Burst error correction in high-speed digital mobile radio communications

In high-speed digital mobile radio communication, transmission performance is severely degraded by frequency-selective fading caused by the delay time spread of multipath propagation. Forward error correction (FEC) is one of the most effective techniques to improve transmission quality. The authors evaluate the effect of FEC on burst errors under frequency-selective fading. The FEC effect with diversity reception and interleaving is also investigated.<>     

Understanding the Time and Frequency Varying Characteristics of a Multipath Wireless Channel

Designing a spectrally efficient wireless channel requires a comprehensive understanding of its time and frequency varying characteristics, making it a stochastic medium of communication. These characteristics become more challenging to cater at the receiving terminal in a multipath transmission. This is because of the fading effect and Doppler shift of the transmitted frequency, specifically in cellular mobile radio systems and vehicle to vehicle communications. This paper presents the modeling, simulation, and then characterization of a cellular mobile radio multipath channel over its time and frequency varying fading gain. For this purpose, a discrete-time Finite Impulse Response (FIR) type filter of such a channel is designed, modeled, and simulated using time and frequency varying characteristics of the received signal. The simulated channel response is further analyzed in terms of coherence bandwidth, coherence time, delay spread, Doppler spread, and symbol time.

     

An investigation of space-path hybrid diversity scheme for basestation reception in CDMA mobile radio

In CDMA mobile radio communication systems, degradation of instantaneous signal to interference power ratio (C/I), which causes impairment of frequency utilization factor, will be significant in multipath fading environments. In this paper, a hybrid diversity scheme for fading reduction combining effects of space diversity and path diversity (represented by the RAKE method) is investigated. A quantitative evaluation of fading reduction effects of the hybrid diversity is performed, comparing them with those of space diversity only and path diversity only. The hybrid diversity scheme is promising in environments where the delay spread of a transmission path is 1 μs or less     

Line-of-sight indoor radio communication using circular polarizedwaves

This paper presents an anti-multipath fading scheme employing a circular polarized wave for line-of-sight (LOS) indoor radio communications. The circular polarized wave transmission/reception significantly mitigates the multipath fading caused by reflections from walls, ceiling, and floor since the energies of single-bounce reflected waves do not appear at the receiver, thereby resulting in significant amplitude fade and delay spread reduction. The received signal intensity variations and delay spread are investigated numerically using a 14-path propagation model and the usefulness is confirmed by comparing with a linear polarized wave transmission/reception under a same LOS condition     

The performance of the PACS radio link and its improvements atvehicular speeds

The personal access communication system (PACS) radio interface is the leading low-tier candidate for standardization in North America. This radio interface was originally conceived to serve pedestrian and fixed-distribution applications; there has been significant recent interest in extending this technology into high-mobility environments. In such environments, rapid channel variations significantly degrade the performance of the preselection diversity scheme proposed for use in PACS handsets. The effects of time-delay spread on the PACS radio link was also included in our investigation. It is found that a received signal strength indicator (RSSI) with a short measurement length can better cope with high fading rates than can a quality measure in a preselection diversity system, although a quality measure has better performance than RSSI at low speeds in the presence of time-delay spread. In the preselection diversity scheme, using short-measurement RSSI provides relatively good performance in both low- and high-mobility environments. However, its performance degrades rapidly for RMS delay spreads larger than about 9% of a symbol time. Postselection diversity using two complete receiver chains is more robust than preselection diversity, both to high fading rates and to delay spread. Postselection diversity is relatively insensitive to changes in the fading rate and can tolerate an RMS delay spread up to 12.5% of a symbol time     

无人机毫米波信道建模及统计特性研究

针对传统移动信道模型不适用于无人机毫米波通信场景的问题,本文基于射线追踪原理构建了一个无人机毫米波三维几何信道模型,并提出了一种针对该模型的信道参数更新算法,该算法考虑了通信距离、信号角度、路径时延和功率等传播参数的随时间演进过程。在此基础上,针对南京航空航天大学校园传播场景,研究分析了时延分布、时延扩展、角度分布和角度扩展等信道统计特性。数值仿真结果表明,本文模型能够有效复现无人机信道的时变非平稳特性,输出信道统计特性与实测结果吻合,可用于无人机毫米波通信系统的方案设计、性能优化和评估验证等领域。      

A Hybrid Multichannel Hardware Simulator for Frequency-Selective Mobile Radio Paths

We present the analysis and design of an efficient hybrid digital/analog multichannel hardware simulator for frequencyselective Rayleigh-fading mobile radio paths. The simulator has eight RF outputs and is linear and reciprocal. All outputs exhibit mutually independent Rayleigh fading over a 45 dB range and have identical time delay spreads. Analog RF techniques are combined with a digital signal processor (DSP) to generate random path modulation. Frequency selectivity from time delay spread is created using several RF path delay elements. Simulated vehicle velocity is continuously variable from 6 to 60 mph. The simulator is thus a flexible tool for the study of mobile radio transmission techniques.     

Low-complexity diversity combining algorithms and circuitarchitectures for co-channel interference cancellation andfrequency-selective fading mitigation

In a high capacity personal communication system (PCS), for a given bandwidth, co-channel interference (CCI) limits the system capacity. Low-complexity diversity combining algorithms and circuit architectures for co-channel interference cancellation and frequency-selective fading mitigation, which do not require training sequences, are introduced. Results obtained from computer simulation of hardware show that two-antenna diversity combining gives wireless communication systems a signal to interference ratio improvement of at least 3 dB over conventional two-antenna selection diversity. The technique is also effective in mitigating frequency-selective fading without using conventional equalization-an average irreducible word error rate (WER) of 2.4% is obtained in computer simulation of hardware for radio channels with normalized delay spread of 0.3. In contrast, for the same WER, selection diversity and single antenna without diversity can sustain normalized delay spread up to about 0.16 and 0.06 respectively     

失真自适应接收机技术及信号形式选择

对流层散射信道是一种典型的多径衰落信道,多径传播造成了信号的深衰落和多径扩散。而多径扩散现象的存在,会使速率高、码元窄、频谱宽的信号遭受频率选择性衰落的影响,造成传输中的信号码元之间产生码间干扰,严重影响信息传输质量。在此提出了一种基于失真自适应接收技术和3/4占空半余弦信号形式的数字接收机方案,能够自适应地跟踪失真信号的变化而进行最佳相干接收,使系统具有稳定的通信性能。     

多状态马尔可夫信道的时延分析

随着无线通信技术的迅速发展,无线通信为移动用户提供了极大的灵活性和便利性。然而,由于多径效应引起的慢/快衰落,无线衰落信道的传输速率是不稳定的,这种不稳定性影响着信道的传输时延和通信质量。在研究中,无线衰落信道也被称为马尔可夫信道,信道状态的切换由马尔可夫链控制。针对一个三状态的马尔可夫信道进行分析。结合数据分组的到达与服务,整个系统被建模为一个M/MMSP/1排队系统。通过定义条件开始服务概率以及条件平均时延,结合状态转移矩阵,给出数据分组的平均服务时间和平均等待时间的解析解。最后在极限情况下对数据分组平均等待时间进行讨论,给出了影响时延的因素。     

Postdetection MRC diversity for pi /4-shift QDPSK mobile radio

Diversity reception using a postdetection maximal-ratio combiner (MRC) was experimentally investigated for pi /4-shift QDPSK signal transmission in Rayleigh fading environments. Two-branch postdetection MRC diversity reception provides a diversity gain, for a required E/sub b//N/sub 0/, of about 1.5 dB over selection diversity. It is also effective in reducing the impact of multipath channel delay spread. The average BER due to delay spread can be further reduced by a factor of about 1.5 times from selection diversity.<>     

The Effects of Time Delay Spread on Portable Radio Communications Channels with Digital Modulation

Frequency-selective fading caused by multipath time delay spread degrades digital communication channels by causing intersymbol interference, thus resulting in an irreducible BER and imposing a upper limit on the data symbol rate. In this paper, a frequencyselective, slowly fading channel is studied by computer simulation. The unfiltered BPSK, QPSK, OQPSK, and MSK modulations are considered first to illustrate the physical insights and the error mechanisms. Two classes of modulation with spectral-shaping filtering are studied next to assess the tradeoff between spectral occupancy and the performance under the influence of time delay spread. The simulation is very flexible so that different channel parameters can be studied and optimized either individually or collectively. The irreducible BER averaged over fading samples with a given delay profile is used to compare different modulation/detection methods, while the cumulative distribution of short-term BER is employed to show allowable data symbol rates for given values of delay spread. It is found that both GMSK and QPSK with a raised-cosine Nyquist pulse are suitable for a TDM/TDMA digital portable communications channel.     

Performance of Multiple-Access DCSK Communication over a Multipath Fading Channel with Delay Spread

The bit error rate (BER) performances of variant delay multiple-access differential chaos-shift keying (VDMA-DCSK) communication systems over a multipath fading channel with delay spread are investigated. The BER formula of the VDMA-DCSK over the fading channel is evaluated. A two-ray Rayleigh fading channel model is used to simulate the VDMA-DCSK system. The theoretical and simulation results are presented and they match each other, which supports the theoretical analysis. The multipath performance of the VDMA-DCSK is compared with that of a benchmark coherent MA-CSK system and with that of an invariant delay MA-DCSK system. The results show that in the multipath fading channel with delay spread environment the VDMA-DCSK system performance decreases least.     

一种用于短波通信的新的频域差分解调器

在短波通信中,传输信号遭受延时`扩展、时变干扰以及接收机加性高斯噪声的影响。常规的差分解调器在恶劣的短波信道环境下性能很差。该文提出一种新的频域差分解调器,通过盲相位估计来消除延时扩展造成的相位误差的影响。仿真了常规差分解调器和新的差分解调器在恶劣短波信道环境下的性能对比。结果表明在快速衰落短波信道环境下,新的频域差分解调器的性能优于传统的差分解调器。     

Causes of burst errors in multipath fading channel [mobile radio]

The so-called irreducible error due to frequency-selective fading is known to have a serious effect on mobile radio communication systems. Thus, the analysis of such errors is a prerequisite for making high-speed digital signal transmission over a fading channel feasible. The authors attempted to elucidate the physical mechanisms causing such errors using laboratory measurements of microscopic bit error rate. The results clarified some sources of burst errors in a multipath fading channel. In particular, large and rapid timing fluctuations of eye pattern due to severe delay distortion of the multipath channel was found to be the most dominant source of error     

Pilot Based Channel Estimation in IEEE 802 16a OFDM System

1　Introdution　IEEE 802 16[1] is a specification for fixed broadbandwireless Metropolitan Access Networks (MAN). The stan dard is expected to bring low cost and more bandwidth prod ucts for broadband wireless access in the next years. Thisstandard specifies the physical (PHY) and Medium AccessControl layer (MAC) of the air interface of interoperablepoint to multipoint and optional Mesh topology broadbandwireless access system. The specification enables access todata, …     

A Wireless Network for Wide-Band Indoor Communications

We propose and analyze a wide-band indoor communication system that uses radio as the transmission medium either on a stand-alone basis or to supplement a hard-wired network for those situations where complete portability is desired. One principal impairment to such a system is intersymbol interference caused by frequencyselective fading. A novel media-access scheme is proposed which permits the use of resource sharing, wherein a small pool of time slots is effectively shared among all users to provide added protection against channel impairments on an as-needed basis. Our results show that the use of resource sharing and diversity provide excellent protection against intersymbol interference caused by frequency-selective fading with negligible impact on throughput. Furthermore, resource sharing plus diversity can permit significantly higher data rates without large queueing delays. For example, a wireless network with a 10 Mbit/s data rate in a 10 MHz bandwidth using four antennas at the base station has a less than 10^-4outage probability at a 10^-4BER in buildings with less than 58 ns rms delay spread. A loading of 75 percent is permitted for a queueing delay of less than 20 packet transmission times all but 0.01 percent of the time.     

Nonorthogonal pulseshapes for multicarrier communications in doublydispersive channels

A new approach to multicarrier digital communication over time-varying, frequency selective fading channels is presented. We propose a transmission signal set whose basic structure is similar to standard orthogonal frequency division multiple access (OFDM)-setups, i.e., a system of functions generated by time and frequency-shifted versions of a pulse-like prototype function known as a Weyl-Heisenberg (WH) system. Unlike previous OFDM studies, however, which are restricted to the case of orthonormal pulses, we consider nonorthogonal pulses that are adapted to realistically available a priori knowledge of the channel. Perfect transmultiplexing in the case of an ideal channel is incorporated as a mathematical side-constraint. We derive the expected intersymbol/interchannel interference of such a nonorthogonal FDM (NOFDM) system under the assumption of a wide-sense stationary uncorrelated scattering (WSSUS) channel. Based on this result, we compare OFDM and NOFDM schemes with regard to robustness against delay/Doppler spread     

小尺度时变多径衰落信道的仿真研究

从理论上分析了多径效应和多普勒效应对无线信道的影响,并分别从时域和频域给出了信号经历信道后的仿真图形,分析了典型时变多径衰落信道理论模型,并在此基础上建立了抽头延迟线（TDL）的确定性仿真模型;仿真分析表明,通过改变时延功率谱和确定性模型的参数计算方法,此模型可以进一步扩展并用于多种传播环境下的小尺度衰落仿真。     

Improved ADPCM voice signal transmission employing click-noisedetection scheme for TDMA-TDD personal communication systems

In personal communication systems, better voice quality and lower terminal price are essential requirements, and adaptive differential pulse-code modulation (ADPCM) codecs are one of the voice codec candidates that have good voice quality and reasonable cost. However, in slow fading environments, burst errors induce click noise in ADPCM voice signal transmission. This paper proposes a novel error-concealment scheme to improve the ADPCM voice signal transmission quality of time division multiple access time division duplex (TDMA-TDD) personal communication systems in slow fading environments. The proposed scheme employs an improved error-detection technique with PCM differential detection and overflow detection coupled with a cyclic redundancy check (CRC) code. To compensate for residual click noise, an expanded CRC error-detection signal is used. Erroneous PCM signals are adaptively suppressed according to the click-noise detection information. Hardware simulation results show that these techniques significantly improve ADPCM voice signal transmission quality by 0.5 points and 1.0 compared to that of the conventional ADPCM muting scheme and no processing, respectively, with the mean opinion scoring (MOS) test in the slow fading environments typical of personal communication systems