Decision feedback equalization for CDMA in indoor wirelesscommunications

Commercial interest in Code Division Multiple Access (CDMA) systems has risen dramatically in the last few years. It yields a potential increase in capacity over other access schemes, because it provides protection against interference, multipath, fading, and jamming. Recently, several interference cancellation schemes for CDMA have been proposed but they require information about all interfering active users or some channel parameters. The authors present an adaptive fractionally spaced decision feedback equalizer (DFE) for a CDMA system in an indoor wireless Rayleigh fading environment. This system only uses information about the desired user's spreading code and a training sequence. An analysis on the optimum performance of the DFE receiver shows the advantages of this system over others in terms of capacity improvements. A simulation of this system is also presented to study the convergence properties and implementation considerations of the DFE receiver. Effects on the performance because of sudden birth and death of users in the CDMA system and bit error rate performance of the DFE receiver is also presented     

Decision feedback equalization

As real world communication channels are stressed with higher data rates, intersymbol interference (ISI) becomes a dominant limiting factor. One way to combat this effect that has recently received considerable attention is the use of a decision feedback equalizer (DFE) in the receiver. The action of the DFE is to feed back a weighted sum of past decision to cancel the ISI they cause in the present signaling interval. This paper summarizes the work in this area beginning with the linear equalizer. Three performance criteria have been used to derive optimum systems; 1) minimize the noise variance under a "zero forcing" (ZF) constraint i.e., insist that all intersymbol interference is cancelled, 2) minimize the mean-square error (MMSE) between the true sample and the observed signal just prior to the decision threshold, and 3) minimize the probability of error (Min Pe). The transmitter can be fixed and the receiver optimized or one can obtain the joint optimum transmitter and receiver. The number of past decisions used in the feedback equalization can be finite or infinite. The infinite case is easier to handle analytically. In addition to reviewing the work done in the area, we show that the linear equalizer is in fact a portion of the DFE receiver and that the processing done by the DFE is exactly equivalent to the general problem of linear prediction. Other similarities in the various system structures are also shown. The effect of error propagation due to incorrect decisions is discussed, and the coaxial cable channel is used as an example to demonstrate the improvement available using DFE.     

The indoor radio propagation channel

In this tutorial survey the principles of radio propagation in indoor environments are reviewed. The channel is modeled as a linear time-varying filter at each location in the three-dimensional space, and the properties of the filter's impulse response are described. Theoretical distributions of the sequences of arrival times, amplitudes and phases are presented. Other relevant concepts such as spatial and temporal variations of the channel, large-scale path losses, mean excess delay and RMS delay spread are explored. Propagation characteristics of the indoor and outdoor channels are compared and their major differences are outlined. Previous measurement and modeling efforts are surveyed, and areas for future research are suggested     

Channel modeling and adaptive equalization of indoor radio channels

The authors analyze the benefits of using a decision feedback equalizer (DFE) in the indoor radio environment and examine the results of performance predictions for different channel modelings. It is found that a QPSK/DFE modem with second-order diversity can operate at a data rate that is an order of magnitude higher than a QPSK (quadratic-phase-shift-keying) modem without equalization. A given set of measured profiles of the channel impulse response is interpreted using continuous and discrete channel models. The continuous channel model is represented by the delay power spectrum and the discrete channel model by the envelope delay power spectrum and the arrival rate of the paths. The sensitivity of the performance to the shape of the delay power spectrum, and the arrival rate of the paths is analyzed     

Broadband TDMA over the indoor radio channel

Prior work has focused on the negative aspects of time-dispersive channels and associated intersymbol interference. The authors show that the inherent diversity introduced by such dispersion is advantageous in reducing the performance-limiting effects of flat fading. This is demonstrated for an indoor QPSK radio link employing a decision-feedback equaliser     

Doppler spread measurements of indoor radio channel

The temporal variations caused by human movement or human-induced motion of the communication equipment, on the otherwise static indoor radio channel, are measured. For the reported narrowband measurements, a maximum Doppler spread of 6.1 Hz and a maximum RMS Doppler bandwidth of 0.87 Hz are observed.<>     

Millimeter-wave indoor radio channel with artificial reflector

Experiments of 94-GHz millimeter-wave propagation were carried out in indoor channels, where the propagation path loss and multipath fading are investigated. The results show that the path-loss exponents in the line-of-sight (LOS) and obstructed (OBS) channel are 1.2-1.8 and 3.6-4.1, respectively. The received power of the OBS path is attenuated significantly as compared with that of the LOS path, thereby requiring an additional link-budget margin as well as battery power. In this paper, the use of an artificial reflector is also suggested under OBS conditions in order to avoid significant propagation loss, and then, a comparative study is conducted     

Optimum diversity combining and equalization overinterference-limited cellular radio channel

This paper presents the comparative analysis of Nth-order diversity combining and equalization over an interference-limited cellular radio channel. The method of combining diversity and equalization has been analyzed previously. However, cochannel interference (CCI) was not considered, and the number of equalization taps was assumed to be infinite. A quadrature amplitude modulation (QAM) is used in our signal analysis. In modeling the multipath radio, we take into account CCI generated by frequency reuse and additive white Gaussian noise (AWGN). The performance evaluations are made of average error probability and outage probability. The average error rate is determined by using a Monte Carlo simulation for a set of channel parameters such as signal-to-noise ratio (SNR), signal-to-interference ratio (SIR), and equalization coefficients determined for this channel. In the error-rate estimation, we analyze and compare the results of system performance obtained by the upper bound approach and the moment estimation method. We also investigate the tradeoff of the performance improvement in terms of average error probability and equalizer complexity (the number of equalization taps)     

Mm-wave indoor radio channel modelling vs. measurements

A propagation model for the prediction of indoor radio channel characteristics at millimetre-wave frequencies, based on geometrical optics is described. This model has been shown to agree well with wide-band measurements carried out at Eindhoven University of Technology [8]. Simulation results for 4th and 6th order reflection are compared with corresponding measured data obtained in small and medium size rooms. The influence of objects located inside the room superstructure, on the indoor radio channel characteristics, is analysed. The agreement between measured and simulated results, confirms the validity of the proposed simulation method.J.J.G. Fernandes would like to thank JNICT — Portugal, for sponsorship.     

A novel stochastic millimeter-wave indoor radio channel model

The paper introduces a model for the stochastic millimeter-wave indoor radio channel. This model relates the stochastic properties of the radio channel to the underlying geometry of the investigated environment. The geometric simplicity of the millimeter-wave channel allows examining fundamental deterministic properties of the wave propagation behavior in environments of predefined randomness, i.e., environments whose dimensions and properties are described by various probability distributions. The influence of the randomness on the radio channel is studied for the down-link of a wireless local area network at 60 GHz. Joint amplitudes of path lengths, angles of departure, and amplitudes, as well as spatial power densities, average power of the direct paths, and k factors are investigated.     

Computerized indoor radio channel estimation using ray tracing

This paper describes a computer tool for site specific indoor radio channel characterization. The behavior of the electromagnetic fields associated with a base station and a portable radio unit is simulated by combining approximate geometric optics and ray tracing techniques. Given a building plan and the transmitter/receiver locations, the set of all[attenuation / time delay /phase] vectors, each representing an electromagnetic propagation path between transmitter and receiver, is computed using ray shooting techniques to generate the channel wideband impulse response. The model takes into account specular reflection and transmission. Linear polarization and various antenna patterns can be simulated as well. The simulated results are shown to correlate well with experimental data. The computer tool can be used to provide estimates of the coverage of a base station and of the channel quality.     

Statistical AR models for the frequency selective indoor radio channel

A statistical model of the indoor radio channel is proposed that is derived from a second order autoregressive process representation of the channel frequency response. The accuracy of the statistical model is examined by comparing the cumulative distribution functions of the RMS delay spread and the 3 dB width of the frequency correlation function computed from the regenerated data with that of the measurements performed in two indoor radio propagation studies in the 0.9-1.1 GHz band.<>     

Performance of decision feedback equaliser on measured indoor radio channels

The measured multipath profiles from five different factories are used for the performance analysis of a binary phase shift keying (BPSK) modem, with and without a decision feedback equaliser (DFE). Both average bit error rate and outage probability are calculated as the performance criteria.<>     

Wide-band measurements of nondeterministic effects on the BRAN indoor radio channel

External nonstatic effects on the indoor radio channel can be classified according to their natures; they can be quasistatic, as are those due to the furnishing and decoration of rooms, or clearly dynamic, as are those forced by the movement of people and pets within the propagation environment. So, it can be stated that people acting and walking nearby the antennas of radio-communication systems introduce nondeterministic effects in the radio channel, whereas walls, ceiling, floors, and even doors and windows are structural elements whose influence can be modeled by ray-tracing tools, the activity of people (both movement and variations in the distribution of elements such as furniture or decorative objects) introduces stochastic components in the channel characteristics prediction. Deterministic tools cannot model these effects, but their experimental knowledge will allow radio-network designers to keep guard values for the different design parameters at the same time that they plan the radio network. In this manner, the quality of service provided by these communication systems could be preserved. This paper shows the results and analysis of measurement campaigns performed in static and dynamic conditions, in order to compare the behavior of both narrow and wide-band parameters of the radio channel, including people activity within the environment.     

Combined linear pre-equalization and BLAST equalization with channel correlation feedback

In this letter, we study linear pre-equalization in conjunction with BLAST equalization. Linear pre-equalization is based on the assumption that the channel state information available at the transmitter is limited to the second order statistics of channel and noise (long term channel state information or LT-CSI). Through simulations, we compare different design criteria for the pre-equalizer and show the advantages of the considered structure with respect to BLAST.     

室内多载波CDMA系统在深度衰落环境下的能量受控均衡策略

多载波CDMA(MC-CDMA)是将多载波调制OFDM与DS-CDMA结合起来的多址访问技术。如何从接收到的信号恢复出发送信号，涉及到频率分集接收和均衡。本文提出的能量受控均衡技术，在多用户的环境下能较好地恢复扩频码的正交性且避免信道深度衰落的影响，而且，实验表明该方法对于不同的信道状况均能很好适应。     

Empirical characterization of wideband indoor radio channel at 5.3GHz

Characteristics of wideband indoor radio channel at 5.3 GHz were defined based on an extensive measurement campaign using a wideband channel sounder with 19 ns delay resolution. Pathloss exponents were 1.3-1.5 in LOS and 2.9-4.8 in non-line of sight (NLOS). Large difference in NLOS exponents was due to different dominating propagation mechanisms in different types of building structures. The delay dispersion was characterized by cumulative distribution functions (CDF) of the RMS delay spreads, the values for CDF=0.9 varied from 20 to 180 ns in different setups in an office building and large hall environments. The correlation functions of the radio channel in spatial and frequency domains were extracted. Small scale models for five typical indoor scenarios were developed using tapped delay lines     

Performance of M-QAM modulations for indoor radio communications with combined diversity and equalization techniques

Digital indoor radio communications systems have become an attractive proposal as they could provide the user high quality voice and low rate data services with total mobility. However, indoor communications are affected by multipath propagation that restricts their application imposing an upper limit on the system data rate. To overcome this drawback diversity and channel equalization could be considered. In this paper the performances of 4, 16 and 64-QAM modulations in a radio communications indoor environment are presented. The results show that a system without protection has very limited performance. Diversity techniques are efficient when the radio channel can be assumed to be non-selective. For systems operating in selective fading channels joint equalization and diversity techniques are necessary to fight the degrading effect introduced by the channel. Moreover, if correlation between the two diversity channels is taken into account, the system performances remain almost unchanged, assuming that the correlation coefficient between the two diversity branches is lower than 0.6 to 0.7 approximately.     

联合信道均衡与时间反演的室内保密通信方法

针对物理层无线通信的窃听信道问题,提出了一种适用于室内频率选择性衰落信道的均衡时间反演(Equalized Time Reversal,ETR)保密通信方法。首先,建立了均衡时间反演窃听模型,在信号发送端将均衡器级联配置,利用信道均衡技术提高信息传输的保密性能;其次,通过传统时间反演(Time Reversal,TR)技术的等效信道分析对均衡器进行了设计,根据均衡后的信道推导了系统保密信干噪比、系统保密容量、误码率的闭合表达式。理论分析和仿真结果表明,与传统的TR技术相比,ETR的保密信干噪比、系统保密容量更高,合法接收用户的误码率更低。     

Measurements and characterization of wideband indoor radio channel at 60 GHz

This paper presents the measurements, the statistical results and channel models extracted by impulse response measurements of an indoor 60 GHz radio channel. The measurements were based on the pulse sounding technique. Multipath parameters that characterize the channel have been extracted and analyzed statistically concerning corridors and offices locations. The mean excess delay is in the range of 3.84 to 8.18 ns for hallways and 3.52 to 14.69 ns for offices. Additionally, rms delay spread varies from 12.34 to 15.04 ns in corridors and from 12.56 to 21.09 ns inside the laboratory. The coherence bandwidth varies between 13.88 and 30.49 MHz in corridors with a mean value of 22.48 MHz. Inside offices the mean coherence bandwidth is 22.80 MHz for LoS locations and drops to 7.05 MHz for NLoS. Small-scale models for all the measured locations were developed using tapped delay lines. The maximum Doppler frequency of the modeled channel remains around 1 Hz, whereas the coherence time is calculated 1.04 s, which indicates that the channel remains, almost stationary, exhibiting very slow fading. Finally, from the models it is derived that the channel preserves WSS and US characteristics giving rise to a WSSUS representation.