Dongxu Shen

Super-Resolution Time Delay Estimation in Multipath Environments

The super-resolution time delay estimation in multipath environment is very important for many applications. Conventional super-resolution approaches can only deal with signals with wideband and flat spectra. In this paper, we propose a novel super-resolution time delay estimation method that can treat signals with narrowband spectra. In our method, the time delay estimation is first transformed into the frequency domain, in which the problem is converted into the parameter estimation of sinusoidal signals with lowpass envelopes. Then a MUSIC-type algorithm taking account of the envelope variation is applied to achieve the super-resolution estimation. Time delay estimation in active and passive systems are considered. Simulation results confirm that the proposed estimators provide better performance than the classical correlation approach and the conventional MUSIC algorithm for separating closely spaced signals with narrowband spectra.

Super-resolution time delay estimation in multipath environments

The problem of super-resolution time delay estimation in multipath environments is addressed in this paper. Two cases, active and passive systems, are considered. The time delay estimation is first converted into a sinusoidal parameter estimation problem. Then the sinusoidal parameters are estimated by generalizing the multiple signal classification (MUSIC) algorithm for single-experiment data. The proposed method, referred to as the MUSIC-type algorithm, approximates the Cramer-Rao bound (CRB) in terms of the mean square errors (MSKs) for different signal-to-noise ratios (SNRs) and separations of multipath components. Simulation results show that the MUSIC-type algorithm performs better than the classical correlation approach and the conventional MUSIC method for the closely spaced components in multipath environments.

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.

UEP for video transmission in space-time coded OFDM systems

We provide a sub-channel partitioning based unequal error protection (UEP) scheme for a space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) system. In such a scheme, video data is partitioned into high-priority (HP) and low-priority (LP) layers according to the importance of the data. At the receiver side, OFDM sub-channels are partitioned into high-quality (HQ) and low-quality (LQ) groups according to the estimated channel qualities. Based on the feedback of sub-channel partitioning results, the transmitter assigns HP and LP video data to the corresponding HQ and LQ sub-channels. Through theoretical analysis, we show there is indeed a significant BER difference between the HQ and LQ sub-channels, which can be exploited by UEP. Based on the analysis, we provide a criterion for determining the appropriate transmission power. Through computer simulations, we show that the proposed scheme offers significant performance gain compared to conventional methods. We also demonstrate that the scheme is the least sensitive to channel estimation errors among all compared schemes, and is hardly influenced by the Doppler spread. The feedback overhead can also be reduced with almost no performance penalty by bundling several neighboring sub-channels together and assigning them to the same group.

Stabilized multi-channel ALOHA for wireless OFDM networks

Multiple access based on orthogonal frequency division multiplexing (OFDM), or OFDMA, enables multiple users to simultaneously access the media by using different subcarriers. This leads to the convenient realization of multi-channel ALOHA, in which each user transmits with a group of subcarriers. In this paper, we first introduce the multi-channel slotted ALOHA algorithm to OFDM, which is called OFDMA-based multi-channel ALOHA (OMC-ALOHA). Since ALOHA is an unstable algorithm, we show OMC-ALOHA is also unstable. To solve this stability problem, we extend the pseudo-Bayesian algorithm to achieve stabilized OMC-ALOHA.

Unequal error protection for MIMO systems with a hybrid structure

We propose a hybrid multiple-input-multiple-output (MIMO) architecture to implement unequal error protection (UEP) for video delivery in MIMO systems. On the transmitter side, some of the transmit antennas are used to implement transmit diversity for delivering video data with high priority, while others are employed for spatial multiplexing to transmit video data with low priority. On the receiver side, a hybrid signal detection mechanism is adopted to separate and decode the mixed data. The goal is to exploit the diversity gain to provide better protection to the high priority data, while transmitting the low priority data with spatial multiplexing to achieve high data rate. Analysis and simulation results demonstrate that the proposed UEP mechanism significantly enhances the quality of video reception with high spectrum efficiency

Towards Opportunistic Fair Scheduling in Wireless Networks

Opportunistic transmission scheduling schemes improve system capacity by taking advantage of independent time varying channels in wireless networks. In the design of such scheduling schemes, the fairness criterion plays an important role in the tradeoff of total system capacity and the achievable throughput of individual users. To meet different fairness demands with a unified opportunistic scheduling scheme, in this paper, we have extended the well known opportunistic scheduling scheme PFS into αPFS, which satisfies arbitrary fairness demands, varying from proportional fairness to maxmin fairness, through adjusting the parameter α. To further improve the achievable diversity gains of αPFS, we extend the αPFS scheme into an αPFS-P scheme. Performances of αPFS and αPFS-P are studied and compared. As demonstrated in the simulation results, both αPFS and αPFS-P can achieve adjustable fairness criteria, varying from proportional fairness to max-min fairness. Compared with αPFS, αPFS-P achieves higher diversity gains with degraded short term performance, which is still better than the performance of PFS.

Effective throughput: a unified benchmark for pilot-aided OFDM/SDMA wireless communication systems

In this paper, we study the uplink performance of an orthogonal frequency division multiplexing (OFDM) wireless system where multiple antennas are utilized at the base station (BS). Further, capacity can be greatly enhanced through spatial division multiple access (SDMA), so that several users can transmit packets simultaneously to the BS. The system performance is determined by various transmission techniques, including methods for channel estimation, modulation, as well as channel coding. Conventional parameters such as packet error rate (PER) and bit error rate (BER) are unable to reflect the actual system performance because no consideration is given to the overheads incurred by the transmission techniques. Therefore, we are motivated to propose a novel concept called effective throughput to characterize the capacity available to users by incorporating all these factors. The effective throughput for a user can be viewed as the average number of successfully received data bits in an OFDM symbol after excluding erroneously received packets and the overheads due to channel estimation and coding. It also directly relates to the transmission delay of a user packet. The system effective throughput is the aggregated effective throughput of all users. Simulation results demonstrate that effective throughput can serve as a useful and more meaningful benchmark parameter in optimizing system performance.

Performance analysis for a stabilized multi-channel slotted ALOHA algorithm

We study the slotted ALOHA with multiple random access channels, the so called multi-channel ALOHA (MC-ALOHA). It is well known that single-channel ALOHA (SC-ALOHA) is unstable. Not surprisingly, MC ALOHA is also unstable. A stabilization algorithm for MC-ALOHA has been proposed in [D. Shen and V.O.K. Li, 2002], in which the pseudo-Bayesian algorithm in SC-ALOHA was extended to achieve stabilized MC-ALOHA. The idea is to estimate the number of attempting users so that user transmission probability can be adjusted accordingly. In this paper, we give a theoretical analysis on the algorithm performance for cases with limited and unlimited number of users by assuming perfect estimate. The theoretical results are validated by simulation, which shows the stabilization algorithm performs close to a system with perfect estimate. The simulation results also show that the performance of the stabilized algorithm is much better than the non-stabilized algorithm. With the stabilized algorithm, the system is always stable when the new packet arrival rate is less than system capacity. Even when the arrival rate is higher than capacity, system throughput can still be guaranteed.

CPLD-PGPS scheduler 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. Based on PGPS, the CPLD scheduler considers both the physical channel condition and the length of packets, and optimally allocates the sub-carriers to different users. The total transmit power is adoptively allocated to each subcarrier. With this scheduler, the system can achieve better system BER performance, and correspondingly superior PER performance. The system throughput is improved, the required bandwidth is guaranteed, and long term fairness for all traffic in the system is provided. In order to reduce the complexity, a simplified algorithm 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 schedulers is demonstrated by simulation with multimedia traffic