Evolution of multiple-access networks — cellular and non-cellular — in historical perspective. Part 4

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SU/MU MIMO technology embedded into LTE network
However, even using a single-carrier (SC) or single-user (SU) FDMA modulation technique for uplink transmission and a SC-OFDMA technique for downlink transmissions (see definitions in [147]), it is difficult to provide a wide range of spectra allocations of different sizes for each subscriber located in various terrestrial conditions, as well as a significant increase in spectrum efficiency compared to previous 2G and 3G cellular networks.This can be achieved only by combining Advanced FDMA and OFDMA technologies with MIMO systems performed on the basis of multi-beam or phased-array ante nnas [135][136][137][138][139][140][141][142][143].The LTE Release 8 was recently expanded from two to 4 antennas in downlink spatial multiplexing from a BS, as shown in Fig. 26 (called also SIMO (single-input-multiple-output)-LTE system).
Here, the layers can be defined as simultaneously transmitted streams of data to multiple UEs using the same time-frequency resource.In such a manner, any transmission of separate data streams is distributed among desired layers.The pre-coder matrix indicator (or selection suggestion matrix) is needed to transfer the selected data for each desired user (see details in reference [143]).The LTE Release 9, as a new dual-layer transmission mode, also was performed for supporting of up to 4 transmitted antennas at the BS in downlink channel.Now we postulate the following question: if both LTE Releases 8 and 9 could be integrated with MIMO, can such a combined LTE-MIMO system satisfy the International Mobile Telephony (IMT) requirements.

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ysis of these requirements with respect to those that satisfy the deployments of LTE Release 8 system.It is clear seen that, even giving better latency for each UE, but using twice-narrower bandwidth, Release 8 cannot support high-rate transmission of data streams for each individual user.A fully-adaptive MU-MIMO transmission mode cannot be realized in cooperation with LTE Release 8 and LTE Release 9 [132,133,142,144].
Recently, a new MU-MIMO antenna system was introduced called the Advanced LTE (A-LTE) or, simply, LTE .We will introduce this advanced technology since, as was mentioned in references [145], it is better equipped to meet the requirements of the modern 4 th generation of wireless networks.
As seen in Fig. 27, the LTE Release 10, or A-LTE, can use at BS with at least 8 separate antennas for downlink MU connections, whereas for uplink, up to 4 UE antennas can be utilized.Here, a layer mapping supports the transfer of individual codes from 2 codebooks to each pre-decoding layer.
At the MU terminals, a de-mapping layer is used for transporting to each individual user its desired data codes.The use of a MIMO system at both end terminals allows for: -fast user channel estimation, selecting and equalization; -reliable cancellation of MU interference; -simplification of complexity of the interference-aware receiver; -reduction of the system's detection complexity; -fitting of each single antenna of UEs in MIMO configuration; -better implementation in the existing hardware, and so on.
In works [134,143,145] were introduced the scheduling algorithms, based on the geometrical alignment at the BS, which can minimize the IUI seen by each UE.In such a configuration, the proposed interference-aware receiver was found as a

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good candidate for the practical implementation in MU-MIMO LTE Release 10 combined system.
To show the difference between the SU-SISO (single user single-input-single-output), MU-MIMO systems and the latter based on multi-beam antennas, we schematically presented them in Fig. 28.The first system is based on point-to-point single BS and single UE antennas, whereas the second one is based on multiple antennas from both side terminals [146].
To show the efficiency of usage of combination of MIMO/LTE network based on multi-beam antennas with respect to SISO network, we present the computations of a normalized maximum sum rate I [in bits/s/Hz] of downlink based on the mathematical algorithm fully described in [146].In simulations, we account for a SU-SISO (that is, for M 1, N 1), for a SU-SIMO (single user single-input-multiple-output) (that is, for M 1, N 4), and for a MU-MIMO (that is, for M 4, N 4) integrated schemes (in the case of antenna correlated elements).The results of the numerical experiment are shown in Fig. 29.
It is clear seen from the presented illustrations that using the MU-MIMO system of various input-output antenna elements integrated as an example, with an Advanced LTE technology, it is possible to increase the spectral efficiency and the data rate in such an integrated MU-MIMO network.Moreover, both SU-SIMO (or MISO) LTE and MU-MIMO LTE integrated systems, with a high correlation between transmitter and receiver multi-beam antennas (Fig. 30), show better performance in spectral efficiency and data stream rate [146].Thus, it can be seen that the LTE-SU Rx gives low spectral efficiency and data rate with respect to the MU-MIMO A-LTE system.The later has a tendency to increase spectral efficiency and data rate per several times with respect to the previous systems and this tendency increases with an increase in SNR.With increase of amount of UE antennas and BS antennas this difference becomes more significant.
Finishing this paragraph, we should outline that by controlling of number of elements of multi-beam antennas at the both end terminals, BS and UEs, and a priory accounting for the real responses of each channel on multipath fading phenomena (by prediction of the real K-factor of fading) [148], we show the same effects, as were obtained in [140,[142][143][144][145], where a set of precoding codebooks (from one to several) was introduces for extension of the LTE Releases 8 to 10, using MIMO configurations with 2 or 4 transmitting antennas, or a dual-code-

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book deployment [134] for MIMO configuration with 8 transmitting antennas at the BS terminal.What is important to notice is that the main limiting factors that can decrease efficiency of the proposed MIMO system integrated, for example, with the Advanced LTE Releases from 8 to 10, observed during numerical computations based on the real experiment carried in built-up area, are the K-factor of fading, as a response of each individual communication channel on data transmission, and number of antenna beams within each terminal of the system, BS and UE.
Despite the fact that the approaches, proposed in [134,140,[142][143][144][145] can significantly reduce a total LTE/MIMO system structure yielding a low-complexity of signal processing against inter-user in-terference, as was shown in [146], without accounting for derivation of the K-factor based on various topographic scenarios of the built-up areas of service, as well as for the effective configuration of MIMO system based on multi-beam antennas [146], it is impossible a priory predict data stream parameters and efficiency of each specific propagation channel "hidden" in the whole system, based on strict analysis of the channel response, and, finally, increase efficiency of the proposed Advanced LTE/ MIMO-R10 network by managing and control of its GoS and QoS.
Such configurations can be extended for the combined femtocell/picocell/microcell/macrocell planning tool design (see Fig. 31).
These schemes can be considered as a best candidate of the convenient configurations that satisfied requirements of the 4 th and 5 th generation networks.

Summary
In Section 1, we introduced the reader into the conventional and current techniques, technologies and systems adapted for 2 nd (2G) and 3 rd (3G) generations of wireless networks, as well as the advanced technologies and their corresponding protocols used to utilize modern networks beyond 3G, such as 4 th and 5 th generations.A new generations, called 4 th (4G) and 5 th (5G), were introduced, which are ex-

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pected to be capable of providing wider bandwidth, higher data stream rates, greater interoperability accords various communication protocols, without any collusion between them, user's security and non-collision communications between users, that is, to provide significant increase in GoS and QoS.Thus, typical 2G standards as GSM (Global System for Mobile Communications) operated at 900 to 1900 MHz frequency band, which used TDMA/FDMA separate or combined digital modulation techniques, have not satisfied the high-data communication requirement.The Universal Mobile Telecommunications Systems (UMTS) standards that were related to 2.5G and 3G mobile systems, dealt with higher voice capacity and higher-speed digital data.The same parameters were expected for 3G communication networks such as WPAN (or Bluetooth), WiFi (or WLAN), WiMAX; all described briefly above.Unfortunately, even integrating and combined the existing 2G and 3G networks, technologies and protocols, was problematic to achieve 200 Mbps-1 Gbps rate, multimedia (video and audio) applications, as well as terminal's heterogeneity related to significant decrease of the network costs, greater mobile signal availability in a "jungle of noises" caused by multipath fading.
For these reasons in the 4 th and 5 th generations, a physical layer was significantly broadened by serving a wide range of frequency bands (see Sections 2 and 3).
In our opinion, recently performed modern LTE and LTE-A networks integrated with MIMO systems based on multi-antenna (multi-beam or phased-array) technology, briefly described in Sections 4, can substantially improve 4G network spectrum efficiency providing three kind of advantages with respect to single antenna LTE system: -transmit time diversity; -beamforming; -spatial multiplexing.All these advantages are shown in Table 3.Moreover, using spatial multiplexing, the number of simultaneously transmitted data streams, as well as the beam pattern for each transmitted data stream, can be managed and controlled by the corresponding protocol to optimize the 4 th and 5 th networks' capacity.
Therefore, such integration of a MIMO system with LTE-A technology allows us to avoid, in practice, all the drawbacks related to the previous generations described above.It also allows designers of modern 4 th generation of wireless networks improve their GoS and QoS, protecting against ISI and ICI caused by multipath fading phenomena, mentioned in Section 1, increase their frequency spectral allocation, and finally, minimize bit error rate and packet error rate.All these aspects fully correspond to the main aim of the authors of this book, that is, show the reader on how should be completely integrated all basic components of the wireless network: -the physical layer, based on multipath fading phenomena; -signal processing, based on modulation techniques; -protocols and accesses of multiuser servicing; -antenna design layer, based on performance of multi-beam and phased-array antennas, and so on.

Fig. 28 .
Single user SISO network (top panel) and multiple user MIMO network (bottom panel)

Table 2
presents a comparative anal- . IMT requirements for 4 th generation vs. the last LTE Releases and WiMAX 1 st and 2 nd generations of networks