We consider the downlink of a multi-cell massive MIMO system suffering from asymptotic rate saturation due to pilot contamination. As opposed to treating pilot contamination interference as noise (TIN), we study the performance of decoding the pilot contamination interference. We model pilot-sharing users as an interference channel (IC) and study the performance of schemes that decode this interferencepartiallybased on rate-splitting (RS), and compare the performance to schemes that decode the interference in itsentiretybased on simultaneous unique decoding (SD) or non-unique decoding (SND). For RS, we non-uniquely decode each layer of the pilot contamination interference and use one common power splitting coefficient per IC. Additionally, we establish an achievable region for this RS scheme. Solving a maximum symmetric rate allocation problem based on linear programming (LP), we show that for zero-forcing (ZF) with spatially correlated/uncorrelated channels and with a practical number of BS antennas, RS achieves significantly higher spectral efficiencies than TIN, SD and SND. Furthermore, we numerically examine the impact of increasing the correlation of the channel across antennas, the number of users as well as the degree of shadow fading. In all cases, we show that RS maintains significant gain over TIN, SD and SND.

We consider a multi-cell massive MIMO system where a time-division duplex protocol is used to estimate the channel state information via uplink pilots. When maximum ratio combining (MRC) is used at the BSs, the re-use of pilots across cells causes the pilot contamination effect which yields interference components that do not vanish as the number of base-station (BS) antennas M → ∞. When treating interference as noise (TIN), this phenomenon limits the performance of multicell massive MIMO systems. In this paper, we analyze more advanced schemes based on simultaneous unique decoding (SD) as well as simultaneous non-unique decoding (SND) of the interference that can provide unbounded rate as M → ∞. We also establish a worst-case uncorrelated noise technique for multiple-access channels to derive achievable rate expressions for finite M. Furthermore, we study a much simpler subset of SND (called S-SND) which provides a lower bound to SND and achieves unbounded rate as M → ∞, and also outperforms SD for finite M. For the special cases of two-cell and threecell systems, using a maximum symmetric rate allocation policy we compare the performance of different interference decoding schemes with that of TIN. Finally, we numerically illustrate the improved performance of the proposed schemes.

We study the multiple-access interference channel (MAIC) where two two-user multiple-access channels (MAC) operate over a shared medium, thus mutually interfering. We first characterize an achievable region for the discrete memoryless MAIC based on the Han-Kobayashi scheme. For the Gaussian MAIC, a computable achievable region is provided by limiting the time-sharing variable to be binary, and depending on its binary state, the transmitters use a fixed private-common message power splitting. Focusing on the weak Gaussian MAIC, where the magnitude of the coefficient (gain) of the cross-channels are smaller than that of the direct-channels, two genie-aided outer bounds to the sum-rate are derived by providing noisy interfering signals as side-information. Numerical examples in the weak symmetric Gaussian MAIC shows that (i) the outer bounds improve upon previous bounds in many cases, and (ii) for a wide range of cross-channel gains, the achievable sum-rate and one of the outer bounds differ by a small numerical gap, thus providing good approximations to the sum-capacity in these cases.

Relay cooperation and integrated microwave and millimeter-wave (mm-wave) dual-band communication are likely to play key roles in 5G. In this paper, we study a two-user uplink scenario in such dual-bands, modeled as a multiple-access relay channel (MARC), where two sources communicate to a destination assisted by a relay. However, unlike the microwave band, transmitters in the mm-wave band must employ highly directional antenna arrays to combat the ill effects of severe path-loss and small wavelength. The resulting mm-wave links are point-to-point and highly directional, and thus used to complement the microwave band by transmitting to a specific receiver. For such MARCs, the capacity is partially characterized for sources that are near the relay in a joint sense over both bands. We then study the impact of the mm-wave spectrum on the performance of such MARCs by characterizing the transmit power allocation scheme for phase faded mm-wave links that maximizes the sum-rate under a total power budget. The resulting scheme adapts the link transmission powers to channel conditions by transmitting in different modes, and all such modes and corresponding conditions are characterized. Finally, we study the properties of the optimal link powers and derive practical insights.

We consider a two-transmitter two-receiver dual-band Gaussian interference channel (GIC) which is motivated by the simultaneous use of both the conventional microwave band and the unconventional millimeter wave (mm-wave) band in future wireless networks where the traditional microwave band is complemented by additional spectrum in the mm-wave band. A key modeling feature of the mm-wave band is that due to severe path loss and relatively small wavelength, it must be used with highly directional antennas, and thus the transmitter is able to transmit to its intended receiver with negligible to no interference to other receivers. For this model, we derive some sufficient conditions on the channel gains under which the capacity of this type of dual-band GIC is determined. Specifically, these conditions are classified as when microwave band channel gains have (a) weak interference, i.e., both the cross channel gains are less than 1 and (b) mixed interference, i.e., only one of the cross channel gains is less than 1, while the channel gains in the dual-band GIC satisfy certain additional conditions in each case.

Energy harvesting (EH) cognitive relays are an exciting solution to the problem of inefficient use of spectrum while achieving green communications and spatial diversity. In a spectrum sharing scenario, we investigate the performance of a cognitive relay network, where a secondary source communicates with its destination over Nakagami-m channels via decode-and-forward EH relays while maintaining the outage probability of the primary user below a predefined threshold. Specifically, we derive a closed-form expression for the secondary outage probability and show that it is a function of the probability of an EH relay having sufficient energy for relaying, which in turn, depends on the energy harvesting and consumption rates of the EH relay and the primary outage probability threshold. We also show that relaxing the primary outage constraint may not always benefit the cognitive EH relay network due to the limitations imposed on the relay's transmit power by the energy constraint.