Elsevier

Signal Processing

Volume 81, Issue 8, August 2001, Pages 1663-1679
Signal Processing

Imperfect orthogonal multiplexing in the forward link of CDMA networks

https://doi.org/10.1016/S0165-1684(01)00078-0Get rights and content

Abstract

The Universal Mobile Telecommunications System (UMTS) will comprise both a terrestrial part (T-UMTS) and a satellite part (S-UMTS). The air interface will include code division multiple access as one of the operating modes, with direct sequence spreading based on scrambling and channelization codes. The latter are used for discriminating among different users/services, and play a fundamental role in the overall system performance. We consider Orthogonal Variable Spreading Factor codes for channelization, and we focus our attention on the forward link from the base station to the mobile terminal. Our aims are to describe analytically the transmitted multiplex signal passed through a high power amplifier (inevitable in S-UMTS), before and after the non-linear amplification, and to analyze the performance loss which derives from imperfect orthogonality induced by the non-linear device. To this end, we introduce a semi-analytic procedure based on the notion of channelization code power ratio, and we validate it through computer simulation. The major finding in our numerical results is that due to the orthogonality loss, it is impossible to fully exploit the forward link power control dynamics, with consequent capacity loss.

Introduction

The European Telecommunications Standards Institute (ETSI) is actively participating into the standardization of the third generation (3G) mobile system, International Mobile Telecommunications-2000 (IMT-2000), within the International Telecommunication Union (ITU). IMT-2000 will provide multimedia services at data rates that range from 16kbit/s all the way up to 2Mbit/s, and will comprise both terrestrial and satellite air interfaces to guarantee worldwide coverage [8]. The ETSI proposal for IMT-2000, named Universal Mobile Telecommunications System (UMTS), contains the so-called wideband code division multiple access (W-CDMA) option for T-UMTS, and a very similar air interface has also been proposed for the S-UMTS component [2], [7]. Owing to this close resemblance of the two air interfaces, the scope of this paper pertains to both T-UMTS and S-UMTS without distinction. In particular, we focus on the forward link from a BS (terrestrial base station or satellite fixed earth station) to a mobile terminal (MT) of the W-CDMA air interface.

In W-CDMA, spectrum spreading is accomplished by overlaying two signature sequences. Namely, on a lower layer a channelization code is used, while on an upper layer a scrambling code is adopted. In the forward link, the channelization codes are used to discriminate between different users, or between different services pertaining to the same user. In other words, the various users/services are mapped onto separate channelization codes which are designed to be orthogonal, so that ideally no mutual interference is present. In W-CDMA, the channelization codes are orthogonal variable spreading factor (OVSF) codes [1]. The scrambling code is necessary to protect the transmitted multiplex against interference coming from other BSs and from other antenna sectors of the same BS, or other satellites/spot-beams in the satellite case.1

Unfortunately, ideal orthogonality among the channelization codes is not achievable in practice due to several impairments, the most important being non-linearity and multipath. For a realistic and complete evaluation of the effectiveness of the W-CDMA air interface, a thorough analysis of the impact of this orthogonality loss is mandatory. Non-linearity is the prevalent impairment in S-UMTS, where on-board amplifiers are driven into saturation to optimize the DC/RF conversion efficiency, while fading for satellite mobile channels follows a Rice statistic with strong direct component and negligible delay spread, and as such multipath effects do not induce any significant orthogonality loss. Conversely, time dispersion due to multipath fading is prevalent in T-UMTS, where no direct component is normally available and the delay spread may be very significant, while amplifier non-linearity can be kept under control to a large extent.

The objective of this paper is the evaluation of the orthogonality loss due to non-linear distortion, to give a quantitative measure of the performance degradation that can incur both on a single link and on the overall system capacity. Future work will address the problem of multipath fading. In order to gain a deep understanding of the problem, we carry out a complete characterization of the multiplex signal. Also, to evaluate performance we pursue a semi-analytic procedure introducing the notion of channelization code power ratio (CCPR), which is an extension for multiplex with variable data rate services of the concept of walsh power ratio (WPR) introduced for the IS-95/Globalstar system [5], [10]. The analysis also includes the impairment due to imperfect compensation of the phase rotation induced by the AM/PM characteristic of the power amplifier. In the numerical results we report CCPR values in different loading and input back-off conditions, and we confirm bit error probability values obtained via the semi-analytic approach with the results of Monte-Carlo simulation. Among the various findings, we will show that the multiplex constellation has a large peak-to-average ratio, although the peak value has very low probability. Also, we will verify that the CCPR decreases linearly with increasing imbalance among the various channels, which is the basic motivation behind the fact that the loss of orthogonality hinders the use of power control to compensate for largely different path losses.

The paper is organized as follows. In Section 2 we describe the system model to be used as a reference for performance evaluation. The characterization of the signal multiplex in terms of constellation shape and associated a priori probability is the subject of Section 3. Next, in Section 4 we introduce the concept of CCPR, which is the essential performance indicator in the presence of non-linear distortion. Numerical results, both semi-analytic and from Monte-Carlo simulation, are reported in Section 5. Finally, our conclusions are drawn in Section 6.

Section snippets

System model

The system block diagram for the problem at hand is shown in Fig. 1. The channel for the ℓth user is identified as dedicated physical channel (DPCH). The DPCH symbols are the output of a convolutional encoder, with coding rate Rc. The DPCH symbols are serial-to-parallel converted in view of the subsequent QPSK modulation. The in phase (I) and quadrature (Q) branches are spread to the chip rate with the same OVSF channelization code ci, for i=0,…,M(ℓ)−1, where M(ℓ) is the code period spanning

Multiplex characterization

The transmitted signal in the forward link is a multiplex of a large number of signals, possibly with different data rates. Furthermore, the power level in each channel can be different due to variable propagation/interference conditions. Therefore the multiplex analytical description, which would be extremely helpful in gaining a deep understanding of the motivations behind the possible weakness with respect to non-linear effects induced by the HPA, is not easily achievable. In this section,

Performance evaluation through a semi-analytical procedure

We introduce here a semi-analytical procedure to evaluate system performance loss due to the impairment caused by HPA non-linearity. In fact, performance prediction through Monte-Carlo simulation is a lengthy process owing to multilevel spreading and the oversampling unavoidable for filtering purposes. Therefore, it is useful to have some means to predict performance analytically, or semi-analytically as in this case. To characterize the impact of the HPA-induced distortion on the system

Numerical results

The simulator, that emulates the complete transmission chain for the downlink dedicated physical data channel, is able to evaluate system performance in various working conditions such as for different number of active channels, up to the full multiplex capacity, with the full spectrum of data rates available. Moreover there is the possibility to change the service characteristics (i.e. the rate of incoming binary data streams) frame by frame in order to simulate real traffic conditions.

To

Conclusions

In this paper, we have addressed the problem of characterizing the performance impairment due to the loss of orthogonality among the signals contributing to the multiplex in the forward link of a S-UMTS system. In particular, we considered the surge of mutual interference induced by the non-linearity of the HPA. To this end, a few analytical and simulation tools were developed and tested. In particular, we produced formulas that describe geometrically and statistically the multiplex signal

Paola Salmi was born in Imola (Italy) in 1971. She received the Dr. Ing. degree in Telecommunications Engineering from the University of Bologna (Italy) in 1996. She started her Ph.D. study in 1998 and received the Ph.D. degree in Electronics and Computer Science in 2001 from the University of Bologna (Italy). Since June 1997, she has been with the Department of Electronics, Computer Science and Systems (D.E.I.S.) at the University of Bologna. Her research activities include digital

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Paola Salmi was born in Imola (Italy) in 1971. She received the Dr. Ing. degree in Telecommunications Engineering from the University of Bologna (Italy) in 1996. She started her Ph.D. study in 1998 and received the Ph.D. degree in Electronics and Computer Science in 2001 from the University of Bologna (Italy). Since June 1997, she has been with the Department of Electronics, Computer Science and Systems (D.E.I.S.) at the University of Bologna. Her research activities include digital communication theory with special emphasis on spread spectrum communication systems, third generation terrestrial and satellite CDMA systems, synchronization techniques and space-time diversity techniques.

Giovanni Emanuele Corazza was born in Trieste (Italy) in 1964. He received the Dr. Ing. degree (cum laude) in Electronic Engineering in 1988 from the University of Bologna (Bologna, Italy), and a Ph.D. in 1995 from the University of Rome “Tor Vergata” (Roma, Italy). In the years 1989 to 1990 he was with the Canadian aerospace company COM DEV (Ontario), where he worked on the development of microwave and millimeter-wave components and subsystems. In the years 1991–1998 he was with the Department of Electronic Engineering at the University of Rome “Tor Vergata”, as a Research Associate. Since November 1998 he joined the Department of Electronics, Computer Science, and Systems (DEIS) of the University of Bologna, where he is presently Associate Professor and Chair for Telecommunications. During 1995 he visited ESA/ESTEC (The Netherlands) as a Research Fellow. During 1996 he was a Visiting Scientist at the Communications Sciences Institute at the University of Southern California (Los Angeles, CA). The same institute invited him as a Visiting Professor to teach a course on Spread Spectrum Systems in the fall of the year 2000. During the summer of 1999 he was a Principal Engineer at Qualcomm (San, Diego, CA). He is Associate Editor for Spread Spectrum for the IEEE Transactions on Communications. His research interests are in the areas of communication theory, wireless communications systems, spread-spectrum techniques, synchronization. He leads the group from the University of Bologna participating to the European Community fifth framework project SATIN.

Dr. Corazza received the Marconi International Fellowship Young Scientist Award in 1995 (ex-aequo). He was co-recipient of the Best Paper Award at the IEEE Fifth International Symposium on Spread Spectrum Techniques & Applications, ISSSTA’98, Sept. 2–4, 1998, Sun City, South Africa.

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