Ashish James

Throughput Optimization in Cooperative Communications Based on Incremental Relaying

In wireless networks, designing transmission schemes that can adapt to time-varying channel conditions is key to improving spectral efficiency. However, the realization of such schemes for multihop wireless networks is challenging, owing to the requirement of coordination among nodes. As coordination through the use of feedback between nodes incurs significant bandwidth penalty, spectral efficiency can be improved by minimizing traffic in the feedback channel. This paper proposes an adaptive rate transmission scheme using incremental-redundancy-based cooperative coding that minimizes the required feedback. By exploiting the inherent implicit feedback channel during relaying, the spectral efficiency of multihop wireless networks can be improved considerably. Rather than allocating dedicated channels to feedback the quality of information at the relay, the implicit feedback channel measures such information and determines the transmitter for the additional coded (redundancy) bits. This optimizes the throughput. The proposed scheme is thoroughly analyzed under different deployment environments. Theoretical bounds for the proposed scheme are presented and supported with results from extensive simulation studies.

Performance Analysis of Fountain Codes in Multihop Relay Networks

Fountain codes have been extensively employed in delay-tolerant networks (DTNs) due to their near-capacity performance with very low encoding/decoding complexity. A decode-and-forward-based relaying strategy is ideally suited for fountain codes in such networks due to its ability to recover the source message from any subset of encoded packets with sufficient mutual information. However, the unreliable nature of the channel may lead to the starvation of some subsequent nodes with good channel conditions. By cooperation among the forwarding nodes, the overall latency of such networks can be alleviated. This paper analytically quantifies the latency of both cooperative and conventional fountain-coded delay-tolerant multihop networks by deriving the exact closed-form equations for the channel usage. The overall latency suffered by such networks forces conservation of the end-to-end delay, particularly for real-time applications. However, by constraining the total delay (the number of encoded transmissions), the performance of fountain codes deteriorates due to the lack of encoded packets for retrieving the entire source message. This degradation can be gauged by the average packet loss experienced with partial decoding of fountain codes. The exact closed-form equation for the average packet loss based on the channel usage for such delay-constrained networks (DCNs) is derived in this paper. The tradeoff between average delay and the channel usage required for successful decoding is also analyzed. It is observed that the average packet loss can be minimized by optimizing the total delay based on the performance across each link. Finally, the pros and cons of using DCNs and DTNs employing fountain codes are evaluated, and theoretical grounding to the simulated results is provided.

Downlink capacity improvement and interference reduction through Reverse Frequency Allocation

Multi-tier networks comprising of macro-cellular network overlaid with less power, short range, home base stations like femtocells provide an economically feasible method for increasing the cellular capacity. Also the benefit of femtocells is found more in downlink since the traffic in downlink is several times more than in uplink. However, the femtocells share the same licensed frequency spectrum as that of the macrocell resulting in cross tier interference which degrades the downlink traffic considerably. This paper investigates a novel method for seamlessly embedding femtocells within a macrocell resource distribution framework to create a two-tier system, which incorporates a cross-tier complementary spectrum sharing known as Reverse Frequency Allocation (RFA) along with Soft Frequency Reuse (SFR) strategy. The proposed scheme guarantees inter-cell orthogonality with reduced interference. It also assures a reasonably high system spectral efficiency, while providing better performance for the users especially in the downlink. Simulation results prove that this scheme can bring about higher system throughput and lower co-channel interference irrespective of the distance of the femtocell from the macrocell base station.

Interference mitigation through reverse frequency allocation in multi-tier cellular network: a downlink perspective

With the introduction of third generation mobile services, femtocells are considered as an economically feasible solution for combining mobile and internet technologies, thereby giving fast and reliable access to data with a better coverage. However, it is well-known that the femtocells and macrocells sharing the same licensed frequency spectrum results in heavy cross-tier interference which degrades the downlink performance considerably. In this paper, we investigate a novel frequency–division duplex allocation strategy which eliminates the downlink cross-tier interference to the femtocell network from the macrocell base station throughout its coverage area. The proposed scheme seamlessly embed the femtocells within a macrocell resource network to create a heterogeneous two-tier system. It makes use of a cross-tier complementary spectrum sharing technique known as reverse frequency allocation (RFA) where the frequency carriers used in the macrocell transmission are reversed and allocated to femtocells. As a result, it better balances the requirement of greater inter-cell orthogonality and reduced inter-cell interference since macrocell and femtocell operates on different bands in uplink and downlink. It also assures enhanced spectral efficiency and the well-known benefit of reduced outage probability, especially for cell-edge users. This work further analytically quantifies and highlights through simulation results that RFA guarantees greater overall network throughput in the downlink and reduced cross-tier interference regardless of the positioning of the femtocell with respect to the macrocell base station. Also it is to be noted that, with recent academic surveys illuminating that the benefit of femtocells is reflected more in downlink, the focus of the current work is on downlink transmission where the traffic is high and the deployment is more beneficial.

Incremental Code Relaying Protocol for Cooperative Communications

In this paper, the application of rate-compatible punctured convolutional (RCPC) code into coded cooperation relay system is studied. As opposed to conventional coded cooperation systems, RCPC is attractive for its better spectral efficiency. In addition, the broadcast nature of relay transmission provides a new dimension to the system in the form of implicit feedback channel from relay to source. This allows the source node to justify the quality of information bits at relay node and react accordingly by taking over relays role to transmit the additional parity. At the destination, selection code combining is then performed, which provides a diversity benefit to the underlying RCPC code. The performance of the proposed scheme in terms of the number of correctly decoded bits (goodput), bit error probability, and the required number of slots for message transmission are then analysed. Extension to the case where there are multiple relays to assist the transmission is also discussed.

Adaptive rateless coding with feedback for cooperative relay networks

Cooperation among nodes is proposed as an effective means of combating fading and for enhancing systems overall capacity and coverage. The robustness of rateless codes makes them particularly attractive for such networks. Yet, the decoding aspects of rateless codes are discarded in most of the traditional distributed networks. In this paper, the intermediate packet decodability of rateless codes is exploited to reduce the number of packets being processed at the relay nodes. This is achieved by harnessing the back channel (from destination to relays) for feedback. The relay transmissions and processing are thereby confined to assist the receiver in decoding the remaining information. It is shown both analytically and through simulation studies that such a scheme achieves significant savings in computation complexity, memory usage and overall energy consumption.

Effects of Delay Constraints on Multihop Networks Using Rateless Codes

When a message is forwarded through a network, a node will have multiple opportunities to reliably receive the message and cooperate in transmission. However, the transmit power of each node in a network is limited and so is the transmission range. Therefore, multihop transmission is a potential technique to deliver the data over large distances. Rateless code properties ideally suit decode and forward based relaying strategies as it improves the outage and bandwidth efficiency. In decode and forward based multihop networks, the messages are decoded and relayed over sequential point-to-point communication links. In a conventional multihop network, the intermediate nodes will have to wait indefinitely to correctly decode the information. To quantify the impact of this unknown waiting time, this paper analyses the performance of a multihop network with strict delay constraints. The average packet loss for such a network is obtained as a function of the total delay. By optimizing the total delay based on the performance across each links, the average packet loss can be minimized.

Spectrally Efficient Packet Recovery in Delay Constrained Rateless Coded Multihop Networks

Rateless codes have been found to be particularly attractive for decode and forward based relaying strategy in delay tolerant multihop networks. The latency performance of such networks is dependent on the worst links that results in the starvation of subsequent nodes with good channel conditions. The total delay suffered by such networks can be constrained by limiting the number of rateless coded transmissions, especially for applications with critical latency requirements. However, the performance of rateless codes deteriorates in such circumstances due to the lack of sufficient mutual information for successfully recovering the entire source packets. The fraction of source packets that can be recovered will depend on the encoded packets received across the transmission channel. This paper investigates the degradation in the performance of such rateless coded networks by deriving the average packet recovery rate. In order to improve the reliability in such delay constrained networks, a novel spectrally efficient transmission scheme for reliable multihop data transfer is proposed. The proposed scheme exploits the broadcast nature of wireless transmissions, which provides an inherent implicit feedback channel, to ensure the reliable delivery of information packets to the nodes in the network. Rather than allocating dedicated channels to feedback the packet recovery information, the implicit feedback channel determines such information which enhances the spectral efficiency. Further, the optimum number of packets recoverable within the specified delay constraint to reduce the reprocessing of lost packets across hops is analytically analysed in this paper.

Performance limits of rateless codes in delay constrained multihop relay networks

Rateless code properties ideally suit multi-node transmissions with decode-and-forward based relaying strategy, and have been extensively employed in delay tolerant networks. The overall latency suffered by such networks forces to conserve the end-to-end delay, especially for real-time applications, by limiting the number of rateless coded transmissions. However, the performance of rateless codes deteriorates in such circumstances due to the lack of sufficient mutual information for successfully recovering the entire source packets. This performance degradation can be gauged by the average packet recovery and cooperation among nodes results in enhanced performance. The exact closed-form equations for the average packets retrieved based on the channel usage for such delay constrained networks (DCNs) is derived in this paper. These are verified and supported with results from extensive simulation studies.

Impact of implicit feedback channel in cooperative relay networks

In a multi-node network, cooperation among nodes is an effective means to enhance coverage and potentially increase the capacity. For such systems, schemes based on incremental relaying have great potential to improve the spectral efficiency by adapting the transmission to time varying channel conditions. The performance enhancement brought about by the presence of relays in such incremental relaying based cooperative systems is dependent on the level of cooperation (based on the relay information quality) and on coordination among the nodes. Coordination is achieved through the use of feedback channels, which incurs significant bandwidth penalty and brings down the spectral efficiency. In order to mitigate this, one can exploit an implicit feedback channel available due to broadcast nature of relay transmissions. Instead of using dedicated feedback channels, the implicit feedback channel is used to measure the relay information quality. Based on this information, the transmitter (source/relay) for the additional coded (redundancy) bits is determined. Such a mechanism enhances the reliability as it ensures the availability of correct information at the destination node for decoding. This paper studies the impact of such an implicit feedback channel by employing powerful codes which exhibit inherent incremental redundancy features, such as rate-compatible codes (rate-compatible punctured convolutional (RCPC) codes and punctured low-density parity-check (LDPC) codes) and rateless codes (Luby Transform (LT) codes). Theoretical analyses of the proposed scheme are presented, and supported with results from extensive simulation studies.