Broadband MIMO-OFDM wireless communications

Orthogonal frequency division multiplexing (OFDM) is a popular method for high data rate wireless transmission. OFDM may be combined with antenna arrays at the transmitter and receiver to increase the diversity gain and/or to enhance the system capacity on time-varying and frequency-selective channels, resulting in a multiple-input multiple-output (MIMO) configuration. The paper explores various physical layer research challenges in MIMO-OFDM system design, including physical channel measurements and modeling, analog beam forming techniques using adaptive antenna arrays, space-time techniques for MIMO-OFDM, error control coding techniques, OFDM preamble and packet design, and signal processing algorithms used to perform time and frequency synchronization, channel estimation, and channel tracking in MIMO-OFDM systems. Finally, the paper considers a software radio implementation of MIMO-OFDM.     

Co-channel interference cancellation for space-time coded OFDM systems

Space-time coded orthogonal frequency division multiplexing (OFDM) is a promising scheme for future wideband multimedia wireless communication systems. The combination of space-time coding (STC) and OFDM modulation promises an enhanced performance in terms of power and spectral efficiency. Such combination benefits from the diversity gain within the multiple-input-multiple-output ST coded system and the matured OFDM modulation for wideband wireless transmission. However, STC transmit diversity impairs the system's interference suppression ability due to the use of multiple transmitters at each mobile. We propose an effective co-channel interference (CCI) cancellation method that employs angle diversity based on -steering beamforming or minimum variance distortion response beamforming. It is shown that the proposed method can effectively mitigate CCI while preserving the space-time structure, thereby, significantly improving the system's interference suppression ability without significant bit-error rate performance degradation. Furthermore, it is demonstrated that the proposed method can significantly combat the delay spread detrimental effects over multipath fading channels without the use of interleaving.     

智能天线与空时编码技术的性能分析

为了研究智能天线自适应波束形成和空时编码技术(等效于MRRC最大比合并分集技术)这两种无线通信系统的关键技术在3G系统中的选择使用或结合使用的可能性及理论依据，比较了它们各自分别在其适用条件下所能达到的对输入信噪比的最优改善性能，得到了从对平均信噪比的改善的广义角度上讲它们是等效和一致的，它们对信噪比的改善都等于做波束形成的阵元数(或相应的分集支路数)的结论．     

Channel estimation for OFDM systems with transmitter diversity inmobile wireless channels

Transmitter diversity is an effective technique to improve wireless communication performance. In this paper, we investigate transmitter diversity using space-time coding for orthogonal frequency division multiplexing (OFDM) systems in high-speed wireless data applications. We develop channel parameter estimation approaches, which are crucial for the decoding of the space-time codes, and we derive the MSE bounds of the estimators. The overall receiver performance using such a transmitter diversity scheme is demonstrated by extensive computer simulations. For an OFDM system with two transmitter antennas and two receiver antennas with transmission efficiency as high as 1.475 bits/s/Hz, the required signal-to-noise ratio is only about 7 dB for a 1% bit error rate and 9 dB for a 10% word error rate assuming channels with two-ray, typical urban, and hilly terrain delay profiles, and a 40-Hz Doppler frequency. In summary, with the proposed channel estimator, combining OPDM with transmitter diversity using space-time coding is a promising technique for highly efficient data transmission over mobile wireless channels     

Adaptive interference suppression in multiuser wireless OFDMsystems using antenna arrays

This paper considers the problem of mitigating fading and interference in wireless orthogonal frequency division multiplexing (OFDM) multiple access communication systems. Applications include cellular mobile radio, wireless local loop, and wireless local area networks. The effect of interchannel interference (ICI) arising from time-selective fading and frequency offsets and co-channel interference (CCI) is analyzed. A loop-timing method that enables a synchronous uplink between multiple mobile transceivers and a base-station is described. Adaptive antenna arrays are utilized at the base for uplink reception, and optimum array combining based on the maximum SINR criterion is used for each subchannel over slowly time-varying channels. For operation over fast time-varying channels, a novel two-stage adaptive array architecture that incorporates combined spatial diversity and constraint-based beamforming is presented. While ICI alone is most effectively overcome by spatial diversity, combined beamforming and diversity are most effective to combat CCI in the presence of fading. The overall method is suitable for real-time implementation and can be used in conjunction with traditional coding schemes to increase the link-margin     

Combined turbo coding and unitary space-time modulation

Space-time coding is well understood for high data rate communications over wireless channels with perfect channel state information. On the other hand, channel coding for multiple transmit antennas when channel state information is unknown has only received limited attention. A new signaling scheme, named unitary space-time modulation, has been proposed for the latter case. In this paper, we consider the use of turbo coding together with unitary space-time modulation. We demonstrate that turbo coded space-time modulation systems are well suited to wireless communication systems when there is no channel state information, in the sense that the turbo coding improves the bit error rate (BER) performance of the system considerably. In particular, we observe that the turbo-coded system provides 10-15 dB coding gain at a BER of 10/sup -5/ compared to the unitary space-time modulation for various transmit and receive antenna diversity cases.     

Adaptive cochannel interference cancellation in space-time coded communication systems

Space-time coding is a powerful scheme that combines channel coding, modulation, and multiple transmit antennas to achieve higher data rates and combat fading in wireless systems. In this letter, we propose a multiple-input multiple-output minimum mean-square error spatial-filtering-based adaptive antenna arrays method to suppress cochannel interference (CCI) in space-time coded systems. It is shown that the proposed method can effectively suppress CCI while preserving the space-time structure, thereby significantly improving the system's interference suppression ability without significant bit error rate performance degradation.     

Representation theory for high-rate multiple-antenna code design

Multiple antennas can greatly increase the data rate and reliability of a wireless communication link in a fading environment, but the practical success of using multiple antennas depends crucially on our ability to design high-rate space-time constellations with low encoding and decoding complexity. It has been shown that full transmitter diversity, where the constellation is a set of unitary matrices whose differences have nonzero determinant, is a desirable property for good performance. We use the powerful theory of fixed-point-free groups and their representations to design high-rate constellations with full diversity. Furthermore, we thereby classify all full-diversity constellations that form a group, for all rates and numbers of transmitter antennas. The group structure makes the constellations especially suitable for differential modulation and low-complexity decoding algorithms. The classification also reveals that the number of different group structures with full diversity is very limited when the number of transmitter antennas is large and odd. We, therefore, also consider extensions of the constellation designs to nongroups. We conclude by showing that many of our designed constellations perform excellently on both simulated and real wireless channels     

Error Analysis of Grouped Multilevel Space-Time Trellis Coding with the Combined Application of Massive MIMO and Cognitive Radio

In today’s scenario, demand for error-correcting codes with minimal error constraints for wireless communications. Multilevel coding scheme with trellis codes as component codes provides flexible data transmission rates, coding gain, diversity gain with improved spectral efficiency and low decoding complexity. This paper investigates the potential improvements by using the Multilevel coding scheme with massive Multiple-Input Multiple-Output in Cognitive Radio Networks with trellis codes as component codes. This paper discussed space-time coding with beamforming and antenna grouping according to the channel state information. Multilevel Space-time coding is based on multi-level Quadrature Amplitude Modulation signaling and beamforming to mitigate the effect of primary users for the enactment of secondary users in Cognitive Radio. The primary users provide channels dynamically to the secondary user for an unknown duration. Our transmission use Quadrature Amplitude Modulation based signals, with an adaptive grouping of antenna which weight according to the optimization, which inherently depends upon the resource allocation of the secondary user. The results show that the proposed coded system achieves Bit error rate/Symbol error rate/Frame error rate and Signal to noise ratio varies according to sources sensing time.

     

Performance Comparison of MIMO-STBC Systems with Adaptive Semiblind Channel Estimation Scheme

Wireless communication is now a part of everyday life in the urban areas. Wireless LAN is mostly utilized communication system as an example. These wireless devices are data rate and range limited, for which the scientists are spending great efforts on finding ways to overcome these limitations. Multi input multi output (MIMO) antenna systems are the example through which these limitations have been reduced upto great extent which provides multilayer beamforming, diversity, and spatial multiplexing. Analysis of adaptive semiblind channel estimation scheme for MIMO antenna array systems with different code rate space time block coding (STBC) has been performed using the adaptive pilot assisted modulation scheme proposed earlier. Semi blind channel estimation method provides the best trade-off in terms of bandwidth overhead, computational complexity and latency. The result after using MIMO systems shows higher data rate and longer transmit range without any requirement of additional bandwidth or transmit power. This paper presents the detailed analysis of diversity coding techniques using MIMO antenna systems. Different STBC schemes have been explored and analyzed with the different code rate STBC using MATLAB environment and the simulated results have been compared in the semiblind environment which shows the improvement even in highly correlated antenna arrays, and is found close to the condition when channel state information is known to the channel.     

Joint transmitter receiver diversity for efficient space divisionmultiaccess

The beamforming problem is studied in wireless networks where both the transmitters and receivers have linear adaptive antenna arrays. Algorithms are proposed that find the antenna array weight vectors at both the transmitters and receivers as well as the transmitter powers with one of the following two objectives: (1) to maximize the minimum signal-to-interference-and-noise ratio (SINR) over all receivers and (2) to minimize the sum of the total transmitted power satisfying the SINR requirements at all links. A numerical study is performed to compare the network capacity and the power consumption among systems having a different number of antenna array elements in a code division multiple access network     

Space-time bit-interleaved coded modulation for OFDM systems

Space-time coding techniques significantly improve transmission efficiency in radio channels by using multiple transmit and/or receive antennas and coordination of the signaling over these antennas. Bit-interleaved coded modulation gives good diversity gains with higher order modulation schemes using well-known binary convolutional codes on a single transmit and receive antenna link. By using orthogonal frequency division multiplexing (OFDM), wideband transmission can be achieved over frequency-selective fading radio channels without adaptive equalizers. In this correspondence, we combine these three ideas into a family of flexible space-time coding methods. The pairwise error probability is analyzed based on the correlated fading assumption. Near-optimum iterative decoders are evaluated by means of simulations for slowly varying wireless channels. Theoretical evaluation of the achievable degree of diversity is also presented. Significant performance gains over the wireless local area network (LAN) 802.11a standard system are reported.     

Adaptive Modulation and Coding with Selective Retransmission under OFDM signaling

In this paper, we present bandwidth efficient selective retransmission method in conjunction with adaptive modulation and coding (AMC) scheme for OFDM waveform. In the proposed method, when a packet failure occurs, receiver requests retransmission of information symbols prone to error corresponding to the low signal-to-noise ratio (SNR) sub-carriers of OFDM modulation. The selective retransmission avoids unnecessary retransmission and AMC chooses a proper modulation and coding scheme with an objective to maximize the throughput. Our method achieves higher throughput as compared to conventional retransmission methods such as Chase combining hybrid automatic repeat reQuest (CC-HARQ) and incremental redundancy hybrid automatic repeat reQuest (IR-HARQ). We also provide the throughput and delay analysis of the proposed method for non-truncated ARQ. The simulation results demonstrate throughput gain without significant impact on delay as compared to the conventional retransmission approaches.     

On the eigenvalue distribution of smart-antenna arrays in wireless communication systems

The recent development of communication theory and smart-antenna techniques such as space-time coding (STC), which exchange efficiency for complexity, has intensified the interest in a general theory of multiple-antenna communication. These techniques use parallel channels to increase the capacity of wireless systems. The parallel channels are formed either in a natural way, due to radiowave propagation (receiving diversity, RD, RAKE), or in an artificial way, due to appropriate pre-processing and coding (transmitting diversity, TD, STC), or in both ways (multiple-input multiple-output, MIMO, systems, STC, BLAST). The demand for ubiquitous, heterogeneous, and sophisticated wireless systems has led to the need for a better understanding of fundamental issues in space-time communication theory, and their implications for the design of reliable, high-bandwidth and high-speed wireless communications. The authors discuss the issue of eigenvalue distribution for antenna arrays. The present article provides an approach that should be useful in simulations, analysis, and design of smart-antenna wireless systems. The results show that this parameter of the spatial channel has an important rcHe to play in analyzing the performance of antenna arrays in wireless systems.     

On a hybrid beamforming/space-time coding scheme

A multiple transmit antenna system based on hybrid beamforming and space-time coding technologies is examined. The reduction factor of the required transmitted energy achievable by the use of hybrid scheme is quantified for any given outage capacity. We show that although a sole space-time coding configuration is superior asymptotically (i.e., for extremely low outage requirements), a hybrid beamforming/space-time coding configuration can be a more effective solution for modest outage requirements. It may provide a useful design guideline for wireless systems, especially for the downlink where multiple transmit antenna scheme is feasible.     

Multiple antenna systems: their role and impact in future wireless access

Multiple antennas play an important role in improving radio communications. In view of this role, the area of multiple antenna communication systems is in the forefront of wireless research. This article reviews two key related aspects of multiple antenna communication systems: multiple access interference mitigation at the receiver via multi-user beamforming; and space-time modulation and coding for MIMO systems. It is shown that both multi-user and MIMO receivers share similar signal processing and complexity tradeoffs.. Following that, a general unified framework for assessing different types of space-time modulation for MIMO systems is introduced. These space-time modulation methods are then compared in terms of Shannon capacity over multipath channels. Key MIMO system performance and implementation issues are also highlighted.     

一种基于空时分组编码的MIMO-OFDM系统接收空间分集方案

文献[1]提出了一种使用正交设计的单输入多输出正交频分复用(SIMO-OFDM)系统的空间分集接收结构,目的是为了减少接收端DFT块的数目以降低系统复杂度和减少功率消耗。由于在线性处理过程中噪声叠加的影响,造成了一定的性能损失。本文提出了一种基于空时分组编码的多输入多输出OFDM(MIMO-OFDM)系统空间分集接收方案,通过在文献[1]提出的分集结构中引入使用空时分组编码的发射分集,弥补了因减少DFT块数目而造成的性能损失。本文对使用空时分组编码后的处理过程进行了推导,并对使用空时编码前后的系统性能进行了仿真和比较。     

Adaptive antenna arrays for OFDM systems with cochannelinterference

Orthogonal frequency-division multiplexing (OFDM) is one of the promising techniques for future mobile wireless data systems. For OFDM systems with cochannel interference, adaptive antenna arrays can be used for interference suppression. This paper focuses on a key issue for adaptive antenna arrays, that is, parameter estimation for the minimum mean square error (MMSE) diversity combiner (DC). Using the instantaneous correlation estimation approach developed in the paper, an original parameter estimator for the MMSE-DC is derived. Based on the original estimator, we propose an enhanced parameter estimator. Extensive computer simulation demonstrates that the MMSE-DC using the proposed parameter estimators can effectively suppress both synchronous and asynchronous interference in OFDM systems for packet and continuous data transmission     

Diversity Embedded Space–Time Codes

Rate and diversity impose a fundamental tradeoff in wireless communication. High-rate space-time codes come at a cost of lower reliability (diversity), and high reliability (diversity) implies a lower rate. However, wireless networks need to support applications with very different quality-of-service (QoS) requirements, and it is natural to ask what characteristics should be built into the physical layer link in order to accommodate them. In this paper, we design high-rate space-time codes that have a high-diversity code embedded within them. This allows a form of communication where the high-rate code opportunistically takes advantage of good channel realizations while the embedded high-diversity code provides guarantees that at least part of the information is received reliably. We provide constructions of linear and nonlinear codes for a fixed transmit alphabet constraint. The nonlinear constructions are a natural generalization to wireless channels of multilevel codes developed for the additive white Gaussian noise (AWGN) channel that are matched to binary partitions of quadrature amplitude modulation (QAM) and phase-shift keying (PSK) constellations. The importance of set-partitioning to code design for the wireless channel is that it provides a mechanism for translating constraints in the binary domain into lower bounds on diversity protection in the complex domain. We investigate the systems implications of embedded diversity codes by examining value to unequal error protection, rate opportunism, and packet delay optimization. These applications demonstrate that diversity-embedded codes have the potential to outperform traditional single-layer codes in moderate signal-to-noise (SNR) regimes.     

Intelligent antenna system for cdma2000

Third-generation (3G) cellular code division multiple access (CDMA,) systems can provide an increase in capacity for system operators over existing second-generation (CDMA) systems. The gain in capacity for the base station to mobile (forward) link can be attributed to improvements in coding techniques, fast power control, and transmit diversity techniques. Additional gains in the mobile to base station (reverse) link can be attributed to the use of coherent quadrature phase shift keyed (QPSK) modulation and better coding techniques. While these enhancements can improve the performance of the system, system operators expect that with increased demand for data services, even greater capacity enhancements may be desired. There are essentially three methods, which we describe, based on diversity, spatial beamforming, and a combination of diversity and beamforming, to improve the performance of system through the use of additional antennas at the base station transmitter and receiver. The performance improvements are a function of the antenna spacings and the algorithms used to weight the antenna signals. We focus on the possibilities for the cdma2000 3G system that do not require standards changes. We highlight the performance enhancements that can be obtained on both the reverse and forward links through use of an antenna array architecture that supports a combination of beamforming and transmit diversity. We focus on the performance enhancements for the forward link