Effect of imperfect channel estimation on transmit diversity in CDMA systems

In this paper, the effect of imperfect channel estimation on the transmit diversity based on space-time block coding for the downlink of a direct-sequence code-division multiple-access system is studied. Two transmit antenna and one receiving antennas are employed. However, the results of this paper can be extended to the system with more receiving antennas. Each channel is modeled as frequency-selective Rayleigh fading and the pair of channels corresponding to two transmit antennas are mutually independent. Both spatial diversity gain and multipath diversity gain are obtained in the system. The system performance is evaluated in terms of bit-error rate under the perfect and imperfect channel estimation. A pilot-assisted channel-estimation scheme with one common spreading code sequence is exploited. It is shown that the inaccurate channel estimates suffering from multiple access and multipath interference significantly degrade the system performance and can be effectively improved by use of a simple low-pass filter. The investigation of the power ratio of pilot to data channels illustrates that the base station should dynamically adjust the transmit power of the pilot channel according to the varying system configurations in order to achieve the best performance.     

Optimal antenna diversity signaling for wide-band systems utilizingchannel side information

Optimal multi-antenna wide-band signaling schemes are derived for multipath channels assuming perfect channel state information at the transmitter. The scheme that minimizes the bit-error probability in the single-user case is a rank-one space-time beamformer which focuses the signal transmission in the direction of the most dominant channel mode. Several suboptimal variations are discussed for multiuser applications. The optimal signaling scheme given channel statistics at the transmitter is also derived. The optimal scheme in this case is a full-rank space-time beamformer that transmits on all channel modes. Analysis and simulation results are used to compare the schemes proposed in this paper. Finally, we discuss the optimal signaling scheme when a delayed version of the channel state is available at the transmitter. It is shown that in this case the optimal scheme is a rank-1 beamformer when the channel variations are sufficiently slow and is a full rank beamformer in a sufficiently fast fading channel     

Merging multicarrier CDMA and oscillating-beam smart antenna arrays: exploiting directionality, transmit diversity, and frequency diversity

In this paper, a novel merger of multicarrier code-division multiple access (MC-CDMA) and smart antenna arrays is introduced. Here, a group of Q carriers in the MC-CDMA system is applied to its own M-element smart antenna array at the base station (BS). The smart antennas are located in close proximity to one another. We generate a transmit diversity gain at the receiver by carefully moving (oscillating) the antenna array's pattern. The pattern oscillation is achieved by applying appropriate time-varying phases to array elements of each smart antenna. The beam pattern oscillation ensures a mainlobe at the position of the intended user and small oscillations in the beam pattern. This beam pattern oscillation leads to a time-varying channel with a controllable coherence time; hence, a transmit diversity benefit, in the form of a time diversity benefit, is available at the receiver. Employing MC-CDMA with the proposed smart antenna at the BS, we achieve: 1) directionality which creates high network capacity via space-division multiple access; 2) a transmit diversity gain which supports high performance at the receiver in the mobile unit; and 3) increased capacity and performance via MC-CDMA's ability to support both CDMA and frequency diversity benefits, respectively.     

Spectrally overlaid narrow-band and wide-band CDMA systems

A spectral overlay of a narrow-band code-division multiple access (CDMA) system and a wide-band CDMA system is proposed. The achievable joint capacity of the overlaid system is examined and the capacity tradeoffs between the two CDMA systems are given in a closed form     

Combined transmit and receive antenna diversity for WCDMA in multipath fading channels

We analyze the performance of various receiver antenna diversity combining schemes in combination with transmit diversity on the downlink of a WCDMA system. The analytical framework developed can handle an arbitrary number of transmit and receive antennas, as well as the study on the effect of dissimilar received signal strengths at the mobile handset on the average symbol error rate performance of a multitude of digital modulation schemes.     

Circular shifted transmit diversity

Three design requirements of space‐time block code, full diversity order, full data rate and orthogonality, cannot be achieved simultaneously when the number of transmit antenna is more than two. In this paper, we propose a transmit diversity scheme—circular shifted transmit diversity (CSTD). CSTD can always achieve the full data rate and has very low decoding complexity. Through the performance analysis and simulation comparison in the case of same spectrum efficiency between CSTD and other well‐known proposed schemes, it can be proved that CSTD always significantly outperforms the other transmit diversity schemes with full data rate. Additionally, compared with the transmit diversity schemes with full diversity order, CSTD has lower complexity and approximately the same performance when channel coding is concatenated. Copyright © 2004 John Wiley & Sons, Ltd.     

Differential multiple-length transmit diversity

We propose a new bandwidth-efficient differential transmit diversity scheme as an extension of differential space-time block codes from orthogonal designs. The information is coded in both the difference of successive unit-length vectors and the length difference of successive transmit vectors. We derive a simple soft-output detector which does not require knowledge of the channel coefficients or of the noise variance. The new scheme outperforms the existing unit-length approach at high bandwidth-efficiency, particularly in time-varying channels. At the same time the detection complexity is reduced.     

CDMA with power control and sequence design: the capacity region with and without multidimensional signaling

We consider the symbol-synchronous code-division multiple-access (CDMA) channel in which every user is assigned a rate at which arbitrarily reliable transmission in the Shannon sense is to be guaranteed. For an overloaded system in which the number of active users exceeds the available processing gain, we optimally design the users' signature sequences and a power-control policy to minimize the required sum-power (i.e., sum of the users' powers) while meeting the rate-tuple constraint with a (joint) maximum-likelihood receiver. This result is extended to find the power-constrained capacity region of the system; this is the set of all achievable rate-tuples over all signature sequences and power-control policies whose sum-power is constrained. Furthermore, it is shown that this capacity region may be substantially and maximally expanded in those regions where there are oversized users whose rate requirements are relatively large compared to those of the other users; this is accomplished by allowing for the flexibility of multidimensional signaling in the sense of a user simultaneously transmitting several different scalar symbols, each modulated by its own signature sequence. From the viewpoint of resource efficiency, this means that a multicarrier approach is essential in systems that support multiple classes of users. Finally, we also address the dual problem of determining the region of valid power-control policies subject to a sum-capacity constraint on the system.     

混合发射天线选择的闭环发射分集系统的优化设计

研究了混合发射天线选择技术的闭环发射分集系统.针对最大率传输和等增益传输两种传输模式,根据信道矢量衰落损耗系数的排序统计分布特性,分别提出了发射天线数和射频链路数的选择算法.这两个算法可分别使系统所需的发射天线数和射频链路数最小.仿真结果显示合理地选择发射天线数和射频链路数可以大大简化系统的复杂度,减少不必要的开销.针对混合发射天线选择的等增益传输提出了相位扇区化量化算法.这种算法只需少量的量化反馈就可以使系统的性能接近全反馈情况.     

The effects of channel-estimation errors on a space-time spreading CDMA system with dual transmit and dual receive diversity

The uplink of a space-time spreading code-division multiple-access system with dual transmit and dual receive antennas is analyzed with the effect of imperfect channel estimation. With the help of pilot signals, the Rayleigh multipath fading channel experienced by the transmitted signal is estimated by simple correlators, and subsequently used to coherently combine the multipath signals in a RAKE-like space-time combiner. As a system becomes more wideband, more multipaths are resolved, and the energy in each path is reduced. This reduction in signal strength causes increased estimation error and impacts the system performance. Through the derivation of the probability of error in a space-time spreading system with channel-estimation errors, this paper studies the tradeoff between diversity and estimation errors. It is shown that an optimal bandwidth exists and the optimal power allocation for the pilot signals is a function of the quality of the channel estimates.     

Compact wide-band multimode antennas for MIMO and diversity

This paper presents broadband multimode antennas for multiple-input multiple-output (MIMO) and diversity applications. The antenna system is not based on spatial diversity, as usual MIMO systems, but on a combination of pattern and polarization diversity. Different modes of self-complementary, thus extremely broadband, spiral and sinuous antennas are used to decorrelate the signals. It is shown that only one antenna is necessary to receive three uncorrelated signals, thus the space required to place the MIMO antenna is very small. Simulation results and measurements of a typical indoor scenario are given.     

Code acquisition in transmit diversity DS-CDMA systems

In this paper, two code acquisition schemes are studied for use in conjunction with transmit diversity direct-sequence code-division multiple access (DS-CDMA). One is a training-based single-user maximum-likelihood (SUML) estimator, which can achieve code acquisition very fast at low computational complexity. The other is a blind estimator based on the multiple signal classification (MUSIC) algorithm. Two recently proposed transmit diversity schemes known as orthogonal transmit diversity simulcast (OTD-S) and space-time selective spreading transmit diversity (STSTD) are considered. While the advantages of transmit diversity from the detection standpoint are well known, less is known about how code acquisition performance is affected by the use of transmit diversity. Through the analysis in this paper, it is proven that the SUMI. estimator should give the same performance in both the OTD-S and STSTD schemes in a single-user environment. In a multiple-user environment, simulation results show that the STSTD system offers slightly better code acquisition performance. It is also seen that the SUML estimators provide significantly better code acquisition performance than the MUSIC estimators in either transmit diversity system. From the standpoint of robustness to carrier frequency offset, it Is found that the training-based SUML estimator is very sensitive to frequency offset, while the MUSIC estimator is quite robust. A simple frequency offset estimator to be used in conjunction with the SUML estimator is also proposed and is shown to make the timing estimator quite tolerant of substantial frequency offsets.     

A semiblind method for transmit antenna arrays in CDMA systems

Using a transmit antenna array (TAA), the downlink beamforming is able to improve the performance of wireless communication systems. We propose a semiblind downlink beamforming method based on channel reciprocity of the wireless channels for code-division multiple-access (CDMA) systems equipped with TAA. In the proposed semiblind method, it is shown that the feedback delay can result in performance degradation under a time-varying fading channel environment. Hence, the performance of the semiblind method is satisfactory only when the maximum Doppler frequency is low or the feedback delay is not significant. To overcome this difficulty, the channel prediction is considered. Using the channel predictor, the semiblind method can provide a reasonable performance for a moderate maximum Doppler frequency (about a few hundred hertz). For theoretical performance analysis, upper, and lower bounds of the bit error probability under a fading channel environment are derived     

Exact capacity analysis of multiuser diversity combined with transmit diversity

Expressions for the average channel capacity of a multiuser diversity system combined with transmit diversity at each link are presented under the assumption of independent Rayleigh fading. The analysis results exactly agree with the simulation results and definitely show the impact of transmit diversity on multiuser diversity.     

Turbo processing in transmit antenna diversity systems

We consider turbo-trellis-coded transmission over fading multiple-input-multiple-output (M1M0) channels with transmit diversity using space-time block codes. We give a new view on space-time block codes as a transformation of the fading MIMO channel towards a Gaussian single-input-single-output (siso) channel and provide analytical results on the BER of space-time block codes. Furthermore, we describe the concatenation of Turbo-TCM with a space-time block code and show that in addition to the transmit diversity substantial benefits can be obtained by turbo iterations as long as the channel is time-varying during transmission of a coded block or frequency hopping is applied. Finally, a double iterative scheme for turbo equalization and turbo decoding of the concatenation of Turbo-TCM and space-time block code in frequency-selective MIMO channels is described.     

Cooperative transmit diversity based on superposition modulation

We propose a new strategy for cooperative transmit diversity based on superposition modulation and multiuser detection. The new scheme can outperform "classical" cooperative diversity based on decode-and-forward by about 1.5-2 dB in the SNR range of interest and at the same computational complexity.     

A differential detection scheme for transmit diversity

We present a transmission scheme for exploiting diversity given by two transmit antennas when neither the transmitter nor the receiver has access to channel state information. The new detection scheme can use equal energy constellations and encoding is simple. At the receiver, decoding is achieved with low decoding complexity. The transmission provides full spatial diversity and requires no channel state side information at the receiver. The scheme can be considered as the extension of differential detection schemes to two transmit antennas     

3G中的STTD发送分集技术

空时发送分集(STTD)方案已经被选做第三代移动通信系统WCDMA的一种标准。STTD是基于空时分组码(STBC)的一种发送分集技术[1],在详细分析了空时分组码的基础上,研究STTD发送分集技术。针对STTD需要很好的估计信道参数,利用导频信号对信道参数估计,最后进行仿真并给出结论。     

宽带CDMA信道中的智能天线测试平台的设计与分析

在宽带CDMA WLL的环境中，使用波束形成模块(BFM)设计智能天线系统，并分析了其性能。波束形成模块是由DSP(TMS320C6701)电板实现的。首先，给出计算机仿真的结果，这些结果通过由基带信号处理设备组成的测试平台系统产生的模拟接收(RX)信号获得，包括模拟接收(RX)信号生成的PC、计算最优化加权矢量的BFM及接口模块。然后，用基站测试平台系统去评估实际信号的自适应波束形成功能。测试平台包括多用户、RF模块，以及其他相关设备。