocation may or may not match the instantaneous traffic situation. TDD in 3GPP Rel-12 [34] will offer flexible deployments without requiring a pair of spectrum resources, hence, facilitating concatenating frequencies used by abandoned technologies. Dynamic TDD is suggested for small cells in future LTE as significant performance benefits are expected by allowing TDD UL-DL reconfiguration based on

Fig. 3 A 2 9 2 MIMO with paths visulized as double wires

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traffic adaptation in small cells. TD-LTE is the choice of major WIMAX providers in their migration from WIMAX to LTE. One motive for this is the ease with which the WIMAX bands can be re-used. On the other hand, flexibilities of the TDD mode are challenged by factors such as its cross-slot interference, inter-operator interference and duplexing delay [54]. • Other promising techniques for future 5G networks include the simultaneous transmission and reception (STR) at the same time and frequency. STR (also called full-duplex) approximately doubles the link throughput and the spectral efficiency [55] and can be useful in cases of wireless backhauls and D2D communications [56]. The co-operation of Wi-Fi and cellular networks in the unlicensed spectrum can, as well, increase the overall capacity of heterogeneous wireless networks [57].

4 Broadband Technologies and Tools Evolution 4.1 Broadband Technologies The following reviews the different broadband technologies over the wireless with points of strengths and deficiencies and their possible role in future broadband emphasized: • WIFI This was standardized in 1997 as IEEE802.11 at data rates of 2Mbps. Later developments enabled higher data rates of 11, 54, 108 Mbps of 802.11b, 802.11 g and 802.11n in 1999, 2003, 2008 respectively. WIFI standards use 20/40 MHz bandwidth in the unlicensed spectrum (mainly 2.4 GHz). It covers homes and businesses of few hundred meters. Air interface of 802.11 utilizes FHSS with DSSS and OFDM in later versions. WIFI paved the way for other developments in wireless broadband provided by the networking industry like WIMAX. However, WIFI is expected to be an integral part of future networks because high density networks need it as a low range low power technology for indoor use that can help in offloading traffic from the external big towers. A new generation of WiFi standards will make its way to the market in the near future that will drive WLAN evolution [55], examples are IEEE802.11ac (Very-high throughput at \6 GHz) and IEEE802.11ah (Machine-to-Machine communications) specifications [55]. • Broadband over 2G technologies Basic GSM provided very limited circuit switched data rates of 9.6 Kbps. GSM received some enhancements to provide higher data rates. The generalized packet radio service (GPRS) and the global evolution (EDGE) enhancements provided data rates in the order of 10 s to 100 s of KHz. GSM is a TDMA technology over narrowband channels of 200 KHz bandwidth. GSM was the 2G technique of Europe and most of the world. In USA, another technique, based on spread spectrum (CdmaOne) is adopted. It also offered limited rate circuit switched data and received similar enhancements for improving the data rates. • Broadband over 3G technologies Increasing data rates of TDMA–GSM to satisfy those required by IMT2000 was not possible without the resort to complex equalization techniques, i.e. more complex transceivers. CDMA serves as a multiple access scheme with no fixed maximum number of users as opposed to TDMA and FDMA schemes [58]. Therefore, most 3G technologies are based on some sort of spread spectrum. Spread spectrum techniques like the WCDMA could satisfy the data rates required of 2 Mbps (peak). WCDMA uses 5 MHz single carrier system with frequency division duplex mode and direct sequence spreading (FDD-DS). It is the technology

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adopted in Europe. In North America a scaled version of cdmaone (multicarrier) called CDMA2000 is adopted while in China a synchronous time division duplex (TDD) version (TD-SCDMA) is adopted. 3G-enhancements High-speed downlink packet access (HSDPA) and High-speed uplink packet access (HSUPA) are two enhancements to 3G WCDMA that enabled peak data rates of peak 14.4 Mbps on the DL and 5.67Mbps on the UL through more efficient ways of using the available spectrum. Combined HSDPA and HSUPA is called HSPA. Evolved HSPA (HSPA?) uses the spectrum even more efficiently allowing for higher peak data rates and fewer delays in data transmission. With HSPA? it is possible to have peak rates on the order of 75 Mbps on the DL and 30Mbps on the UL with more enhancement coming. 3G/4G WIMAX: (Worldwide Interoperability for Microwave Access); is a TDD technology standardized by IEEE and uses OFDM as its modulation technique and OFDMA as its multiple access technique. The first in a series of IEEE wireless metropolitan access network (MAN) standards, started in 2000, that found real applicability was IEEE 802.16d in 2004 [59]. This standard aimed to provide high throughput, last-mile wireless broadband to fixed users, which formed a real competitor to digital subscriber line (DSL) and cable data providers. Mobility is introduced in IEEE 802.16e, known as WIMAX R1.0, in 2005 [60]. Lately, IEEE 802.16 m in 2011, known as WIMAX R2.0 offers many folds higher data rates than Rel-1.0. It is considered a 4G technology satisfying the ITU requirements of 1 Gbps downlink peak data rates for low mobility users and 100 Mbps for high mobility users [1]. 3G/4G LTE (Long Term Evolution) It is a mobile communication technology standardized by 3GPP. It is the biggest jump on the evolution path from 3G UMTS and CDMA2000 towards 4G, with ambitious requirements for data rates, capacity and latencies. LTE, like WIMAX uses OFDM on the downlink. On the uplink it uses SCFDMA, a sort of OFDM which gives 2-3 dB improvement for the difficult uplink channel as it results in reduced peak to average power ratio (PAPR) of the composite transmitted signal. The advanced 4G version of LTE, LTE-Advanced (LTE-A), based on 3GPP UMTS Rel-10 in 2011, is also a 4G recognized mobile technology. The physical layer of LTE-A is designed to have lots of what future networks will have intensively. For example, LTE-A supports heterogeneous deployments where low-power nodes comprising picocells, femtocells, relays, remote radio heads, and so on are placed in a macrocell layout [61]. The two technologies, WIMAX and LTE, are competing while having much technical similarities and little differences. However, It seems that the LTE will dominate as a one unified standard in the near future [62]. Plans are set for WIMAX to migrate/integrate with LTE in a multiple heterogeneous access technology mode [62]. Future 3GPP enhancements Traditionally, broad-band over the wireless cared to provide the required link data rates, latency and QoS within certain network capacity and coverage targets. However, broadband provisioning is challenged by varying load and traffic conditions. This impacts the ability of satisfying the required link performance metrics all the time and everywhere; especially for users far away from the base station. Typically, 3G and 3G? systems aim at satisfying peak rates requirements. Hence, the broadband provided by these systems is characterized as being patchy in coverage and erratic in data rates. Standards for 4G tried to rectify this by enforcing new requirements such as setting limits on data rates peaks versus averages and total throughputs as well as cell centre versus cell boarders data rates while at the same time providing for hot-spot reliable service. New 3GPP releases (Rel13 and Rel-14) will tackle much of the issues on the evolution toward 5G, e.g. more

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spectral efficiency via full dimension MIMO (FD-MIMO) with large number of antennas at the base, licensed assisted access (LAA) for utilizing unlicensed spectrum while guaranteeing coexistence with existing devices, enhanced carrier aggregation (eCA) with up to 32 component carriers with inter-band carrier aggregation, mission critical enhancements, LTE support for V29 (vehicle-to-vehicle, infrastructure and pedestrian) services, and more [63]. Keith Mallinson, in his paper ‘‘2020 Vision for LTE’’ [64] following 3GPP TSG meeting in Slovenia in 2012 heralded an improvement figures of 3 9 6 9 56 = 1008 as a result of the new releases. These are: 39 increase in spectrum employed, 69 improvement in spectral efficiency, and 569 higher average cell density. Tools enabling such massive improvements [34, 53, 64, 65] include factors of more bandwidth, higher order 3D-MIMO and vertical sectors using 3D-beamforming, higher order QAMs, small cells, more reliance on WLAN (WIFI) networks to support broadband in hotspots and indoors, more reliance on TDD mode for LTE, and the use of cognitive radio techniques to exploit the spectrum more efficiently.

4.2 Tools Evolution in the Different Technologies This section outlines the evolution of the use of broadband tools categories throughout the different technologies until current 3GPP enhancements (Fig. 4). We comment below on the use of tools categories shown in the figure: • Hierarchy of users and cell size: The regular cellular structure prevailed in all technologies. The current trend is the use of multi-tier networks with small cells of different sizes under the umbrella of the bigger macro-cell. • Interference avoidance: Every technology has its own way of avoiding the interference. CDMA is the technology which rather confront it. Future technologies will have to 2G GSM / GPRS MIMOs & Modulation

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Improved Link Tools Tools for More BW

Cell Edge Tools

Interference Avoidance

Cell Size

Hierarchy of users

3G WCDMA

Re-use

Repeaters

3G HSPA+

4G WIMAX 2 & LTE-A

Beyond 4G LTE Rel 12-14

No MIMOS AMC

coop. +Small Cells coop. +Small Cells

Segregation in NoneTime & Spreading in Frquency Time & Freq (TDMA/FDMA) (CDMA)

None Spreading in Time & Freq (CDMA)

None Spreading in Time & Freq (CDMA)

Macro-Cell

Macro-Cell

Macro-Cell

Macro-Cell

Cellular

Cellular

Cellular

Cellular

Fig. 4 Tools evolution in the different generations

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WIMAX 1 & LTE

DL MIMO+ AMC+ 4x4 MIMO 8x8 MIMO DL-64QAM+ UL3D MIMO 16QAM, Multi4, 16, 64QAM 4, 16, 64QAM 4, 16, 64, 256 QAM Carrier(2x5, 4x5MHz) Short TTI, Short TTI, Fast PC Narrow banding + Narrow banding + Narrow banding + Permutation+ Improved Sched.(Dedicat Sched.(Dedicated Permutation+ + Rake Permutation+ Sched.& Sched. & short ed Channels) Receiver Scheduling short frame frame Channels) Re-use + Refarming +Carrier Aggr + vertical Re-use+FFR+ sectorization + Cogn. Re-use Re-use Re-use Re-use/ FFR Carrier Aggr. Radio + Dynamic TDD +Wifi Internetworking Enh. Repeaters+ Enh. Repeaters+ Repeaters+ FFR+ICC Repeaters Repeaters Repeaters FFR+ICC+BS +BS coop. +Small Cells FFR+ICC+BS

No MIMOS No MIMOS No High Order 8PSK in Mod. HSCSD Power Control(PC)+ Equalizer

3G HSPA

Segregation in Freq & Time (OFDMA)

Segregation in Freq & Time (OFDMA)

Segregation in Freq& Time (OFDMA)

Macro-Cell , Femto & other Small Cells Cellular + Multi-Tier Cellular+ Multi-Tier Cellular+ Multi-Tier w Small Cells (Hetro. w Small Cells w Small Cells Networks) Macro-Cell Femto & other Small Cells

Macro-Cell Femto & other Small Cells

Wireless Broadband Tools and Their Evolution Towards 5G…

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accept some sort of less orthoganility due to factors of increased heterogeneity and network density. Cell edge tools Repeaters are traditional solution in all systems to handle rate and coverage problems of cell edge users. Recent solutions include the cooperation of base stations, the use of small cells and to have the network and radio resources more distributed and in the vicinity of the users. Tools for more bandwidth Traditionally the tool for more bandwidth, besides licensing, was increased frequency re-use through cell splitting, sectorization and macro cell size reduction. More recent tools are FFR, carrier aggregation, and TD-LTE mode, Tools for improved link Improving link conditions is the job of equalizers and Rake receivers. In the HSPA enhancement of WCDMA, link rates are increased for links with good channel conditions through shared high speed channels, shorter transmission time interval (TTI) and fast scheduling operations. OFDMA technologies of WIMAX and LTE utilize even lower TTI and flat architecture for all of its connections. Additionally, OFDM uses narrowband channels that can be spread (like WCDMA) through subchannels’ hopping (permutation) without the pitfalls of direct spreading in WCDMA that complicates the receiver. MIMOs and higher order modulation tools Adaptive modulation and high order modulations are used in 3G DL and UL enhancements and in systems of LTE and WIMAX. QAM64 is the highest used in current technologies while QAM256 is suggested in future releases of LTE-3GPP. MIMOs are suggested for HSPA? , LTE, WIMAX and all 4G. Future systems will even have higher MIMOs.

5 5G Challenges It is clear by now that the basic motive for a new generation is the continuously increasing demand for connectivity and the exponential growth of traffic. Additionally, networks in the near future will have to handle traffic of widely varied characteristics as they will have to serve humans originated traffic as well as machine originated traffic. This variety is the result of the multitude of use cases of coming IoT expected; e.g. smart industry, e-health, entertainment, automotives, smart homes and offices, environment monitoring, security and public safety and intelligent transportation and smart cities [66, 67]. Tens of billions of devices will generate traffic of widely varied QoS requirements; i.e. low or high data rates, delay/jitter tolerant or delay/jitter intolerant, etc. These will present different challenges for 5G as to increase its capacity to accommodate such traffic explosion, to exploit the spectrum as possible to accommodate needs for high data rates, to ensure the connectivity of these billions of devices of diverse requirements at minimum energy consumption, complexity, and infrastructural costs as possible. Hence, main challenges and aims of the coming 5G broadband are: • Capacity 5G aims at 1000 times increase of capacity compared to current networks [6] to satisfy the expected growth. • Spectrum The increased demand for capacity means that 5G will require improved creative solutions, especially on the spectrum side taking into consideration the limited spectrum available to current broadband systems. The spectrum for future 5G (and 4G as well) will not be a continuous one but rather a compilation of different frequency bands. This results in another challenge for transceivers to handle such discrete wide-

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band spectrum. Compressed sensing (CS) approach is suggested to alleviate the problem of sampling such wideband signals [68]. In fact, CS is suggested as an approach to handle many of the other challenges of 5G such as channel estimation, security and energy efficiency [68]. Energy 5G needs to provide this 1000-fold capacity increase to billions of devices in an affordable and sustainable way with low energy consumption [2]. It is worth noting that over the past three decades of wireless revolution data rates has witnessed 1000 times increase while the possible power reduction remains limited [52]. Hence, with the expected future explosion of traffic and increased data rates more advanced techniques will be needed to inflect a corresponding decrease in energy consumption. Enabling communication with energy limited devices like the wireless sensors is also a challenging aspect of the energy issue in the future. Connectivity 5G needs to connect anything, anybody, anywhere, anytime with any other thing or body using heterogeneous wireless networks. For example, the Everything is Alive (EiA) project at the University of Arkansas aims at exploring pervasive computing in a future smart, semantic world where every object can have identity and can communicate with humans and other smart objects [69]. This will bring up the problem of scalability; flexibility of adding and removing nodes, as another dimension of the connectivity challenge. Services and performance There is a continuous increase in the demand for new and improved services which in turn necessities improved technologies. In ITU vision for 2020 [4] services include mobile internet services of UHD video streaming, cloud storage, augmented reality, and virtual reality in addition to internet of things (IoT) services like smart home, intelligent transport systems (ITS), surveillance and smart grid. These services will also have to be guaranteed in conditions of high density, high traffic and high mobility scenarios. Hence, this multiplicity of services and the absence of the centralized decision in many scenarios will necessitate that multiple radio access technologies (RAT) will have to exist together in order to serve their varied requirements, Complexity and cost The connectivity, services and performance requirements of 5G will result in an increased complexity in comparison to the centralized nature of current technologies. Complexity of future networks may impose extra infrastructural costs on the customers’ side, as much of this complexity will be mainly within the user jurisdiction; i.e. his smart mobile device and the access node in his premise. Many factors can, on the other hand, result in reduced cost such as the increased density of the base stations and the flexibility and convenience of using mm-Waves for backhaul networks.

As an example, the services under the healthcare use case of 5G networks, may include, ultra-high-definition (UHD) content. delivery to aid health workforce, remote patient monitoring, medical telesurgery and healthcare centres [70]. Such usages may necessitate ensured real time connectivity at high data rates, low latencies with scalable network architecture and reliable and secure communication [70]. However, for the industrial automation use case there will be sensory monitoring, machinery and robotic remote control, asset and product tracking and connectivity with suppliers and maintenance. Such usages calls for time critical and none time critical applications, connectivity to different mediums and technologies, low data rates communications (e.g. sensors) as well as high data rates communications (e.g. the use of augmented reality) and high reliability and security for control applications [67, 71]. The smart city use case will include wide

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spectrum of services, which can range from efficient solutions to traffic congestion and transportation and mobility management, to providing services to autonomous and assisted cars through vehicle to vehicle and vehicle to infrastructure communication to many other city solutions such as smart parking and robust internet provision everywhere. Hence, smart city applications vary widely in their requirement for throughput and delay, must ensure reliability and security and must provide scalability and cater for intensive handover requirements Mobile edge computing (MEC) in which storage and computation resources are placed at the network edge, in the proximity of users in order to accelerate data streaming and shorten delays in smart cities context [72]. Generally, to tackle the above issues it will be necessary ‘‘to ensure tight whilst at the same time scalable integration of the various technology components into a single, seamless end-to-end networking experience’’ [67]. Hence, 5G network functions will have to be reconfigurable and many parts of the network to be virtualized/software defined [73]. Also, core network functionalities shall be sliced to enable serving the different use cases of varied QoS requirements [74]. In particular, the technologies and key solutions suggested for broadband and capacity provisioning (the focus of this survey article) in future 5G or beyond 2020 will resort mainly to either increasing the bandwidth or increasing its re-use. Bandwidth increase is mainly suggested through invasion of millimetric wave bands in the excess of 10 GHZ and up to 300 GHz and more. The use of massive MIMOs of large number of antenna elements is another method to largely increase the rates out of the same bandwidth. Additionally, the aforementioned concept of frequency re-use can be extensively utilized through more reliance on small cells and high density networks with separation of indoor and outdoor users using controlled power emission, mm-Waves frequencies, and optics. These technologies will necessitate that more roles to be assigned to mobile nodes in managing and optimizing the network. These key technologies and concepts for 5G are discussed more in the next section.

6 5G Tools In addition to what has been discussed in Sect. 3, the following paragraphs presents the major tools of 5G broadband and capacity provisioning.

6.1 mm-Waves A main difference between 5G and 4G will be the use of much greater spectrum allocations at untapped mm-wave frequency bands with highly directional beamforming antennas at both the mobile device and base station [75]. Communicating using the mm radio waves can significantly increase the bandwidth available resulting directly in higher data rates. The mm-wave spectrum for future cellular and backhaul networks and mobile users will result in multi-Giga bits per sec data rates [76]. Certain bands of the mm-Waves are suggested for future radio, e.g. 28 GHz and 38 GHz. This is because the frequencies at these bands are less attenuated than the frequencies in neighbouring bands [77]. Highly attenuated bands of 60, 120, 183, 325 and 380 GHz are as well suggested for short-range communications in office applications [77] as they can be limited in coverage. Indoor Tera bits/sec is possible by moving to the Tera Hz range [78]. For 1 Tb/s transmission under the constraint of a reasonable signal-to-noise ratio limit, the minimum required bandwidth is

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around 0.2 THz [78] which can be provided only in the Tera Hertz range. The use of mmWaves can improve energy efficiency as it is shown that the relative power consumption of wireless devices decreases as the RF bandwidth increases [79]. The mm-Waves also enable the use of complex antennas at reasonable size and cost, e.g. an on chip CMOS antennas is suggested for use in personal area communication systems in [80]. The challenge of using these mm-waves poses a challenge for developing suitable channel models for the 5G. The work in [76] aims to create a 28 GHz statistical spatial channel model for future 5G cellular networks while [81] handles channel estimation in massive MIMO systems. Backhauling will be critical to 5G. The mm-waves spectrum could simultaneously support mobile communications and backhaul. The backbone networks of 5G will move from copper and fiber to mm-wave wireless connections, allowing rapid deployment and mesh-like connectivity with cooperation between base stations [75].

6.2 Massive MIMOs The need for higher rates and capacities of future networks can be satisfied by further exploiting more the basic principle of MIMO systems; i.e. more multiple streams or more antenna elements. MIMO systems with large number of antennas at both the transmitter and the receiver are suggested to enhance point to point links [82]. An alternative is the use of multi-user MIMO in which the base station is equipped with large number of antennas while it communicates with a number of receivers of a single or limited number of antennas [83]. There are still many open issues with respect to the behavior in realistic channels that need further research and understanding, but the overall system performance seems very promising [83]. Some of the challenges to large MIMO systems are: • Energy efficiency of large MIMOs: Though MIMO systems will produce the required power using smaller values of each antenna, as the number of antennas increases, the overhead in low load situations increasingly impacts the overall energy consumed by a network [84]. Additionally baseband processing can increase energy consumption of large MIMO systems [84]. • Complexity of large MIMO systems which can be attributed to the huge processing of large number of data streams and the complexity in channel estimation. To reduce the complexity in channel estimation, techniques based on compressed sensing in which sampling is done at sub-Nyquist rates are suggested [85]. • Sharing of pilot carriers between neighboring small cells in multiuser systems with large MIMOS will result in pilot contamination problem [86]. Sharing pilots will result in interference causing less reliable channel estimation assuming no coordination between neighboring cells exists so as to share the pilots say at different times thereby avoiding pilots contamination. Pilot contamination limits the spectral efficiency of large MIMOs [87]. Spatial Modulation (SM) is a new modulation and MIMO technique [88] proposed to reduce the complexity in both signal processing and channel estimation. In these systems only one antenna element is activated at any single instant in time. Hence, the space is used as another dimension for modulation, in addition to the phase/and amplitude dimensions, therefore increasing the signal constellation dimension. In SM, one of the base station antennas is selected over which one symbol of the M-ary signal is transmitted. This is called Space Shift Keying. As a result, SM has the advantage that the interference between antenna elements is no more a problem as only one element is activated at a time. The

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drawback of using SM is reduced rates as data rates using SM increase logarithmically, rather than linearly with the increase in the number of antenna elements [88].

6.3 Indoor–Outdoor Separation A key design issue of 5G is to separate outdoor and indoor scenarios so that penetration loss through building walls can somehow be avoided thereby enhancing data rates and spectral efficiency while reducing energy consumption [7]. An example of a suggested 5G structure based on the separation idea is given in [7]. Integrating technologies such as complex antenna systems, mm-Waves communication and visible light communication can help create the separation.

6.4 Ultra-dense and Cooperative Networks Small cells provide capacity where it is needed whereby the spectrum is re-used more frequently. Hence future networks will not be limited by spectrum [89]. These networks will be primarily limited by the energy and infrastructural costs [89]. Resort to more spectrum and spectrum re-use can result in less energy but more infrastructural costs. Therefore, future ultra-high-capacity 5G networks will have to balance infrastructure deployment, spectrum, and energy cost components [89]. At the network level, the potential for reducing energy consumption lies in the layout of networks, their management [84], and the collaboration between neighboring cells. A more wider concept of collaboration, which will necessitate more powerful UE, is the collaboration between different systems of communication networks; e.g. mobile networks, WIFI, and GPS, to enhance services and/or achieve objectives of throughput, energy efficiency, and connectivity. As an example, the work in [90] presents an energy-efficient collaborative and opportunistic positioning system (ECOPS) for heterogeneous mobile devices. ECOPS facilitates mobile devices with estimated locations using Wi-Fi in collaboration with a few available GPS broadcasting devices in order to achieve high-energy efficiency and accuracy within available energy budget constraints. As the RF carrier is increased, the signals are absorbed by the environment in non-LOS scenarios, hence, the employment of cooperative solutions becomes more crucial [52].

6.5 Increased Roles of User-Terminals As a result of the multiplicity of network nodes and the availability of different RATs, 5G mobile networks will rely heavily on the role of the users’ terminals to manage and optimize the network performance. This is especially encouraged by the improved capabilities of the users’ smart devices. Some of these roles are: • Future terminals need to access and combine flows from different wireless technologies based on a pre-defined policy in a user-centric mobile environment [91]. Future mobiles architecture will be based on an open wireless architecture (OWA) platform that can enable a single terminal to seamlessly and automatically connect to different wireless mobile communications and high-speed wireless access systems whether at home, work, or in open areas [92]. OWA can ensure highest data rates, optimized QoS, and efficient utilization of the spectrum and other network resources [92], Generally, the user terminal in a 5G network is expected to endure changes at all network layers providing more flexibility and open architecture [93].

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• Transceivers in access points, base stations, or mobile units will be redesigned based on SDR with reconfigurable channels, modulation techniques, error-control schemes, etc., to enable connecting to different systems. Similarly, the whole network can be reconfigurable. This is called software defined networking [94]. • Future terminals will need intelligence to sense, share and use the spectrum opportunities through the concept of the cognitive radio. Cognitive technology is a key enabling technology of next generation networks [95]. SDR platforms are also needed to enable reconfiguring cognitive radio systems [96]. • The cognitive radio terminals will be involved in much of the communications with other terminals in D2D communication mode [97]. Also terminals will be involved in communications with multitude of wireless sensor networks (WSNs). • Additionally, there is an urging demand for flexible, bandwidth-efficient as well as power efficient transceivers. In [52], a few general design guidelines for the MIMO transceiver for next generation wireless transceivers are offered.

6.6 Tools for Machine Type Communication (MTC) As discussed in the previous section, different use cases in 5G may have devices which deliver high throughputs, which is, to some extent, similar to human type communication. On the other hand, billions of sensor/actuator devices will be exchange small data packet bursts in the order of bytes or hundreds of bytes. These in particular, present special challenges to 5G networks with synchronous type of communications based on 4G LTEAdvanced. Device attaching and synchronization require massive signalling overhead compared to the data load which result in energy inefficiency and battery discharge. Multitier solutions are suggested in research where devices are optimally grouped and an aggregator collects the small data bursts from the devices in the group via D2D communication. The aggregator in turn uploads the aggregated data efficiently to the macro eNodeB [98]. Naturally, this approach incurs extra delay due to aggregation cycle which makes this method suitable for delay tolerant applications. Another approach is the standardization of Narrow Band-IOT (NB-IOT) air interface in 3GPP Rel-13 to support efficiently low throughputs cellular IoT devices. The NB-IOT is LTE based and can be deployed as stand-alone as a dedicated carrier (with a bandwidth of 200 kHz), in-band within the occupied LTE carrier (occupies one resource block of 180 kHz), and within the guardband of an existing LTE carrier [99]. NB-IoT enables refarming of the 200 kHz GSM carriers for the usage by 5G IOT devices. NB-IoT carrier accommodates massive number of devices in one cell of more then 52,000 devices per carrier, ensures longer battery life and enhances indoor coverage [99]. The NB-IOT suits delay tolerant IOT devices such as smart metering and industrial applications where the devices are buried indoors.

7 Comparing 5G with Current Networks Table 1 below compares current and future networks from many aspects such as air interface, network architecture, mobile terminal functionalities, antennas, etc.

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Wireless Broadband Tools and Their Evolution Towards 5G… Table 1 Comparison between current and future networks Feature

Current

Future

Air interface

Currently dominated by OFDM

LTE/OFDM will continue. Non orthogonal OFDM and other new air interfaces suitable for low latency and short burst types of communication

Network architecture

Macro cell based

Heterogeneous, ultra-dense small cells networks with Macrocell umbrella

Isolated network nodes of UEs, APs, and BSs

Cooperative networks in which more than two nodes cooperate to establish the communication

3-sectors architecture

3-D sectorization to serve hot spots and 3-D beamforming

Macro size of the order of kilometers or hundreds of meters

All sizes will exists from the Macro sizes to few meters to body area networks

Base-station centered with most radio functionalities within the base station

User-Terminals centered with more reliance on the role of complex mobile terminals

Isolated communication networks, e.g. mobile, GPS, and WIFI

Collaborated communication networks for more services, energy efficiency, etc.

Wired and fiber backhauling

mm-Waves backhauling added to support heterogeneity and scalability

Hardware Defined Networks

Reconfigurable SDNs are likely to exist as well

RAT

Single RAT or a combination of WIFI and different mobile generations (e.g. 2.5G/ 3G/ 4G)

No single RAT suffices but rather a flexible combination of evolved existing technologies and new radio concepts [9]

Mobile Terminal

Closed-Proprietary wireless architecture

Open Wireless Architecture (OWA)

Hardware built radio

Software Defined Radio (SDR)

Rigid spectrum access

Flexible spectrum access with cognitive radio feature

Antennas

Mainly in the lower bands of up to 5 GHz

Invasion of higher bands in the mm-Waves frequencies such as 28 GHz and 38 GHz for indoor usage and small cells in outdoors

Spectrum bands

Mainly Licensed/Some unlicensed bands used by WIFI and other services

Cognitive technology will allow unlicensed users to use the licensed spectrum of others as secondary users

Spectrum License

Mainly Point-to-Multipoint

Point-to-Multipoint and Point-to-Point

Limits

Spectrum limited

Infrastructural costs and energy limited

Capacity

Mainly outdoor with indoor served by outdoor systems

Separation between the outdoor and the indoor with indoor served by proprietary/small base indoor stations and access nodes

8 Conclusions This paper reviewed the different tools of the past and current wireless technologies that were used to offer capacity and the requested data rates from these technologies. The tools are classified as either tools to improve signal conditions or tools to increase the spectrum

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or improve its usage. The potentials of these tools to assist in providing rates and capacity requested from future networks is also examined. The paper also reviewed broadband provisioning evolution over the different wireless technologies and the tools that distinguished each technology in its utmost capability to offer capacity and the required data rates. Finally, the challenges and the tools for future 5G networks are presented and a comparison between present and past technologies and the expected 5G is finally given. Acknowledgements The authors like to thank Albaha University and the Deanship of Scientific Research for their support under research Grant No. 1435/228.

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I. Aldmour 92. Lu, W. W. (2008). An open baseband processing architecture for future mobile terminal design. IEEE Wireless Communications, 15(2), 110–119. 93. Janevski, T. (2009). 5G mobile phone concept. In 6th IEEE consumer communications and networking conference (CCNC) 2009 (pp. 1–2). 94. Kim, H., & Feamster, N. (2013). Improving network management with software defined networking. IEEE Communications Magazine, 51(2), 114–119. 95. Akyildiz, I. F., Lee, W.-Y., Vuran, M. C., & Mohanty, S. (2006). Next generation/dynamic spectrum access/cognitive radio wireless networks: A survey. Computer Networks, 50(13), 2127–2159. 96. Haykin, S. (2005). Cognitive radio: Brain-empowered wireless communications. IEEE Journal on Selected Areas in Communications, 23(2), 201–220. 97. Doppler, K., Rinne, M., Wijting, C., Ribeiro, C. B., & Hugl, K. (2009). Device-to-device communication as an underlay to LTE-advanced networks. IEEE Communications Magazine, 47(12), 42–49. 98. Orsino, A., Araniti, G., Militano, L., Alonso-Zarate, J., Molinaro, A., & Iera, A. (2016). Energy efficient IoT data collection in smart cities exploiting D2D communications. Sensors, 16(6), 836. 99. Ratasuk, R., Vejlgaard, B., Mangalvedhe, N., & Ghosh, A. (2016). NB-IoT system for M2M communication. In IEEE wireless communications and networking conference (WCNC), 2016 (pp. 1–5). Ismat Aldmour is currently in Albaha University in Saudi-Arabia in its Computer Engineering and Science department. He got his PhD from the University of Glamorgan (currently University of South Wales), Wales, UK, in the area of mobile wireless communications systems in 2008. His MSc and BSc are from the university of JordanJordan in the field of Electrical Engineering/Communications. He assumed a number of a non-academic positions including a research engineer/Head of Electrical testing at the Royal Scientific Society of Jordan. His current research interests include resource management in future 5G networks, femtocells, WSNs and engineering education.

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