We utilize the orthogonality and channel hardening properties of massive multiple-input multiple- output (MIMO) systems to propose an efficient uplink transmission scheme for a heterogeneous network (HetNet). Such a network consists of multiple user-equipments (UEs) communicating with a macro-cell base station (MCBS) through a small-cell BS (SCBS) where both BSs have a large number of antennas and deploy zero-forcing (ZF) detection. The SCBS helps relay UEs' information using quantize-forward (QF) relaying with Wyner-Ziv (WZ) binning and multiple-timeslot transmission for the binning indices to the MCBS. The MCBS then deploys separate and sequential decoding for each UE's message. To maximize the rate region, we optimize the quantization levels through geometric programming and further obtain the optimal transmission timeslot durations in terms of the optimal quantization. We show that the proposed scheme has linear codebook size and decoding complexity in the number of UEs, while it achieves the same rate region of other QF schemes that employ joint transmission at the SCBS and/or joint decoding at the MCBS, all of which have exponential complexity. Furthermore, simulation results show that the SCBS should employ finer quantization for UE signals that have strong UE-SCBS links compared with the UE-MCBS links, and the proposed scheme can substantially outperform several existing alternatives under a wide range of parameter settings.

We investigate the impact of small-macro cell cooperation (SMC) in improving the spectral efficiency and reliability of uplink transmission in a heterogeneous network. We consider a network of two user equipments (UEs), a macro-cell base station (BS) and a small-cell BS. Joint SMC involves macro-to-small quantized feedback and decode-forward relaying from small to macro cell. This cooperation utilizes full-duplex transmission and intra-network spectrum sharing. We first propose a transmission scheme based on superposition block Markov encoding at each UE, coherent decode-forward relaying and sliding window decoding at the small-cell BS, and quantize-forward relaying and backward decoding at the macro-cell BS. Second, we derive the optimal macro-cell quantization to maximize the whole spectral efficiency. Third, for a certain non-fading scenario, we prove that the proposed scheme asymptotically achieves the capacity (maximum spectral efficiency) by reaching the cut-set bound as macro-cell power approaches infinity. Fourth, we formulate the outage probability over block fading channels, considering the outage events at the small and macro-cell BSs and the channel variations over different blocks. Last, we generalize the proposed scheme to an N (>2)-UE case. As macro-cell power increases, the results show that the proposed scheme achieves a full diversity order of two and outperforms all existing non-SMC schemes. These strong results suggest the utility of the proposed scheme for potential deployment in 5G cellular networks.

We propose a composite decode-forward (DF) scheme for the two-way relay channel in the full-duplex mode by combining coherent, independent, and partial relaying strategies. The relay partially decodes each user's information in each block and forwards this information coherently with the source user to the destination user in the next block as in block Markov coding. In addition, the relay independently broadcasts a binning index of both users' decoded information parts in the next block as in independent network coding. Each technique has a different impact on the relay power usage and the rate region. We further consider the independent and partial DF scheme for its more practical channel state information requirements, and derive in closed-form link regimes when this scheme achieves a strictly larger rate region than just time sharing between its constituent techniques, direct transmission, and independent DF relaying, and when it reduces to a simpler scheme. The analytical approach is based on maximizing the weighted composite DF sum rate and comparing with the outermost time-sharing line connecting corner points of rate regions of the constituent techniques. Numerical results demonstrate significant rate gains by performing link adaptation of the composite scheme based on the identified link regimes.

We investigate the outage and decoding-delay performances for full-duplex (FD) decode-forward (DF) relaying with backward and sliding window decoding. In our analysis, we consider a block fading channel with full channel state information (CSI) availability at receivers and with limited CSI at transmitters. For backward decoding where the destination starts decoding from the last transmission block, we derive the average block decoding delay. For sliding window decoding, we analyze both joint and sequential decoding where the destination utilizes two received blocks either simultaneously or sequentially to decode the source data. We then derive their average block decoding delays and outage performances by considering channel variation over two transmission blocks and outage events at both the relay and the destination. When comparing FD relaying with backward decoding and half-duplex (HD) transmission, numerical results show that joint sliding-window decoding is the preferred choice in terms of complexity, decoding delay and outage performances.

In this paper, we examine message prioritization in terms of both rate and reliability in the two-way relay channel using decode-forward relaying. Each source sends both a low priority message and a high priority message. We design a scheme that routes the high priority messages through the relay and direct link but the low priority message is only decoded by the destination. For fixed message priorities, the rate-optimal power allocation is analytically determined based on the link state. The high priority message is guaranteed to have greater reliability than that of the low priority message. We analytically formulate this outage probability from both the perspective of a single user and that of the overall system, and numerically examine the increased reliability for the high priority message.

We study the multiple access relay channel (MARC) with relay-destination cooperation (MARC-RDC). This channel resembles the uplink transmission in heterogeneous network where two user equipments (UEs) communicate with macro-cell base station (BS) through small-cell BS. We propose a coding scheme where the transmission is carried over B blocks and each UE performs superposition block Markov encoding. The relay (small-cell BS) first jointly decodes both UEs information using sliding window decoding over two transmission blocks and then forwards these information to the destination (macro-cell BS) coherently with UEs. The destination quantizes its received signal in each block and forwards the quantization index to the relay. The destination then decodes both UEs information using backward decoding. For this scheme, we derive the achievable rate region and compare it with existing schemes and the cut-set bound. Results show that relay-destination cooperation enlarges the rate region as the destination power increases. We further show that the proposed scheme asymptotically achieves the capacity by reaching the cut-set bound when the destination power approaches infinity and the ratio of one UE-destination to UE-relay link amplitudes is equal to that of the other UE. These results make the proposed scheme appealing for deployment in 5G cellular networks.

High reliability and low latency are critical performance targets in the fifth-generation cellular networks. How does a full-duplex decode-forward relay fare in this context? To answer this question, we analyze the outage and (average) decoding-delay for both joint and sequential sliding window decoding. For comparison, we also analyze decoding delay of backward decoding and consider its existing outage analysis. In our analysis, we consider a block fading channel with full channel state information (CSI) availability at receivers and with limited CSI at transmitters and outage events at both relay and destination and channel variation over different blocks in sliding window decoding. Moreover, by analyzing the asymptotic performance at high SNR, we prove that both joint and sequential decoding achieve a full diversity order of two and derive the coding gain gaps between backward decoding and joint and sequential sliding window decoding. To see the benefits of full-duplex relaying, we also include the performance of half-duplex schemes and conclude that the preferred scheme depends on the rate, outage, and delay requirements for a specific service.

In this paper, we re-examine the relay channel under the decode-forward (DF) strategy. Contrary to the established belief that block Markov coding is always the rate-optimal DF strategy, under certain channel conditions (a link regime), independent signaling between the source and relay achieves the same transmission rate without requiring coherent channel phase information. Furthermore, this independent signaling regime allows the relay to conserve power. As such, we design a composite DF relaying strategy that achieves the same rate as block Markov DF but with less required relay power. The finding is attractive from the link adaptation perspective to adapt relay coding and relay power according to the link state. We examine this link adaptation in fading under both perfect channel state information (CSI) and practical CSI in which nodes have perfect receive and long-term transmit CSI, and derive the corresponding relay power savings in both cases. We also derive the outage probability of the composite relaying scheme, which adapts the signaling to the link regime. Through simulation, we expose a tradeoff for relay placement showing that the relay conserves the most power when closer to the destination but achieves the most rate gain when closer to the source.

In this paper, we consider a composite independent and coherent decode-forward (DF) relaying scheme and analytically evaluate its outage performance over Rayleigh fading channels. We examine the link-state regimes in which the optimal strategy is either direct transmission, independent DF (IDF), or coherent DF (CDF), depending on the relation among the links. Outage probabilities at both the relay and destination are determined in order to analytically derive the overall outage performance. With full channel state information (CSI), the relay can save power without affecting the overall outage performance. Specifically, by using only a portion of its maximum power for transmission, the relay conserves power in the IDF regime while still achieving the maximum transmission rate. Even with just statistical CSI, the relay can employ these link regime results to reduce transmit power significantly while maintaining the same outage performance as that of using full power.

We analyze the outage performance of coherent partial decode-forward (pDF) relaying over Rayleigh fading channels in both half- and full-duplex transmissions. In coherent DF relaying, the relay either partially or fully decodes the source message, then coherently forwards the decoded message with the source to the destination. This coherent transmission creates a beamforming gain from the source and relay to the destination which improves the transmission rate. We analyze the impact of this beamforming gain on the outage performance, considering outage events at both the relay and the destination. We derive analytical expressions for the overall outage probability, assuming full CSI at receivers and only limited CSI at transmitters. We further show that at high SNR, coherent pDF relaying converges to full DF relaying and achieves the full diversity order of 2. These analyses provide a fundamental understanding of the reliability of coherent pDF relaying and form the basis for analyzing performance in larger network settings. Numerical results show that partial decoding at the relay outperforms full decoding for low SNR and high target rates in the half-duplex mode. Comparison with existing results further shows that source-relay coherent transmission and joint decoding at the destination both improve the outage performance.

We propose a time-division uplink transmission scheme that is applicable to future-generation cellular systems by introducing hybrid device-to-device (D2D) and infrastructure cooperation. Compared with existing frequency-division schemes, the proposed time-division scheme achieves the same or better spectral efficiency and outage performance with simpler signals and shorter decoding delay. These advantages come from sending a different message in each transmission block without block Markovity as in existing schemes. Using time-division, the proposed scheme divides each transmission frame into three phases with variable duration. The two user equipment units (UEs) partially exchange their information in the first two phases and then cooperatively transmit to the base station (BS) in the third phase. We further formulate the end-to-end outage probabilities, considering outages at both UEs and the BS. We analyze this outage performance in Rayleigh fading environment assuming full channel state information (CSI) at receivers and limited CSI at transmitters. Results show that user cooperation improves the achievable rate region even under half-duplex transmission. Moreover, as the received SNR increases, this uplink cooperation significantly reduces outage probabilities and achieves the full diversity order despite additional outages at the UEs. These characteristics make the proposed scheme appealing for deployment in future cellular networks.

We analytically evaluate the outage performance of the half-duplex partial decode-forward (PDF) relaying over Rayleigh fading channels. In PDF relaying, the source splits its information into cooperative and private parts, and the relay decodes only the cooperative information then forwards it to the destination coherently with the source. Assuming full CSI at the receivers and limited CSI at the transmitters, we analyze the outage performance by jointly considering outages of the cooperative and private parts at both the relay and the destination. In spite of additional outage at the relay and limited CSI at the transmitters, we show that PDF relaying achieves the full diversity order of 2 at high SNR. Numerical results confirm the analysis and show the advantage of the considered PDF scheme, with coherent transmission and joint decoding at the destination, over the existing DF and PDF schemes.

We consider the impact of destination cooperation on improving the achievable rate region in a multiple access channel with joint source-destination cooperation (MAC-SDC). Such cooperation may be appealing in systems with a more powerful destination than the sources, such as the uplink of cellular networks. We propose a coding scheme in which each source employs superposition block Markov encoding and partial decode-forward relaying, while the destination employs quantize-forward relaying and backward decoding. The sources partially exchange their messages using not only the direct links between them as in source cooperation, but also the feedback links from the destination, hence they are able to exchange more information at the same power by utilizing the destination as a relay. We analyze the condition on channel and power parameters such that involving destination cooperation is strictly better than just having source cooperation. Results for Gaussian channels show improvement especially when the inter-source link qualities are close to source-destination link qualities. These results provide guideline for practical systems such as cellular networks in determining when it is beneficial for the base station to cooperate with mobile users in the uplink communication.

We derive the optimal resource allocation of a practical half-duplex scheme for the Gaussian multiple access channel with transmitter cooperation (MAC-TC). Based on rate splitting and superposition coding, two users transmit information to a destination over 3 phases, such that the users partially exchange their information during the first 2 phases and cooperatively transmit to the destination during the last one. This scheme is near capacity-achieving when the inter-user links are stronger than each user-destination link; it also includes partial decode-forward relaying as a special case. We propose efficient algorithms to find the optimal resource allocation for maximizing either the individual or the sum rate and identify the corresponding optimal scheme for each channel configuration. For fixed phase durations, the power allocation problem is convex and can be solved analytically based on the KKT conditions. The optimal phase durations can then be obtained numerically using simple search methods. Results show that as the inter-user link qualities increase, the optimal scheme moves from no cooperation to partial then to full cooperation, in which the users fully exchange their information and cooperatively send it to the destination. Therefore, in practical systems with strong inter-user links, simple decode-forward relaying at both users is rate-optimal.

We study a cooperative communication system consisting of two users in half duplex mode communicating with one destination over additive white Gaussian noise (AWGN). Cooperation is performed between the two users by partial decode-forward relaying over 3 time slots with variable duration. During the first two slots, each user alternatively transmits and partially decodes while during the last time slot, both users cooperate to forward information to the destination. We establish the achievable rate region of this scheme. Then using the Lagrangian method, we analyze the optimal power allocations and the optimal time duration that maximize its individual and sum rates for the symmetric channel. Results show a significant improvement of the rates compared to the classical multiple access channel (MAC) when the inter-user channel quality is better than that between each user and the destination.

In this paper, we present a new cooperative communication scheme consisting of two users in half-duplex mode communicating with one destination over a discrete memoryless channel. The users encode messages in independent blocks and divide the transmission of each block into 3 time slots with variable durations. Cooperation is performed by partial decode-forward relaying over these 3 time slots. During the first two time slots, each user alternatively transmits and decodes, while during the last time slot, both users cooperate to send information to the destination. An achievable rate region for this scheme is derived using superposition encoding and joint maximum likelihood (ML) decoding across the 3 time slots. An example of the Gaussian channel is treated in detail and its achievable rate region is given explicitly. Results show that the proposed half-duplex scheme achieves significantly larger rate region than the classical multiple access channel and approaches the performance of a full-duplex cooperative scheme as the inter-user channel quality increases.

This paper derives a simple general closed form expression of the bit error probability for differentially detected signals with post-detection equal gain combining in additive white Gaussian noise and slow frequency nonselective fading channels. The derived expression is then used to study the performance of Gaussian minimum shift keying (GMSK) and quadrature Differential phase sift keying (QDPSK) signals when transmitting over non-identical and correlated Weibull fading channels in which each diversity branch can have unequal signal to noise ratio (SNR) and different severity parameter β as well.