Cooperative communications fundamentally changes the abstraction of a wireless link and offers significant potential advantages for wireless networks. However, such technologies are evaluated with idealized scenarios where many control costs are assumed to be negligible. Moreover, even if these costs appear small in simple network scenarios, they increase significantly with the size and traffic level of the network, requiring careful evaluation, or even innovative design, of cooperative protocols to ensure their usefulness in realistic networks. This article describes a realistic evaluation of cooperative communications in a networking context. Insights obtained from this evaluation help guide work on cooperative communications toward practical and potentially beneficial protocols.

In this paper, we investigate resource allocation for a multi-user OFDMA cooperative system. Aiming at minimizing the total transmission power under target rate constraints for each user, we propose centralized and decentralized subchannel, bit, and power allocation schemes. First, assuming knowledge of the instantaneous channel gains for all links in the entire network, centralized resource allocation algorithms with subchannel permutation, in which the subchannels are reallocated at the relay nodes, are considered. Then, a decentralized resource allocation scheme is proposed for ad hoc network. Simulation results show that significant performance gains can be achieved, especially when subchannel permutation is employed.

In this paper, we investigate two selective relaying schemes in cooperative OFDM systems. Selective OFDMA relaying, where the relay selection is performed in a per-subcarrier manner, and selective OFDM relaying, where one best relay among the L potential relays is selected to relay the entire OFDM block, are compared in a two-hop random network. The outage performance of equal bit allocation (EBA), where each subchannel has the same number of bits, and bit loading (BL), where bits are adaptively allocated to each subchannel, are analyzed and compared for these two approaches. The outage analysis clearly shows that a significant performance gain can be achieved by selective OFDMA relaying, whether EBA or BL is employed, compared with selective OFDM relaying. The performance gain remains the same for different relay locations. With EBA, the performance gain increases with an increase in L and N, the number of independent subchannels. For BL, the performance gain also increases with an increase in R, the average number of bits per subchannel, in addition to L and N. Centralized and decentralized implementation issues are also considered. For EBA, selective OFDMA relaying scheme is preferred because of its superior performance and simple decentralized implementation. For BL, selective OFDMA relaying scheme is a good choice for centralized systems and selective OFDM relaying is more suitable for decentralized systems at the expense of a loss in performance.

Information theoretic studies have shown the significant performance improvements of cooperative communications. However, these studies ignore both the overheads incurred in real implementations of the cooperative techniques at the physical layer and their interactions with higher layer protocols in a networking context. In this paper, we study the performance of realistic networking scenarios facilitated by cooperation by taking overheads incurred at the physical, MAC, and network layers into account. In particular, (1) we modify the physical layer model of the QualNet network simulator to incorporate decentralized distributed space-time block coding into all SINR calculations and to combine signals transmitted concurrently from multiple relays, (2) we implement a path-centric MAC protocol to both reserve a multihop path between source and destination nodes and coordination relay nodes, and (3) we modify the DSR protocol to support path reservation at the network layer. Preliminary simulation results demonstrate that significant performance improvement can be achieved by employing cooperation. We also demonstrate the overheads which challenge their effectiveness in real networks.

We investigate the resource allocation problem for an OFDM cooperative network with a single source-destination pair and multiple relays. Assuming knowledge of the instantaneous channel gains for all links in the entire network, we propose several bit and power allocation schemes aiming at minimizing the total transmission power under a target rate constraint. First, an optimal and efficient bit loading algorithm is proposed when the relay node uses the same subchannel to relay the information transmitted by the source node. To further improve the performance gain, subchannel permutation, in which the subchannels are reallocated at relay nodes, is considered. An optimal subchannel permutation algorithm is first proposed and then an efficient suboptimal algorithm is considered to achieve a better complexity-performance tradeoff. A distributed bit loading algorithm is also proposed for ad hoc networks. Simulation results show that significant performance gains can be achieved by the proposed bit loading algorithms, especially when subchannel permutation is employed.

Two fundamental properties of wireless networks, the variable link quality and the broadcast nature of the transmission, have often been ignored in the design of routing protocols. In this paper, we address the routing issue in broadband systems from a link-layer point of view. We focus on a clustered multihop wireless network. A link quality metric is first proposed to measure broadband links; then, three routing strategies, which were proposed in our previous study for flat fading scenarios, are re-designed to achieve cooperative and frequency diversity gain. The outage analysis and simulation results of the proposed three routing strategies show that all three protocols can achieve L(dc + 1)-order diversity, where L is the number of relays in each relay cluster and dc is the error correcting capability of the linear block code. The outage of optimal routing remains constant with an increase in the number of hops, M. While, the outage of ad-hoc routing increase linearly. N-hop routing, where a joint optimization is performed every N hops, can achieve a good complexity-performance tradeoff.

The fading characteristics and broadcast nature of wireless channels are usually not fully considered in the design of routing protocols for wireless networks. In this paper, we combine routing and cooperative diversity, with the consideration of a realistic channel model. We focus on a multihop network with multiple relays at each hop, and three routing strategies are designed to achieve the full diversity gain provided by cooperation among the relays. In particular, an optimal routing strategy is proposed to minimize the end-to-end outage, which requires the channel information of all the links and serves as a performance bound. An ad-hoc routing strategy is then proposed based on a hop-by-hop relay selection, which can be easily implemented in a distributed way. As expected, ad-hoc routing performs worse than optimal routing, especially with a large number of hops. To achieve a good complexity-performance tradeoff, an N-hop routing strategy is further proposed, where a joint optimization is performed every N hops. Simulation results are provided which verify the outage analyses of the proposed routing strategies.

There has been growing interest in the integration of multihop (or relaying) capability into conventional wireless networks. In this paper, we propose an OFDM-based selective relaying scheme, where the relay selection at each hop is performed on a per-subcarrier basis and joint selection is adopted at the last two hops. The outage analysis clearly shows that full spatial diversity gain can be achieved with this proposed selective OFDMA relaying. In contrast, no diversity gain can be obtained if the entire OFDM block chooses the same relay with the highest combined SNR. It is also demonstrated that with coding among the subcarriers, superior performance can be achieved by selective OFDMA relaying with only symbol detection at each relay. This is highly attractive as the processing complexity and decoding delay incurred are very small.

In this paper, we investigate the use of OFDM to facilitate cooperation among relays in a wireless network. In particular, we consider different relay and subchannel assignment and combining schemes. Based on the amount of channel state information, resources, such as subchannels, can be allocated to relays to improve the end-to-end performance. Simulation results are provided to compare the performance of these schemes in terms of block error rate. It can be seen that significant performance gains can be obtained even with little channel state information at the relays.

The system for VBLAST-OFDM based on LCP preceding was proposed for wideband wireless communications. Frequency diversity was obtained by LCP preceding over OFDM sub-carriers. A combined VBLAST-LCP decoder was proposed. It combined successive cancellation of classical VBLAST with Maximum Likelihood decoding of LCP preceding. The simulation result showed that VBLAST-OFDM-LCP can gain at least 8.5 dB compared with VBLAST-OFDM.

Detection algorithms for the Vertical Bell Laboratories Layered Space Time (V-BLAST) system in a flat fading environment were studied. The concept of segmented detection was proposed. The segmented detection method was used to perform the maximum-likelihood (ML) detection for the first layers and the DFE procedure for the remaining layers. A segmented detection algorithm based on minimum mean square error (MMSE) criterion was given. Compared with DFE method, the segmented detection improved the detection performance with the increase of limited complexity. The computer simulation verified it.

Several detection algorithms for the vertical Bell Laboratories layered space time (V-BLAST) system are compared. We propose the concept of segmented detection on the observation that the overall performance of decision feedback equalization (DFE) is limited by the performance of the first detected subchannel. We perform maximum-likelihood (ML) detection for the first several subchannels and use the DFE procedure to detect the remaining subchannels. Also, we propose a novel segmented detection algorithm based on the minimum mean square error (MMSE) criterion. Compared with DFEs, segmented detection improves the detection performance with little increase of complexity. Computer simulation verifies it.

A VBLAST-OFDM system with linear constellation precoding (LCP) tool is proposed for wideband wireless communications. Frequency diversity is obtained through the linear constellation precoding over OFDM subcarriers. And a joint VBLAST-LCP decoder is also proposed. The decoder combines classical VBLAST successive cancellation with maximum likelihood decoding. Computer simulation shows that VBLAST-OFDM-LCP has a performance gain of at least 8.5 dB compared with VBLAST-OFDM.

In this paper a VBLAST-OFDM system with linear constellation precoding (LCP) is proposed for wideband wireless communications. Frequency diversity is obtained through the LCP precoding across OFDM subcarriers. Also a joint VBLASTLCP decoder which combines classical V-BLAST successive interference cancellation with Maximum Likelihood decoding for the precoding is proposed. Computer simulation shows that VBLAST-OFDM-LCP has a performance gain of at least 8.5dB at a certain symbol error rate compared with VBLAST-OFDM.