MMSE Pre-distortion Scheme for Multiuser UWB

The paper proposes a new formulation of a minimum mean square error pre-distortion scheme for multiple-input single-output impulsive ultra-wideband in multiple user environment. For the performance evaluation, scenarios CM1 and CM3 of the IEEE 802.15.3a channel model are considered. Results show that such a scheme has a good trade-off between intersymbol interference and multiple access interference reduction and signal-to-noise ratio preservation, performing better than time reversal and zero forcing pre-distortion schemes.     

多用户跳时超宽带系统串行码捕获性能分析

根据超宽带系统多径延迟的特点,该文给出一种易于实现的超宽带跳时串行捕获方法;并在此基础上,着重分析了该系统在多用户环境下的系统捕获性能;最后通过理论推导和数值仿真分析了用户数量、检测概率、虚警概率、判决门限以及平均捕获时间之间的相互制约关系。从结果中看到,随着用户数的增加,检测概率将降低,而判决门限及虚警概率提高,串行捕获时间也将从微秒级增加到毫秒级。     

空形编码技术在超宽带通信系统中的应用

提出一种基于正交Hermite脉冲的超宽带通信系统方案.该方案将波形域分集概念引入到多天线超宽带系统中,拓展了常规空时编码的概念,有效提高了短距离、高速率无线通信的性能;采用合适的脉冲波型,在不同天线之间构成渡形正交.代替常规的编码正交,简化了发射机和接收机的复杂度.仿真结果表明,该方案性能良好,不仅适用于块衰落信道,同样适用于快衰落信道.     

Multiuser detection for DS-CDMA UWB in the home environment

We demonstrate the effectiveness of multiuser detection for an ultra-wideband (UWB) pulse based direct sequence spread spectrum system using code division multiple access. Extensive simulations were run using channel soundings of the 2-8 GHz band collected in a residential setting and characterized by a high level of multipath fragmentation. We show that the adaptive minimum mean square error (MMSE) multiuser detection (MUD) receivers are able to gather multipath energy and reject intersymbol and interchip interference for these channels to a much greater extent than RAKE receivers with 4 and 8 arms. We also demonstrate the adaptive MMSE is able to reject a narrowband IEEE 802.11a OFDM interferer, even for signal-to-interference ratio as severe as -30 dB. We show the adaptive MMSE exhibits only a 6 dB penalty relative to the single user case for the heavy multi-access interference (number of asynchronous users equal to spreading code length). The practical RAKE receivers were incapable of effectively rejecting either the strong narrowband interference or the heavily loaded wideband interference. Even more moderate levels of interference caused significant degradation in the performance of the practical RAKE receivers.     

Complex Field Network Coding for Multiuser Cooperative Communications

Multi-source relay-based cooperative communications can achieve spatial diversity gains, enhance coverage and potentially increase capacity when multiuser detection is used to effect maximum likelihood demodulation. If considered for large networks, traditional relaying entails loss in spectral efficiency that can be mitigated through network coding at the physical layer. These considerations motivate the complex field network coding (CFNC) approach introduced in this paper. Different from network coding over the Galois field, where wireless throughput is limited as the number of sources increases, CFNC always achieves throughput as high as 1/2 symbol per source per channel use. In addition to improved throughput, CFNC- based relaying achieves full diversity gain regardless of the underlying signal-to-noise-ratio (SNR) and the constellation used. Furthermore, the CFNC approach is general enough to allow for transmissions from sources to a common destination as well as simultaneous information exchanges among sources.     

基于Turbo编码的超宽带系统性能分析

为了降低严重的时间弥散影响,提出了一种Turbo信道编码方案引入超宽带系统中,分析和仿真了在不同无线室内环境下基于Turho编码的超宽带系统的误比特率性能。无线室内环境是由IEEE802．15．3a提出的修正的SV信道模型。为了降低迭代译码的复杂度,采用了LOG-MAP算法。仿真结果表明,相比于无编码的系统,具有Turbo编码的超宽带系统在不同无线室内环境下提供了可观的编码增益,随着迭代次数的增加,超宽带系统的性能得到了改善。     

Space–Time Coding for Multiuser Ultra-Wideband Communications

In this paper, we present the construction of full rate, fully diverse, and totally real space-time (ST) codes for ultra-wideband (UWB) transmissions. In particular, we construct two families of codes adapted to real carrierless UWB communications that employ pulse position modulation, pulse amplitude modulation, or a combination of the two. The first family encodes adjacent symbols and is constructed from totally real cyclic division algebras. The second family encodes the pulses used to convey one information symbol, and permits achieving high performance levels with reduced complexity. The first family of codes achieves only a fraction of the coding gain of the second one. Moreover, these coding gains are independent from the size of the transmitted constellation. For time-hopping multiple-access channels, the amplitude spreading code associated with the second family of codes is taken to be user-specific. In this case, a simple design criterion is proposed, and spreading matrices constructed according to this criterion permit reducing the level of multiple-access interference (MAI). Simulations performed over realistic indoor UWB channels verify the theoretical claims and show high performance levels and better immunity against MAI     

空时编码在单天线UWB通信系统中的应用

超宽带(UWB)适用于基带多用户通信、战场无线通信和高数据率多媒体业务等通信系统,其数据传输速率高、功耗低、多径分辨能力强。但超宽带脉冲信号时域支撑区极窄,信道为密集多径,将空时编码技术引入超宽带通信系统,能够提升无线通信系统的信道容量与抗误比特率性能。在对UWB空时分组编码系统模型性能理论分析的基础上,对空时分组码在单天线UWB系统应用方案与UWB空时分层码方案进进行了简要介绍,利用Matlab对IEEEUWB信道模型进行仿真,提出了空时编码在UWB通讯技术中应用后提升短距高速率无线通信的性能的结论 。     

Space–Time Coding for Multiuser Ultra-Wideband Communications

In this paper, we present the construction of full-rate, fully diverse, and totally real space–time (ST) codes for ultra-wideband (UWB) transmissions. In particular, we construct two families of codes adapted to real carrierless UWB communications that employ pulse position modulation, pulse amplitude modulation, or a combination of the two. The first family encodes adjacent symbols and is constructed from totally real cyclic division algebras. The second family encodes the pulses used to convey one information symbol, and permits achieving high performance levels with reduced complexity. The first family of codes achieves only a fraction of the coding gain of the second one. Moreover, these coding gains are independent from the size of the transmitted constellation. For time-hopping multiple-access channels, the amplitude spreading code associated with the second family of codes is taken to be user-specific. In this case, a simple design criterion is proposed, and spreading matrices constructed according to this criterion permit reducing the level of multiple-access interference (MAI). Performance over realistic indoor UWB channels verify the theoretical claims, and show high performance levels and better immunity against MAI.     

Joint Downlink Beamforming and Time-Reversed Matched Filtering for Multiuser UWB Signals

This paper proposes a simple delay-line (DL) downlink beamformer for pulsed ultra-wideband (UWB) systems to compensate for propagation delay, which is detrimental to UWB signals. Moreover, we propose a prefiltering-based transmission scheme at the fixed access point (i.e., base station) by shifting the signal processing needs from the receiver at the mobile unit to the transmitter where power and computational resources are plentiful. The DL transmitter array can steer signals to the direction of the desired mobile user such that the desired signals stemming from each antenna are coherently combined and the multiuser interference (MUI) is averaged out. Furthermore, the prefiltering scheme is composed of a set of time-reversed matched filters (TR MFs) subject to multiple-input single-output (MISO) finite impulse response (FIR) channels. A simple correlation receiver is proposed at the mobile unit to extract the information of the time-hopping (TH) UWB signal. The performances under different scenarios are extensively evaluated.     

Differentially-Encoded Di-Symbol Time-Division Multiuser Impulse Radio in UWB Channel

Ultra-wideband (UWB) communication systems are expected to operate in a highly frequency-selective multipath fading environment. To exploit multipath diversity gains in a multiuser scenario, we developed a differentially-encoded, di-symbol time-division multiuser impulse radio (d²TD-IR) system with delay-sum autocorrelation receivers. In traditional time-division multiple access systems, each user transmits a single pulse during a symbol duration in a pre-assigned chip which is longer than maximum excess delay of the channel. However, due to the exponential decay property of UWB channel, we proposed the use of much shorter chip duration, which significantly increases the transmission rate. Because dense pulse transmission will induce multiuser interference, two time-hopping access sequences, which alternately encode the odd- and even-index symbols, are employed with delay-sum autocorrelation receivers to maximally suppress the interference. It was shown that when the chip duration is properly chosen, the proposed system outperforms the conventional time-hopping impulse radio system at high signal-to-noise ratio. This paper also proposed a method to estimate the optimal chip duration when only the average power decay profile of the UWB channel is known.     

Cross-Layer Design Using Superposition Coding Scheme for Multiuser OFDM Systems

In this paper, a cross-layer design is proposed for downlink of orthogonal frequency division multiplexing systems which uses superposition coding (SC) scheme. SC theorem allows two users to share the same subchannel. Firstly, the subchannel will be allocated to degraded user (who is far away from the base station) and then this subchannel will be allowed to be shared by potential user (who is near to the base station). We also employed a packet dependent scheduling at the medium access control layer which decides the transmission order of packets according to the delay, size and quality of service priority level of packets. The weight of each user can be calculated by summing up the weights of all packets in the queues of that user. We have considered that each user is having multiple heterogeneous traffic queues. Simulation results show that the algorithm proposed in this paper is better than the previously reported algorithm in terms of total throughput and packet delay with the same computational complexity.     

Source and Channel Coding for Correlated Sources Over Multiuser Channels

Source and channel coding over multiuser channels in which receivers have access to correlated source side information are considered. For several multiuser channel models necessary and sufficient conditions for optimal separation of the source and channel codes are obtained. In particular, the multiple-access channel, the compound multiple-access channel, the interference channel, and the two-way channel with correlated sources and correlated receiver side information are considered, and the optimality of separation is shown to hold for certain source and side information structures. Interestingly, the optimal separate source and channel codes identified for these models are not necessarily the optimal codes for the underlying source coding or the channel coding problems. In other words, while separation of the source and channel codes is optimal, the nature of these optimal codes is impacted by the joint design criterion.      

Receiver Design for Digital Fiber Optic Transmission Systems Using Manchester (Biphase) Coding

An analysis of the sensitivity of an optical receiver in a digital communication system using Manchester (biphase) coding is performed. Both cases of p-i-n and avalanche photodiodes are considered. Experimental results for the sensitivity of a Manchester receiver operating at 250 Mbits/s are reported. Two types of low noise receiver amplifiers, namely the high impedance and the transimpedance amplifier, are designed and implemented for use in the receiver. A receiver sensitivity of -49.8 dBm in terms of detected optical power is obtained (at a 10^-9bit error rate and 0.1 laser extinction ratio), which corresponds to only 175 average photons per bit. It is shown that in contrast to the NRZ code, the Gaussian approximation theory tends to underestimate the Manchester receiver sensitivity. Tradeoffs between Manchester and NRZ coding are also discussed in terms of receiver sensitivity and ease of implementation. It is shown that Manchester coding is an attractive alternative to NRZ coding for optical transmission systems, particularly when an avalanche photodiode is used.     

基于机会式网络编码的多用户信息交换

在两对用户通过一个公共中继节点的协作进行数据交换的无线通信环境下,该文研究了基于机会式网络编码的中继策略,即中继节点可以转发编码的分组,也可以转发未编码的分组的中继策略,说明了该中继策略与传统的复制重传中继策略相比能极大的减少系统功率消耗。传统的中继策略几乎必然导致分组的递交时延,而在机会式网络编码的中继策略下,中继节点为提高能量使用效率也可能导致某些分组递交时延增加。通过建立一个马氏链,该文刻画了在机会式网络编码的中继策略下每个分组的平均时延和平均功率消耗。仿真结果说明平均时延和平均功率消耗之间存在着某种折衷关系。对于缓冲区大小为1的情形,该文给出了这种折衷关系。      

码分多址系统中的多用户检测

CDMA通信系统是一个自干扰受限系统，如何克服CDMA系统中的多址干扰问题已成为提高CDMA系统容量的关键。多用户检测是克服CDMA多址干扰最有潜力的关键技术之一。本文研究了CDMA移动通信系统中几种常见的多用户检测技术，分析了他们的原理，并对各种多用户检测的优缺点进行了比较。     

Bit Error Rate of TH-BPSK UWB Receivers in Multiuser Interference

It is well known that the Gaussian distribution is not an accurate model for approximating the probability density function (PDF) of the multiple access interference (MAI) in time-hopping UWB (TH-UWB) systems. An exact theoretical model which explains the key features of the PDF of the MAI in TH-UWB systems is discussed. These features, which can be precisely anticipated by the proposed model include impulses, singularities, and the tail behaviour in the distribution of the MAI. The model reveals in quantitative terms why a Gaussian approximation for the MAI in TH-UWB systems is highly imprecise even in an environment with a large number of independent interferers. Based on the model obtained for the PDF of the MAI and exploiting the maximum a posteriori (MAP) receiver design rule, the optimal attainable BER performance of binary TH-UWB receivers in additive white Gaussian noise channels is numerically determined. The performances of some recently proposed UWB receivers are benchmarked against the optimal performance showing that some of them achieve near-optimal performance.     

Performance of a Biphase Bandpass Communication System

A binary bandpass communications system is considered, in which the ratio of bandwidth to intermediate frequency (IF) is not necessarily small. Closed-form expressions are given in the case when the receiver (IF) filter hasNdistinct poles. Explicit results, namely probabilities of error, are computed when the filter is a bandpass Butterworth filter. The effect ofk = f_{0}/f_{r}(ratio of half filter bandwidth to IF),f_{0}T(product of half filter bandwidth and bit duration),N, phase and time jitter on probability of error, is investigated.     

Exact Outage Probability Analysis for a Multiuser MIMO Wireless Communication System With Space–Time Block Coding

A multiuser multiple-input-multiple-output (MIMO) system with orthogonal space-time block coding (OSTBC) is analyzed for the uplink of a wireless communication system in a Rayleigh fading environment. In the first part of this paper, a simple and exact closed-form expression for the outage probability of the signal-to-interference-and-noise ratio (SINR) is derived at the input of the base station (BS) receiver by making the following two assumptions: 1) All the users transmit their data by using the same OSTBC; and 2) the users are power controlled by the same BS so that the interfering users are requested to transmit with the same power. In the second part of this contribution, the outage probability of the signal-to-interference ratio (SIR) is calculated at the output of the BS receiver, which, in our case, is a spatial matched filter. To be able to analytically solve the latter problem, the presented analysis is restricted to the case of a single interferer and a 2 2 MIMO system with Alamouti coding. Monte Carlo simulations are carried out to verify the proposed analytical expressions for the outage probability.