Zhifeng Diao

Analysis of pilot-assisted channel estimators for OFDM systems with transmit diversity

In this paper, we analyze the performance of pilot-assisted least square (LS) and minimum mean squared error (MMSE) channel estimators for orthogonal frequency division multiplexing (OFDM) systems with transmit antenna diversity. We first provide a design of orthogonal pilot sequences to simplify the estimators. We then analyze the mean squared error (MSE) performance, and study the leakage effect. When a channel tap is not sample-spaced, our analysis shows that the power of the channel tap will leak to not only other taps of the same antenna, but also taps belonging to other antennas. The leakage across antennas is mainly determined by the phase separation between pilot sequences, which is further related to the ratio between the number of pilots and number of antennas. We demonstrate that the MSE performance can be improved if more pilots are used, or fewer channels are estimated simultaneously.

An adaptive packet scheduling algorithm in OFDM systems with smart antennas

To maximize system throughput and guarantee the quality of service (QoS) of multimedia traffic in orthogonal frequency division multiplexing (OFDM) systems with smart antennas, a new packet scheduler is introduced to consider QoS requirements, packet location in the frame, and modulation scheme. In OFDM, several consecutive subcarriers are grouped as a frequency subband. Each subband in a frame can be reused by several users with smart antennas. In this paper, based on the best-fit algorithm proposed for TDMA and the physical features of OFDM, a new packet scheduler is proposed to allocate different BER-classified traffics into the frame. Adaptive modulation is also applied in the scheduler. When compared with existing schedulers, our scheduler achieves higher system capacity with much reduced complexity. The use of adaptive modulation further enhances the system capacity. Simulation results demonstrate that as the traffic load increases, the new scheduler has much better performance in system throughput, average delay, and packet loss rate

CPLD-PGPS scheduling algorithm in wireless OFDM systems

In this paper, we propose a new scheduler for orthogonal frequency-division multiplexing (OFDM) wireless communication systems, called channel-condition and packet-length dependent packet generalized processor sharing (CPLD-PGPS) scheduler. The CPLD scheduler considers the condition of the physical channel and the length of packets at the same time, and optimally allocates the subcarriers in a frame. With this scheduler the system can achieve better system BER performance, and correspondingly superior PER performance. The system throughput is improved, at the same time the required bandwidth is guaranteed, and long term fairness for all the traffic in the system is provided. In order to reduce the algorithm complexity, a simplified CPLD is proposed, which maintains the system throughput as in the original scheduler, and guarantees the system performance with properly set system parameters. The superior performance of the proposed scheduler is demonstrated by simulation results.

A New Cross-Layer Designed Multipolling Mac Protocol Over WLANs

This paper develops a multipolling MAC protocol which exploits cross-layer information to support delay-sensitive multimedia services over WLANs. A user detection module and a multi-rate adaptation module are proposed in the physical layer to assist in link differentiation. With these two modules, our new multi-polling MAC protocol, named PALD-MPMP not only reduces the polling overhead, but also provides an effective polling scheduler by allocating transmission priorities to users according to their delay requirements or levels and channel gains. Simulation results show that our proposed protocol outperforms the standard point coordination function (PCF) for delay-sensitive services. All the performance improvements are mainly contributed by the awareness of cross-layer channel state information, and the consequent multi-rate adaptation schemes.

A channel-condition and packet-length dependent scheduler in wireless OFDM systems

In this paper, a new scheduler, called channel-condition and packet-length dependent packet generalized processor sharing (CPLD-PGPS) scheduler is proposed for orthogonal frequency-division multiplexing (OFDM) wireless communication systems. Based on PGPS, the CPLD scheduler considers the condition of the physical channel and the length of packets at the same time, and optimally allocates the sub-carriers in a frame. With this scheduler, improved system BER and correspondingly superior PER performance can be achieved. The system throughput is improved, while guaranteeing the required bandwidth, and providing long term fairness for all the traffic in the system. In order to reduce the algorithm complexity, a simplified CPLD is proposed, which maintains the same system throughput as the original scheduler and guarantees the system performance with properly set system parameters. Simulation results demonstrate the superior performance of the proposed scheduler.