Li-Der Jeng

Adaptive receiver for DS/CDMA multiuser communications

In code division multiple access (CDMA) mobile communication systems with time-varying multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) must be considered. The multipath effect usually change the characteristics of the spreading codes. Modification of the conventional receiver structure is needed to account for the interference of the multipath fading. This paper proposes an adaptive receiver for such multiuser direct sequence (DS) CDMA systems in multipath fading. We also analyze the mean square error (MSE) performances of the proposed receiver and the conventional receiver. Both the MSE analysis and simulation results show that the performance of the proposed receiver are better than that of the conventional receiver.

Performance of low-density parity-check coded FFH/BFSK systems under band multitone jamming

Frequency-hopping spread-spectrum (FH-SS) is an efficient technique to combat jamming. Previous works in jamming channels pay more attention to partial band noise jamming (PBNJ). In this paper, Low-Density Parity-Check (LDPC) coded fast frequency hopping binary frequency shift keying (FFH/BFSK) spread-spectrum (SS) systems are investigated under band multitone noise jamming (BMTJ). We take the advantage of both diversity gain and coding gain for enhancing the system performance. We consider maximum-likelihood (ML) diversity combing and square-law with equal gain combing schemes which are used to compute the codeword bit decision reliability. The sum-product algorithm (SPA) based on belief propagation is exploited for iterative decoding. Simulation results reveal that our proposed system provides excellent performance against band multitone noise jamming.

Image restoration for Reed-Solomon coded SFH/MFSK System With Partial-Band Noise jamming

The application of Reed-Solomon (RS) codes for slow frequency-hopping systems has been extensively studied in recent years. Previous works focused on improving the bit error probability (BER) by forward error-correction coding (FEC) or other communication techniques. None of them utilized the property of the transmitted data, such as the correlations among image pixels. Consequently, in this paper, we employ this characteristic of image and propose the scheme combining FEC and image processing techniques to enhance the anti-jam (AJ) capability. We use the Reed-Solomon code and the image filters to verify the quality of the transmittted images. By evaluating the peak signal to noise ratio (PSNR) values of the transmitted images, the simulation results show that the performance of our system is better than that only use FEC under partial band noise jamming (PBNJ).

Low-density parity-check codes for FFH/BFSK systems with partial-band noise jamming

The performance of low-density parity-check (LDPC) codes is investigated for fast frequency hopping / binary frequency shift keying (FFH/BFSK) in partial-band noise jamming channels. We employ different diversity combining schemes to compute the codeword bit decision reliability. In this paper, we consider the maximum-likelihood (ML) diversity combining scheme and two low-complexity suboptimal diversity combining schemes: square-law with adaptive gain control (AGC) combining and square-law with equal gain combining. Simulation results show that, by combining both of the diversity and coding gain, our proposed system can provide excellent performance against partial-band noise jamming.

Interference rejection and equalization in band-limited channels

1988 was summarized in [21 while recent efforts in In addition to thermal noise and intersymbol interference (ISI), a transmitted wideband signal of a wireless communication system may sufler from interference caused b y other cochannel narrowband users or other radio frequency interference (RFI). Although spread spectrum (SS) signals is inherently resistant to interference the system performance would be greatly enhanced i f we can eliminate most of the interference before despreading or demodulation takes place. This paper provides a workable solution for direct sequence systems when the channel can be modeled as an FIR filter a n d the narrowband interference (NBI) is either multiple CW tones, or an A R process driven b y a white Gaussian sequence. Unlike earlier solutions, the proposed NBI rejection scheme is based on a cost function evaluated at the output of the PN despreader. It is simpler to implement and is capable of removing ISI. When the de lay spread of the multipath channel is comparable to a bit time our proposal renders better BER performance.

A robust image transmission scheme for FFH/MFSK systems under PBNJ

In this paper, we consider a wavelet domain diversity combining method for image transmission. We propose a new image compression algorithm called modified-SPIHT (MSPIHT). Compared to the set partitioning in hierarchical trees algorithm (SPIHT), the MSPIHT is more appropriate for unequal error protection technique (UEP). Besides, our system considers a diversity combining to improve the received image quality for fast frequency-hopping (FFH) system. Simulation result shows that the performance of our system is better than that use standard SPIHT algorithm with or without diversity combining method.

DSP implementation of narrowband interference cancellation algorithms in MB-OFDM based ultra-wideband communication system

The multiband orthogonal frequency division multiplexing ultra-wideband (MB-OFDM UWB) system suffered from the narrowband interference (NBI) due to the presence of coexistence communication system, such as worldwide interoperability for microwave access (WiMAX). If we deal with the NBI before the receiver, the performance of MB-OFDM UWB system can be significantly improved. The NBI cancellation algorithms have been proposed and verified, which could mitigate the interference efficiently [1]. In this paper, the Digital Signal Processor (DSP) implementation of three NBI cancellation algorithms are presented to improve the performance of MB-OFDM UWB system. We implemented the algorithms in both floating-point and fixed-point arithmetic using the TMS320C6713 DSP Starter Kit (DSK) from “Texas Insturment Inc.”, and the algorithms are optimized to meet the realistic restriction in terms of execution speed, code length and preciseness. Finally, some statistics such as resource occupied and time consumed are compared in Code Composer Studio (CCS) profile.

Zero Forcing and Minimum Mean Square Error Equalization for OFDM-CDMA Multiuser Detection in Multipath Fading Channels

This paper introduces four block joint detection (JD) receivers which combining the orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) techniques, i.e., OFDM-CDMA receiver. The performance of the OFDM-CDMA receiver with orthogonal and non-orthogonal spreading sequences is investigated in mobile radio channels. The sequences considered as spreading codes are orthogonal Walsh-Hadamard, Orthogonal Gold, Gold and maximim length sequences or m - sequences. The JD algorithms for the OFDM- CDMA receiver are Zero Forcing Block Linear Equalizer (ZF- BLE), Minimum Mean Square Error Block Linear Equalizer (MMSE-BLE), with and without decision feedback (DF). Time- domain equalization does not require DFT or FFT operations and can be used to compensate the channel fading effect. Hence for hardware implementation, the time domain interpretation can lead to a more efficient structure when compared with the frequency domain approach. The bit error rate (BER) performance comparisons of different spreading codes and equalizers are shown in the simulations.

A new time-hopping/direct-sequence biorthogonal PPM UWB communication system

In order to increase the capacity and diminish the multiple access interference (MAI) of an ultra-wideband (UWB) system, we propose a new time-hopping/direct-sequence (TH/DS) scheme using N-ary biorthogonal pulse position modulation (BPPM). In contrast with the conventional TH/DS systems employing fixed partition of the TH slots (Shen and Ueng, Proceedings of the IEEE VTC-Spring, 2010), the proposed TH/DS system can put the start location of the TH slot in any one of Q available pulse positions within the frame. In the proposed TH/DS system, the modulation level of BPPM can be increased and the multiple access capability can be improved without degrading the system performance. Compared with the existing TH-UWB system that employs the whole frame to carry out TH process (Shen et al. IEEE Trans. Veh. Technol. 59(2), 742-753, 2010), the proposed system has the merits of smooth power spectral density and low receiver complexity. In this article, we also derive the probability distribution of MAI for each correlators output of the proposed TH/DS system based on the analytic characteristic function technique. In order to verify the correctness of the performance analyses and to demonstrate the effectiveness of the proposed TH/DS system, some simulation results are given in both the additive white Gaussian noise channel and the realistic UWB fading channels. From the simulation results, we find that the proposed TH/DS system outperforms the conventional TH/DS scheme.

Smart Antennas for Multi-Rate MC-CDMA Communication Systems

This paper proposes and compares three multi- rate receiver structures with post-fast Fourier transform (FFT) type and the pre-FFT type for orthogonal frequency division multiplexing code-division multiple access (OFDM-CDMA) communication systems. In multipath fading channel, the transmitted signal suffers from the effects of inter-chip interference (ICI) and multiple access interference (MAI). In order to solve these problems and support multimedia traffic, we propose three multi- rate methodologies- variable processing gain (VPG), variable chip rate (VCR) and multi-code systems to investigate the performance of the OFDM-CDMA communications in multipath fading environments. Simulations confirm the good performance of the proposed receiver structures.