To Cooperate or Not: A Capacity Perspective

Abstract

Abstract-It is widely recognized that differential decode-andforward (DDF) cooperative transmission scheme is capable of providing a superior performance compared to classic direct transmissions employing differential detection, where no channel coding is used. However, the diversity gains achieved by the cooperative system become modest in practical channel coded scenarios, where the interleaving and channel coding gains dominate. Therefore, when a cooperative wireless communication system is designed to approach the maximum achievable spectral efficiency by taking the cooperation-induced multiplexing loss into account, it is not obvious, whether or not the relay-aided system becomes superior to its direct-transmission based counterpart, especially, when advanced channel coding techniques are employed. Hence in this paper the capacity of the single-relayassisted DDF based cooperative system was studied in comparison to that of its direct-transmission based counterpart in order to answer the above-mentioned dilemma. I. BACKGROUND AND MOTIVATIONThe technological advances in integrated circuits and radiofrequency electronics facilitate the employment of ever more sophisticated signal processing and coding algorithms. At the same time, it is increasingly important to find energy-and bandwidth-efficient solutions for reliable digital communication over time-varying wireless channels. Multiple antenna aided diversity techniques [1] constitute powerful arrangements of mitigating the deleterious effects of fading, hence improving the end-to-end system performance, which is usually achieved by multiple co-located antenna elements at the transmitter and/or receiver. However, it is often impractical for the mobile to employ a large number of antennas for the sake of achieving a diversity gain due to its limited size. Fortunately, in multi-user wireless systems cooperating mobiles may share their antennas in order to achieve uplink transmit diversity by formig a virtual antenna array (VAA) in a distributed fashion [2,3]. On the other hand, in order to avoid channel estimation for a VAA-aided system, which may impose both an excessive complexity and a high pilot overhead, especially in mobile environments associated with relatively rapidly fluctuating channel conditions, differentially encoded transmissions combined with non-coherent detection and hence requiring no channel state information (CSI) at the receiver becomes an attractive design alternative, leading to differential modulation assisted cooperative communications [4][5][6].In point-to-point communication systems using a singleantenna or co-located multiple antennas, it is feasible to achieve a high spectral efficiency by approaching Shannon's capacity limit with the aid of channel coding [7,8]. However, in contrast to the well-understood limitations of point-to-point single-user transmissions, researchers are only beginning to understand the fundamental performance limits of wireless multiuser networks, such as, for example the differentially The authors are with...

Hybrid Automatic-Repeat-reQuest Systems for Cooperative Wireless Communications

“…(2.103), the unwanted noise is also amplified at the relay, imposing a degraded performance at the destination, when compared to the corresponding co-located MIMO. According to [23,129], the relay should be close to the source in order to reduce the effects of noise amplification.…”

“…Therefore, an adaptive relaying scheme should be employed, instead of a fixed strategy at each relay. Such an adaptive scheme was proposed in [125,131], while the condition of switching between the AF and DF was further investigated in [23,129].…”

“…Jing and Jafarkhani [234] further developed the differential philosophy for employment in cooperative wireless communications, demonstrating that diverse space-time codes, such as Alamouti's scheme [47], the orthogonal design of [59], and the so-called Sp(2) design of [235], may be employed in relay-aided networks. Furthermore, Wang and Hanzo [129] proposed differentially encoded and Multiple-Symbol Differential Sphere-Detection (MSDSD) aided schemes for improving the system's performance, while reducing the decoding complexity of the classic Multiple-Symbol Differential Detector (MSDD) [236]. By contrast, Farhadi and Beaulieu [237] advocated a non-coherent low-complexity detection scheme based on M-FSK modulation.…”

“…Wang and Hanzo [129] Proposed sphere-detection for the AF cooperative UL in order to reduce complexity of classic multiple-symbol differential detectors.…”

The Capacity of Successive DF Relaying and Using Soft Multiple-Symbol Differential Sphere Detection

“…providing that the successive relaying induced interference has been sufficiently mitigated by the DS-CDMA technique and using a large spreading factor Q. Fortunately, the noncoherent DCMC capacity of the DF based conventional single-relay aided two-phase cooperative network has been evaluated in [11]. Additionally, in order to achieve perfectly correct detection at the RN and thus to avoid any potential error propagation, the source transmission rate should be lower than the noncoherent information rate of the SR link.…”