Abstract
The paper focuses on the two-way relay channel (TWRC) and the multi-way wireless network with three terminals, where all three want to exchange or share data and have to do that with the help of a relay. This paper shows how it is possible to significantly decrease the number of time slots required to exchange messages between terminals in networks based on time-division multiple access (TDMA), by taking into consideration new techniques at the physical (PHY) layer. The paper considers a PHY layer where physical-layer network coding (PLNC), multiple-input multiple-output (MIMO), and in-band “full-duplex” (IBFD) with loopback interference cancellation are all integrated, so that it is possible to significantly increase the overall throughput of the network. This is entirely attained by transferring the burden from the time domain to the spatial domain, via spatial multiplexing and by simultaneously resorting to non-orthogonal multiple access, which is the consequence of using both PLCN and IBFD. For the TWRC, it is shown that, if a massive MIMO relay is used, a simple lattice-based PLNC can be directly applied and, with typical IBFD interference cancellation amounts, a TWRC can effectively use only one time slot instead of the four needed when adopting the traditional TDMA exchange. In the case of the Y-network (i.e., with three terminals), a technique is presented that allows all the information exchange between terminals to be cut from the six time slots required in TDMA to only one time slot, provided that the information packets are not too short. The error performance of these systems is measured by means of simulation using MIMO Rayleigh fading channels.
Highlights
In wireless networks where a relay node intervenes, the traditional way of exchanging messages between two or more terminals either involves time-domain multiplexing (TDMA) or dedicated frequency-domain disjoint channels, at the expense of high bandwidth inefficiency
Lemos et al EURASIP Journal on Wireless Communications and Networking (2017) 2017:92 of wireless communications have not yet been translated to the way that message exchange protocols make use of the PHY layer [2, 3]. Some of these recent developments in the PHY layer are (i) physical-layer network coding (PLNC), based on the idea that information packets can be superimposed and still recovered as long as the receiver knows part of the information that was superimposed; (ii) multiple-input multiple-output (MIMO) terminals and relays, which allow to boost the time-usage efficiency by transferring the burden from the time domain to the spatial domain, exploiting the spatial multiplexing permitted by having multiple antennas at the terminals; (iii) massive MIMO, where a very large number of antennas create an important channel orthogonality; and (iv) in-band “full-duplex” (IBFD) technology, which allows terminals and relays to transmit and receive at the same time in the same frequency band by applying several layers of loopback interference (LI) cancellation
The same strategy is used for both configuration A and configuration B, using virtual MIMO: the signals are transmitted simultaneously by the three terminals, and the relay applies a robust detection technique such as a lattice reduction-aided (LRA) detector, followed by ordered successive interference cancellation with minimum mean square error (OSIC-MMSE) [41]. It is well known in the MIMO literature that the performance attained with LRA captures the full diversity order available in the MIMO spatial multiplexing, i.e., the slope of the symbol error rate (SER) curves is the same as the one provided by the maximum likelihood (ML) detection, they exhibit some power penalty in respect to the ML performance curves
Summary
In wireless networks where a relay node intervenes, the traditional way of exchanging messages (symbols or packets) between two or more terminals either involves time-domain multiplexing (TDMA) or dedicated frequency-domain disjoint channels, at the expense of high bandwidth inefficiency. The third (digital) stage is required in the signal processing domain in order to provide a fine mitigation of the residual interference still present after the first two steps [5] Another cornerstone technology in 5G is the use of massive MIMO arrays (possibly employing hundreds of antennas) at the base stations and relays, which allows serving more users, i.e., increasing the overall system’s capacity. In [4], a multipair DF “full-duplex” relay that combines massive antenna array techniques is presented to mitigate the self-interference borne by the relay Those authors propose a method where the relay station receives pilots to estimate the loopback channel and processes the signal using ZF or maximum-ratio combining/maximum-ratio transmission (MRC/MRT) detection and precoding. The terminals transmit and receive simultaneously and in the same frequency band
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