Yong Xi

Adaptive Multirate Auto Rate Fallback Protocol for IEEE 802.11 WLANS

An adaptive multirate auto rate fallback (AMARF) protocol is proposed for IEEE 802.11 WLANs. The key idea is to assign each data rate a unique success threshold, which is a criterion to switch one rate to the next higher rate, and the success thresholds can be changed dynamically in an adaptive manner according to the running conditions, such as packet length and channel parameters. Moreover, the proposed protocol can be implemented without any change to the current IEEE 802.11 standards. Our in-depth simulation shows that AMARE yields significantly higher throughput than other existing schemes including auto rate fallback (ARF) scheme and its variants, in various running conditions

Modeling intra-flow contention problem in IEEE 802.11 wireless multi-hop networks

This letter proposes a new approach to model the intra-flow contention problem in multi-hop networks. The model takes into consideration of neighboring interference, hidden-node collision and multi-rate scenario. It can be easily used to do admission control or to calculate the end-to-end capacity of a given multi-hop route. Simulation results validate its accuracy.

On the Throughput and Optimal Packet Length of an Uncoded ARQ System over Slow Rayleigh Fading Channels

This letter presents a simplified expression for the average packet error rate (PER) for an uncoded packet transmission system with large packet length over slow Rayleigh fading channels, by assuming that the bit error rate (BER) has an exponential form. Then the throughput performance of an uncoded packet transmission system with automatic repeat request (ARQ) is investigated, and the optimal packet length is derived to maximize the throughput. The theoretical analysis is validated by simulations.

Multiple carrier frequency offsets tracking in co-operative space-frequency block-coded orthogonal frequency division multiplexing systems

This study addresses the problem of carrier frequency offset (CFO) tracking in co-operative space-frequency block-coded orthogonal frequency division multiplexing (OFDM) systems with multiple CFOs. Considering that the inserted pilot tones are decayed by data subcarriers in the presence of multiple CFOs, a novel recursive residual CFO tracking (R-RCFOTr) algorithm is proposed. This method first removes CFO-induced inter-carrier interference from data subcarriers, and then updates the residual CFO (RCFO) estimation of each OFDM block recursively. When used in conjunction with a multiple CFOs estimator, the proposed R-RCFOTr can effectively mitigate the impacts from the multiple RCFOs with affordable complexity. Finally, simulation results are provided to validate the effectiveness of our proposed R-RCFOTr algorithm, which has performance close to that of perfect CFO estimation at moderate and high signal-to-noise ratio, and significantly outperforms conventional CFO tracking algorithm for large CFOs.

Performance Analysis and Optimum Power Allocation for Packet Decode-and-Forward Cooperative Relaying System

Despite the very substantial body of research on the performance analysis of cooperative relay systems, most studies focus on either the symbol error ratio (SER) or outage behavior. This paper first analyzes the asymptotic average packet error probability (PEP) of uncoded packet cooperative relay system for DF (Decode-and-Forward) schemes in the high signal-to-noise ratio (SNR) region. In particular, the effect of packet length on optimum power allocation r_opt is studied. It is found, with a symmetrical links assumption, the optimum power allocation ratio is in the region of 1/2<r_opt<(3-sqrt(3))/2 regardless of modulation scheme; and when packet length becomes large enough, the optimum r_opt value converges to (3-sqrt(3))/2.

A rate-adaptive cooperative MAC protocol based on RTS/CTS scheme for MANETs

In this paper we propose a novel relay-enabled MAC protocol based on IEEE 802.11 RTS/CTS scheme for MANETs. The protocol enable the middle node lies between the source and destination to decide whether to relay for them adaptively based on the current channel state information (CSI). Because the channel with better quality can transmit data with higher rate, and distance is one of the primary factors that determine wireless channel quality, the throughput will be greatly improved if there is a higher rate supporting relay between source and destination. Besides, the throughput can be farther improved due to its ability to cooperative retransmit by taking advantage of the spatial diversity. When there is a transmission error, the relay will retransmit the signal that it received during the error slot to the destination in the following slot. By processing the originally received error packet and the packet retransmitted by the relay, the destination node can recover the original packets. The proposed approach is approved to have good performance by simulation based on NS2 platform.

Performance Evaluation, Improvement and Channel Adaptive Strategy for IEEE 802.11 Fragmentation Mechanism

We first examine the impact of channel errors and collisions on the performance of legacy IEEE 802.11 fragmentation, and find the optimal fragment size depends on both channel condition and network size. Then, we present an improved contention-free fragmentation burst (CFF) scheme, by which the source immediately retransmits the failed fragment without backoff procedure except for the first one. Finally, a channel adaptive fragmentation (ADF) scheme based on CFF is proposed to combat both channel errors and collisions. ADF can distinguish frame transmission failure caused by collision from that by channel error in moderate noisy channel. Simulations show our CFF and ADF have distinct performance improvements in noisy channel and large size networks.

Packet Error Rate Analysis and Power Allocation for CC-HARQ Over Rayleigh Fading Channels

In this letter, a novel approach is developed to evaluate the average packet error rate (PER) for chase combining (CC) based hybrid automatic repeat request (HARQ) protocol over block Rayleigh fading channels for the long packet system. A simple approximated expression for the average PER at the high signal-to-noise ratio (SNR) region is further proposed. Based on the approximation, the optimal power allocation (OPA) scheme for CC-HARQ is studied. The closed-form optimal assignment solution at the high SNR is derived to minimize the average total power consumption under packet loss rate (PLR) and maximum number of transmission rounds constraints. Furthermore, the impact of transmission parameters on the OPA scheme is investigated. It is found that the ratios among the optimal power sequence just depend on the targeted PLR and the maximum number of transmission rounds.

Asymptotic performance analysis of packet cooperative relaying system over quasi-static fading channel

Despite the very substantial body of research on the performance analysis of cooperative relay systems, most studies focus on either the symbol error ratio (SER) or outage behavior. This paper analyzes the asymptotic average packet error rate (PER) of the packet cooperative relay system for both AF (Amplify-and-Forward) and DF (Decode-and-Forward) schemes in the high signal-to-noise ratio (SNR) region, and studies the effect of packet length on average PER performance. It is shown that the system achieves the same diversity gain in terms of PER as in terms of SER, but with different coding gain depending on packet length. If we consider practical packet lengths, the DF scheme always achieves better performance than the AF scheme; with shorter packet length, the advantage of DF over AF is more significant. For large enough packet length, AF performance approaches that of DF.

Energy Efficient Optimization for CC-HARQ Over Block Rayleigh Fading Channels

In this letter, the energy efficiency (EE) of Chase combining based hybrid automatic repeat request (CC-HARQ) over block Rayleigh fading channels is analyzed and optimized from a cross-layer perspective. Specifically, we derive the EE of CC-HARQ in a tractable closed-form expression using a new log-domain linear threshold approximation method. Then the optimum payload length and average transmission energy per bit are identified to maximize the EE. In addition, to achieve a flexible tradeoff between EE and spectral efficiency (SE), the unified EE-SE tradeoff metric is introduced and optimized. The analytical and simulation results verify the effectiveness of our design.