Buffer-Aided Relaying With Adaptive Link Selection—Fixed and Mixed Rate Transmission

Abstract

Abstract-We consider a simple network consisting of a source, a half-duplex decode-and-forward relay with a buffer, and a destination. We assume that the direct source-destination link is not available and all links undergo fading. We propose two new buffer-aided relaying schemes with different requirements regarding the availability of channel state information at the transmitter (CSIT). In the first scheme, neither the source nor the relay have full CSIT, and consequently, both nodes are forced to transmit with fixed rates. In contrast, in the second scheme, the source does not have full CSIT and transmits with fixed rate but the relay has full CSIT and adapts its transmission rate accordingly. In the absence of delay constraints, for both fixed rate and mixed rate transmission, we derive the throughput-optimal buffer-aided relaying protocols which select either the source or the relay for transmission based on the instantaneous signal-to-noise ratios (SNRs) of the source-relay and relay-destination links. In addition, for the delay constrained case, we develop buffer-aided relaying protocols that achieve a predefined average delay. Compared to conventional relaying protocols, which select the transmitting node according to a predefined schedule independent of the instantaneous link SNRs, the proposed buffer-aided protocols with adaptive link selection achieve large performance gains. In particular, for fixed rate transmission, we show that the proposed protocol achieves a diversity gain of two as long as an average delay of more than three time slots can be afforded. Furthermore, for mixed rate transmission with an average delay of E{T } time slots, a multiplexing gain of r = 1 − 1/(2E{T }) is achieved. As a by-product of the considered link adaptive protocols, we also develop a novel conventional relaying protocol for mixed rate transmission, which yields the same multiplexing gain as the protocol with adaptive link selection. Hence, for mixed rate transmission, for sufficiently large average delays, buffer-aided half-duplex relaying with and without adaptive link selection does not suffer from a multiplexing gain loss compared to full-duplex relaying.

Radio Resource Allocation Over Fading Channels Under Statistical Delay Constraints

“…Such adaptive relaying efficiently schedules the S-R link and R-D link depending on their channel conditions in each frame. As a result, adaptive relaying can attain significant throughput gains over fixed relaying since it can exploit the link diversity by transmitting over the link with more favorable channel condition [45]- [47]. One disadvantage of buffer-aided adaptive relaying is that it introduces random queuing delay at the relay, which is not present under non-buffer relaying.…”

“…The work [47] considers the similar buffer-aided relaying model as in [46], and studies two adaptive link scheduling schemes with different requirements regarding the availability of CSIT. In the first scheme, neither the source nor the relay has full CSIT, and consequently, both nodes are forced to transmit with fixed rates.…”

“…Recently, the works [43], [44] introduce the idea of buffer-aided relaying, where the relay employs buffer to store the received data from the source for future transmission to the destination. Thanks to the buffer-aided relaying capability, adaptive link selection relaying is possible, where either the source-relay link or the relay-destination link is active depending on the channel state information (CSI) in each transmission frame [45]- [47]. While adaptive relaying is able to exploit the link fading diversity, one disadvantage is the (queueing) delay incurred at the relay buffer, which is assumed to be unconstrained or under average delay constraint in existing works [43]- [47].…”

“…Thanks to the buffer-aided relaying capability, adaptive link selection relaying is possible, where either the source-relay link or the relay-destination link is active depending on the channel state information (CSI) in each transmission frame [45]- [47]. While adaptive relaying is able to exploit the link fading diversity, one disadvantage is the (queueing) delay incurred at the relay buffer, which is assumed to be unconstrained or under average delay constraint in existing works [43]- [47]. Alternatively, in Chapter 5, we study the optimal adaptive relaying problem under delay-outage constraint.…”

Buffer Behaviour in Massive MIMO Systems

“…By adding buffers to relays it is increased the spectral efficiency of the systems. Because of better understanding, firstly, we will consider a relay network with three nodes [3]- [4]. So, there is a source of information, a relay that is equipped with a buffer, and a destination.…”