A-OFDM系统中的空时块码设计与应用

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"本文主要探讨了在异步正交频分复用(A-OFDM)系统中的空间时间块编码(STBC)设计，旨在解决OFDM系统的高峰均功率比(PAPR)问题，并提高MIMO多载波通信的性能。" 在无线通信领域，正交频分复用(OFDM)因其高效利用频谱资源和抗多径衰落的特性而被广泛采用。然而，OFDM系统的一个显著缺点是其高PAPR，这可能导致功率放大器效率降低并增加发射机的设计复杂性。为了解决这个问题，异步OFDM(A-OFDM)系统通过减小快速傅里叶变换(FFT)/逆FFT(IFFT)的大小来降低PAPR。本文作者Lin Luo基于FFT算法的分层概念和Alamouti空间时间块编码思想，将A-OFDM扩展到具有全分集的MIMO多载波系统中，特别应用于车辆间通信。Alamouti STBC是一种简单的二天线空间分集编码技术，可以提供两倍的信道容量而不增加信道条件的复杂性。提出的A-OFDM系统的一大优势在于其灵活性，可以根据不同的应用和场景需求任意调整子信道的数量（即每个子信道的长度）。这使得系统能够在保证高可靠性的前提下，根据实际需要降低复杂性，或者在需要高速率传输时提高频谱效率。理论分析和在车辆间通信环境中的仿真结果表明，该提议的系统无论子信道数量从1到N（N为传输块长度）如何调整，都能保持良好的性能。这证明了该系统的适应性和潜力，特别是在动态、高要求的通信环境中。 "Space-Time Block Code Design for Asymmetric-OFDM systems"这篇论文提出了一个创新的A-OFDM系统设计方案，结合了STBC的优势，以实现更灵活、高效且低PAPR的无线通信。这一工作对于优化MIMO-OFDM系统，特别是应对车辆通信等应用场景中的挑战，提供了有价值的理论和技术支持。

Space-Time Block Code Design for

Asymmetric-OFDM Systems

Lin Luo

Institute for Telecommunication Research, University of South Australia, Australia

Email: Lin.Luo@unisa.edu.au

Abstract—A considerable drawback of OFDM systems

is their high peak-to-average power ratio (PAPR). The

Asymmetric-OFDM (A-OFDM) system addresses this problem

by reducing the size of fast Fourier transform (FFT) / inverse

FFT (IFFT). Based on the layered concept in FFT algorithms

and Alamouti-like space time block code (STBC), we extend

the A-OFDM to an full diversity MIMO multicarrier system

and apply it to vehicle-to-vehicle communications. Comparing

with the conventional OFDM with STBC, the proposed system

can be arbitrarily adjusted in the number of subchannels

(or the length of each subchannel), typically in order to

suit requirements for high reliability or low complexity, in

different applications and scenarios. Theoretical analysis in

a frequency selective channel and simulations in a vehicle-

to-vehicle environment show that the proposed system, with

any adjustable size of subchannels from 1 to N , N being

the transmission block length, has higher reliability than an

equivalent MIMO-OFDM system.

I. INTRODUCTION

The asymmetric OFDM system (A-OFDM) [1], [2] is

a low-complexity trade-off solution between OFDM and

SC-FDE systems in terms of peak-to-average power ratio

(PAPR) and carrier frequency offset (CFO) sensitivity, as

well as bit error rate (BER) performance. The A-OFDM

can be freely adjusted in the length of subchannels, P ,

in a broad range between OFDM (P =1) and SC-FDE

(P = N, N being the transmission block length), according

to requirements of PAPR, CFO sensitivity, BER performance

and complexity. In [2] it has been shown that the PAPR

reduces for higher P . On the other hand, the complexity

decreases with lower P [2].

A reliable transmission is required in safety applications

of vehicle-to-vehicle communications. In order to improve

the reliability, i.e. reduce the BER, one option is to use

multiple antennas at the transmitter and receiver in order to

beneﬁt from space diversity. Multiple-input multiple-output

(MIMO) systems for OFDM have already been well studied

[3], [4], [5]. A ﬁrst proposition of a MIMO version of

the Quadrature-OFDMA (Q-OFDMA) system, which is a

multiple access version of A-OFDM, has been given in

[6], using an Alamouti-like space time block code (STBC).

Based on [6], this paper elaborates a general multi-carrier

MIMO system with adjustable length of subchannel, variable

PAPR and variable complexity of transmitter and receiver,

and analyze its BER performance based on zero forcing (ZF)

and minimum mean square error (MMSE) equalizations. The

proposed system has reduced PAPR and outperforms the

OFDM system in terms of BER performance. This makes it

an interesting candidate for applications in dedicated short

range communications (DSRC).

The rest of this paper is organized as follows. In Section II

we further develop the combining scheme at the receiver for

Alamouti-like STBC to achieve maximal diversity, and give

a general system for a 2 × n

STBC using ZF and MMSE

equalizations, where n

is the number of receive antennas. In

Section III, the BER performance is theoretically analyzed

in a Rayleigh fading channel for uncoded MIMO A-OFDM

systems. In Section IV this new MIMO A-OFDM system

will be simulated for vehicle-to-vehicle communications on

a geometry-based stochastic channel model. We compare it

to a single antenna system as well as to conventional MIMO-

OFDM systems. Finally, Section V concludes this paper.

II. S

PACE-TIME BLOCK CODING FOR A-OFDM

A. System model for A-OFDM

Given two N-point time-domain symbols x, h, and their

noiseless circular convolution output y = x  h, their

frequency-domain correspondences via DFT have the rela-

tionship

y =

x 

h. If we rearrange the frequency domain

symbols

h and

y into P × Q matrices (PQ = N)row-

wise according to the layered IFFT structure concept [2],

the vectors

and

from the q-th column of the

matrices retain that



, where [

]

pQ+q

[

]

pQ+q

, [

]

pQ+q

, and p =0, 1, ···,P − 1.

We now introduce the intermediate-domain symbols

{

} as the IDFTs of {

}, given by

= F

, where F

is the normalized

P -point IDFT matrix. According to the convolution property

of DFT, we have



, which establishes the

relationship of symbols in the intermediate domain, and can

be expressed in matrix form as

, where the P ×P

circulant matrix

represents the dispersive channel, with

[

]

i,j

h (((i − j) modP ) Q + q), where

h(·) denotes the

channel response in the intermediate domain.

At the receiver of the A-OFDM system, in order to realize

a one-tap equalization, the weighting outputs are transformed

from the intermediate domain to frequency domain as

= F

+ F

= D

, (1)

where we now include

and

, the white Gaussian noise

in intermediate and frequency domain respectively, and D

the diagonal matrix with the channel frequency coefﬁcients

p,q

on its diagonal. In the above formulas we consider a

GC'12 Workshop: The 8th Broadband Wireless Access Workshop

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