Junli Zheng

Study of the TCP upstream/downstream unfairness issue with per-flow queuing over infrastructure-mode WLANs

Fairness is an important issue in WLANs due to their shared media nature. Essentially, IEEE 802.11 MAC protocols have been designed to provide fair access for all the competing mobile hosts. However, the fairness at MAC layer cannot be maintained at TCP layer in the presence of both mobile senders and receivers accessing to wired networks. In this paper, we investigate the TCP upstream/downstream unfairness issue over WLANs with per-flow queuing employed at the access point (AP). The interactions between the IEEE 802.11 MAC protocol and TCP are evaluated through analysis and simulation. Based on the derived analytical model, an efficient solution is proposed to be implemented at the AP for fairness achievement. Copyright

Companding transform for the reduction of peak-to-average power ratio of OFDM signals

A novel companding transform scheme, which compresses large signals while enhancing small signals, is proposed to effectively reduce peak-average-power-ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals. The proposed method has a low implementation complexity while achieving a significant reduction of PAPR. For validating the method, linear piecewise linear, nonlinear-symmetrical and nonlinear-quasi-symmetrical companding transform schemes are studied. It is shown by computer simulations that the proposed method can outperform the well-known clipping-filtering scheme substantially.

Nonpilot-aided carrier frequency tracking for uplink OFDMA systems

A carrier frequency offset (CFO) tracking scheme without the use of any pilot subcarriers for the uplink orthogonal frequency division multiple access (OFDMA) system is proposed in this paper. We first analyze the interference due to CFO in uplink OFDMA systems, then, an offset tracking scheme is presented. The proposed scheme employs multiple time-frequency decision-feedback carrier recovery loops for individual users to restore the original symbols. Moreover, a modularized and flexible structure design is achieved for a low-complexity implementation of the loop. Simulation results show that the proposed scheme is resistant to multipath fading, while the BER (bit-error-rate) performance is very close to that of an ideal reception system without CFO.

A time-frequency decision-feedback loop for carrier frequency offset tracking in OFDM systems

Acquisition and tracking are two crucial stages necessary to the carrier frequency synchronization in orthogonal frequency division multiplexing (OFDM) systems. In this letter, by employing the rotation property of OFDM data subcarriers, a simple time-frequency decision-feedback loop without the use of pilot subcarriers is proposed for the fine carrier frequency offset (CFO) tracking. Specifically, with proper loop parameters, a residual CFO less than 10% of the subcarrier spacing may be well tracked for quarternary phase-shift keying (QPSK) modulation in the presence of noise, while for systems using QPSK, 16-QAM, and 64-QAM modulation schemes, the bit-error rate (BER) performance very close to that of an offset-free system may be achieved in both additive white Gaussian noise (AWGN) and frequency selective fading channels. Moreover, a hardware implementation in a practical OFDM system is fulfilled which verifies the effectiveness of the proposed scheme.

Piecewise-scales transform for the reduction of PAPR of OFDM signals

In this paper, we propose a piecewise-scales transform (PST) for the reduction of peak-to-average power ratio (PAPR) of OFDM signals. By transforming the signal with different scales according to the statistical property of OFDM signals, this method can compress large signals while enhancing small signals, achieving significant reduction in PAPR with low complexity. Specifically, with the complementary cumulative distribution function (CCDF) of PAPR of OFDM signals, this paper proves that the proposed method can compromise well the reduction in PAPR with BER performance in practical OFDM systems by choosing the transform parameters properly. It is confirmed by computer simulations that the proposed PST can outperform the well-known partial transmit sequence and the clipping-filtering methods substantially.

A dynamic utility-based radio resource management scheme for mobile multimedia DS-CDMA systems

Radio resource management is one of the key ways for providing quality-of-service (QoS) to users in wireless networks. In this paper, we propose a utility-based radio resource management scheme for mobile multimedia DS-CDMA systems. The algorithm is aiming at the maximization of the system overall utilities. In order to reduce the computational complexity, the radio resource management is decomposed into two levels of subproblems. With the proposed algorithm, transmit power and data rate assigned to users are dynamically adjusted according to their QoS requirements, channel conditions and current system load, so that the optimal resource allocation is achieved. Simulation results show that our scheme is flexible and efficient for managing radio resources in mobile multimedia DS-CDMA environments.

Utility optimization and fairness guarantees for multimedia traffic in the downlink of DS-CDMA systems

Radio resource control is one of the key technologies for providing quality-of-service (QoS) in wireless communication systems, where power and data rate are schedulable resources. In this paper, we propose a utility-based resource control scheme for multimedia traffic in the downlink of DS-CDMA systems. The goal is to achieve the social optimal resource utilization. Fairness is guaranteed by allocating each user with a minimal transmission data rate. Simulation results show that the proposed resource control scheme is flexible and efficient for the downlink of multimedia DS-CDMA systems.

Downlink transmit power minimization in power-controlled multimedia CDMA systems

Since in CDMA systems, to transmit with less power is essential for reducing neighbor cell interferences as well as saving the battery of user terminals, the transmit power minimization problem is an important issue to be considered. Sampath, Kumar and Holtzman (1995) presented a model of the transmit power minimization problem in CDMA systems uplink . In this paper, we first review previous work, then investigate the transmit power minimization in CDMA systems downlink. The sufficient and necessary conditions for the existence of the feasible solutions are analyzed. We also provide the optimal solutions to uplink and downlink models. The optimal solutions can be used as a reference for power control and radio resource allocation in multimedia CDMA systems.

A vacation model with setup and close-down times for transmitter buffer of ARQ schemes

This paper develops a finite-capacity single-vacation queueing model with close-down/setup times and batch Markovian arrival process (BMAP) for the statistic behavior of the typical ARQ schemes (Stop-and-Wait, Go-Back-N, Selective Repeat) at transmitter buffer. We obtain the queue length distribution, the average waiting time of arbitrary data frame and the server utilization ratio of the transmitter for all the ARQ strategies by model analysis. Through numerical examples, the influences of the batch arrival process and the link frame transmission error process on system performance are investigated. Numerical results show that an appropriate trade-off between the users service-of-quality (e.g. average waiting time) and the system efficiency (e.g. server utilization ratio) has to be considered in the design of implementation value for the close-down time.

A network-based solution for TCP in wireless systems with opportunistic scheduling

We investigate the impact of wireless opportunistic scheduling on TCP throughput. It shows that the optimization of the wireless link layer mechanisms need to be maintained at the transport layer by cooperation of the adjacent layers. We propose a new ACK reservoir method to combat against the spurious timeout caused by the specific scheduling behaviors. The performance enhancement of the proposed method is evaluated based on analysis and simulation.