Search

The CSMA/CA driven MAC protocols withhold packet transmissions from exposed stations when they detect carrier signal above a certain threshold. This is to avoid collisions at other receiving stations. However, this conservative scheme often exposes many stations unnecessarily, and thus minimizes the utilization of the spatial spectral resource. In this paper, we demonstrate that remote estimation of the status at the active receivers is more effective at avoiding collisions in wireless networks than the carrier sensing. We apply a new concept of the interference range, named as n-tolerant interference range, to guarantee reliable communications in the presence of n (n ≥ 0) concurrent transmissions from outside the range. We design a distributed interference preventive MAC (IP-MAC) using the n-tolerant interference range that enables parallel accesses from the noninterfering stations for an active communication. In IP-MAC, an exposed station goes through an Interference Potentiality Check (IPC) to resolve whether it is potentially interfering or noninterfering to the active communication. During the resolve operation, IPC takes the capture effect at an active receiver into account with interfering signals from a number of possible concurrent transmissions near that receiver. The performance enhancement offered by IP-MAC is studied via simulations in different environments. Results reveal that IP-MAC significantly improves network performance in terms of throughput and delay.

Orthogonal modulation provides low probability of bit error, however its bandwidth efficiency is very low. Biorthogonal code may double the bandwidth efficiency, but its required bandwidth grows exponentially with the number of input bits as in orthogonal modulation. In this paper, we propose a multi-code biorthogonal code keying (MBCK) scheme that significantly reduces the signal bandwidth with the benefit from orthogonal waveform coding maintained. The system consists of multiple waveform coding blocks, and the sum of output codewords is transmitted. A problem with MBCK is that output signal is multi-level, which requires amplifier with high linearity. So it may not be an appropriate scheme for portable unit where power efficiency is highly important. We also propose a modified MBCK scheme that guarantees constant amplitude output. The transmitter of the proposed scheme contains a redundant waveform coder whose input is generated by encoding the information bits. Adding the codewords from all constituent waveform coding blocks, the composite signal has constant amplitude. It is also shown that the redundant bits are not only used to make constant amplitude signal but also used to improve the BER performance at the receiver.

Multicode code-division multiple access (MC-CDMA) and variable processing gain CDMA (VPG-CDMA) are very appealing techniques for achieving high bit-rate data transmission and providing significant capacity gain in wireless channels. This letter provides the peak-to-average power ratio (PAPR) reduction scheme by using a code selective allocation (CSA) in a MC VPG-CDMA system. From simulation results, the MC VPG-CDMA system experiences a reduced PAPR thanks to the CSA algorithm and maintains an acceptable performance degradation even if encountered with nonlinear distortions introduced by a level clipper.

This letter presents a timing synchronization method of high-rate wireless personal area network (WPAN) systems encountered with a multipath fading channel. Two-stage timing synchronization using constant amplitude zero auto-correlation (CAZAC) training symbols and correlation techniques is adopted. At the same time, a simple way of setting a reliable threshold used to estimate WPAN timing is provided.

This letter proposes a low-complexity single frequency estimator for flat fading channels. The simplified estimator decreases the number of computations in the calculation of the autocorrelation function (AF) when compared to AF-based conventional estimators. The simplified estimator yields a comparable estimation performance to the existing estimators, while retaining the same frequency range.

In this letter, we present a simple way of estimating the integer frequency offset of orthogonal frequency division multiplexing (OFDM) system over a rapidly time-varying channel. By utilizing the channel responses of neighboring subcarriers within one pilot symbol, the frequency offset estimator is derived. We show by simulation that the proposed estimator can accurately estimate the integer frequency offset with reduced computational burden.