ISSN:1124-318X    

DOI:10.1002/ett.4460110602

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2000

数据来源: WILEY

摘要:

AbstractA plethora of single‐carrier and multi‐carrier (MC) CDMA systems have been proposed recently to mitigate intersymbol interference (ISI) and eliminate multiuser interference (MUI). We present a unifying all‐digital Generalized Multicanier CDMA framework which enables us to describe existing CDMA schemes and to highlight thorny problems associated with them. To improve the bit error rate (BER) performance of existing schemes, we design block FIR transmitters and decision feedback (DF) receivers based on an inner‐code/outer‐code principle, which guarantees MUI/ISI‐elimination regardless of the frequency‐selective physical channel. The flexibility of our framework allows further BER enhancements by taking into account the load in the system (number of active users), while blind channel estimation results in bandwidth savings. Simulations illustrate the superiority of our framework over competing MC CDMA alternatives especially in the presence of uplink multi

ISSN:1124-318X    

DOI:10.1002/ett.4460110603

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2000

数据来源: WILEY

摘要:

AbstractIn OFDM‐DS/CDMA system, frequency distortion caused by multipath propagation occurs at different frequencies in the bandwidth and generally affects selected subcarriers within the larger bandwidth. It has been investigated that each carrier transmission power control is effective to compensate for subchannel variation due to multipath, by assigning different transmission power level to different subcarriers based on the attenuation level suffered by them. The effectiveness of power control is mainly affected by the accuracy of the transmission power adjustment at the transmitter. As a matter of fact, however, the dynamic range of transmission power is very limited, due to the restriction on power consumption of a mobile station (transmitter side of reverse link). When a subcarrier is subjected to strong fading, power control is not able to compensate for channel attenuation by transmitting signals with large power, and this condition is very potential to degrade system performance. In this paper, we propose to combine each carrier transmission power control with antenna‐carrier diversity reception to lighten the burden imposed on power control. This system allows the base station (receiver side) to select the best combination among the carriers and antennas after receiving signals from mobile station (transmitter side), thus power control only compensates for smaller attenuation level. System performance is evaluated in terms of Bit Error Rate (BER) performance as a function of Doppler frequency, number of users, number of branches, updating period and dynamic range. The result of computer simulations clearly show that the proposed system reduces significantly the burden imposed on power cont

ISSN:1124-318X    

DOI:10.1002/ett.4460110604

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2000

数据来源: WILEY

摘要:

AbstractIn this paper, various detection techniques are studied in the synchronous case of a multiuser MC‐CDMA system operating in frequency selective Rayleigh channel. First of all, an improved linear detection approach named MMSE per user detection technique, which takes into account the despreading process, is presented. Simulation results over a Rayleigh fading channel show that this novel technique offers for non full load systems a significant gain compared to the MMSE equalization per camer method. Then, the performance of various parallel and successive interference cancellation techniques is compared to the performance of the linear MMSE per user scheme. When the power of the interferers is roughly equal to the power of the useful signal, the performance of the linear MMSE per user is really close to the performance of the non linear PIC and SIC scheme

ISSN:1124-318X    

DOI:10.1002/ett.4460110605

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2000

数据来源: WILEY

摘要:

AbstractA method to generate transmit signal sets is proposed which are especially suited for transmission over Rayleigh fading channels. It is shown that this method in combination with a suitable iterative receive algorithm allows the asymptotical transformation of a Rayleigh channel into a set of parallel additive white gaussian noise (AWGN) subchannels. Therefore, coding methods designed for the AWGN channel are now applicable also to Rayleigh channels resulting in a coding gain that is separable into the known AWGN coding gain and an additional diversity gain attributable to the orthonormal transform.Starting from this result, the method is extended to a transmission over frequency and/or time‐selective channels. This leads to an extension of the recently introduced multicamer code division multiplexing (MC‐CDM) scheme. A unified transmission model is derived which allows the application of the iterative reception algorithm also in the latter case and to transform time‐ and frequency‐selective channels under certain assumptions asymptotically into a set of parallel AWGN subchannels. The performance of these transformation methods for an uncoded scheme and together with high rate trellis codes designed for the AWGN channel is investigated and compared to the matched filter bound (MFB). For the calculation of the MFB for channels with a separable time‐variant transfer function a method is introduced that significantly reduces the complexity of co

ISSN:1124-318X    

DOI:10.1002/ett.4460110606

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2000

数据来源: WILEY

摘要:

AbstractThis paper is concerned with a randomized OFDM (R‐OFDM) scheme. The basic principle is to apply a scrambling operation to several OFDM frames. With a proper design strategy, the scrambling operation will not affect intra‐cell orthogonality of OFDM (even considering multi‐path effect), For signals from different cells, mutually random scramblings are used to reduce their correlation. This alleviates the worst case inter‐cell interference problem. R‐OFDM combines some advantages of both OFDM and CDMA, namely, intra‐cell interference avoidance, cross‐cell interference mitigation as well as simple recei

ISSN:1124-318X    

DOI:10.1002/ett.4460110607

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2000

数据来源: WILEY

摘要:

AbstractMulti‐camer spread‐spectrum (MC‐SS) modulation is a new kind of spread‐spectrum modulation[1],[2]. The spreading code is designed in the frequency domain. Hence, the system is dual to a direct‐sequence spread‐system (DS‐SS) where the spreading code is given in the time domain. There exist two different views: the OFDM view and the DS‐SS view. Many publications deal with the frequency domain processing. Transmitter and receiver structures are similar to OFDM. Therefore all these implementations use a guard interval to avoid ISI. The disadvantage of this guard interval is the higher hardware complexity and the waste of symbol energy. In the following paper the effect of ISI on a MC‐SS system is investigated. Both types of implementation (time or frequency domain) are compared and the dependencies of the performance from the spreading code are demonstrated. It is shown that with proper code selection and a time domain implementation without guard interval (RAKE receiver) it is possible to outperform equivalent frequency domain receivers with gua

ISSN:1124-318X    

DOI:10.1002/ett.4460110608

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2000

数据来源: WILEY

摘要:

AbstractWe analyze the effects of Doppler spread in mobile channels on Orthogonal Frequency Division Multiplex (OFDM) systems. This is important, since channel variations during one OFDM symbol cause Inter Sub‐carrier Interference (ICI) in OFDM systems, which degrades the performance, since ICI can be seen as additional near‐Gaussian noise. The analysis is presented and closed‐form results are given for numerous important practical Doppler spread encountered in mobile channels and applications such as mobile reception of DVB‐T. We also show that for the case of asymmetrical Doppler spreads, the frequency correction of the receiver can be adjusted so as to minimize the ICI. To extend our work to OFDM(A) mobile radio systems where many users' contributions to the ICI overlap, we have introduced the concept of an equivalent single‐user which causes the total ICI, in order to use the same analytical framework, and compared the uplink and downlink cases. Simulation results confirm our analysis and show the importance of the analytical tool to perform realistic system analysis without having to resort to time consuming time‐domain

ISSN:1124-318X    

DOI:10.1002/ett.4460110609

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2000

数据来源: WILEY

摘要:

AbstractThis paper deals with the application of spread spectrum techniques to MultiCarrier HF modems. OFDM modems are a special option for the transmission over the high dispersive HF medium in front of single carrier modems. Our present contribution is now focused on the application of Spread Spectrum techniques to improve its fading robustness. The transmission bandwidth is the standard voice band channel in the HF band where the spread spectrum characteristic means that one symbol is transmitted overNtimes the spectrum in the classical N‐OFDM scheme. The motivation of this idea is the observation of the OFDM modems behaviour over fading channels with deep nulls showing a quite different performance between carriers: those in the vicinity of deep nulls provide a much higher Bit Error Rate (BER) than others far away of the nulls. Our proposal intends to spread different symbols over all the subcarriers simultaneously in order to homogenize performances. This approach, performed by orthogonal codes, shows an evident link with MC‐CDMA techniques. In addition, Multi User Detection (MUD) strategies as Interference Cancellation (IC) with coherent detection are also applied in order to improve its performa

ISSN:1124-318X    

DOI:10.1002/ett.4460110610

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2000

数据来源: WILEY

摘要:

AbstractThis article will emphasize on how to efficiently implement multicamer (MC‐) systems different from and alternative to OFDM with polyphase filterbanks. It will show that when using polyphase filterbanks this is an incomplex and flexible solution. It will be derived that with polyphase filterbanks an additional MC‐system design parameter exists; not only spacings between adjacent subcarriers in integer multiples of the inverse MC‐symbol duration are possible but that also non‐integer multiples of the inverse MC‐symbol duration are easily r

ISSN:1124-318X    

DOI:10.1002/ett.4460110611

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2000

数据来源: WILEY