Canonical multipath-doppler coordinates in wireless communications

Abstract
1 Introduction
Code division multiple access (CDMA) has emerged as one of the most promising technologies for meeting the challenges of the physical network layer, as evident from its
This research was supported in part by the Wisconsin Alumni Research Foundation.
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prominence in the wireless infrastructure envisioned for the third and future generation systems. Innovative DSP techniques are playing a progressively important role in the design of CDMA systems due to the high complexity of processing required in the physical layer. The major DSP challenges in CDMA system design stem from three key factors that have a signi cant impact on performance: Channel propagation dynamics manifested as multipath dispersion, multipath fading, and temporal variations or Doppler e ects. Multiaccess interference due to multiple simultaneous users, which is further accentuated by multipath propagation e ects. Complexity of the DSP algorithms. Even though the above factors are clearly interrelated, existing techniques re ect a piecemeal approach due to the lack of a framework connecting the various physical layer facets. In contrast, the progressively stringent performance demands imposed by future wireless systems dictate an integrated approach to deliver fully optimized systems. In this paper, we propose DSP in canonical multipath-Doppler coordinates as an integrated framework for combating time-varying multipath distortion, suppressing multiaccess interference, and managing the complexity of the resulting algorithms. The canonical coordinates are derived from a fundamental characterization of the channel propagation dynamics in terms of discrete multipath-delayed and Doppler-shifted copies of the transmitted spread-spectrum waveform which constitute a xed canonical basis for representing the received signal 1]. A well-known advantage of CDMA, by virtue of spread-spectrum signaling, is its remarkable ability to combat fading by exploiting multipath propagation e ects 2]. Our framework promises a new DSP innovation: further exploiting the time-varying multipath e ects to suppress multiaccess interference via subspace-based processing in the canonical coordinates. The next section introduces the generic receiver structure in the canonical coordinates. Section 2 describes the fundamental channel characterization that underlies the proposed framework. Sections 4 and 5 discuss the basic ideas behind receiver design in canonical coordinates, emphasizing channel modeling and interference suppression issues. Concluding remarks are presented in Section 6.
2 The Canonical Coordinates
Figure 1 describes DSP in canonical multipath-Doppler coordinates that is at the heart of our framework. The front-end processing corresponds to projecting the received waveform onto the canonical coordinates. Each user corresponds to unique coordinates de ned by its spreading code, which are computed for each received symbol. Canonical coordinates, taken together for all users and symbols of interest, constitute su cient statistics for demodulation|all DSP can be performed in the canonical coordinates. The coordinates for each symbol of a particular user are computed by correlating the received signal with time- and frequency-shifted copies of the user spreading waveform ! Z l e?j 2 Tmt dt; zml = r(t)q t ? o ? B (1) l = 0; ; L ; m = ?M; ; 0; ; M; where r(t) denotes the received waveform, o denotes the user delay, and q(t) is the user spreading waveform of duration T and bandwidth B . In e ect, the canonical multipath2
Canonical Multipath-Doppler CoordinatБайду номын сангаасs in Wireless Communications
Akbar M. Sayeed
Department of Electrical and Computer Engineering University of Wisconsin{Madison 1415 Engineering Drive Madison, WI 53706 akbar@ Tel: (608) 265-4731, Fax: (608) 262-1267 /ece/ 36th Annual Allerton Conference, September 1998.
Code division multiple access (CDMA) has emerged as a dominant technology for meeting the physical layer challenges of future wireless communication systems. Signal processing requirements in the physical layer are dictated by three major factors: multiaccess interference, multipath dispersion and fading, and transceiver complexity. Existing CDMA system designs re ect a piecemeal approach due to the lack of an e ective framework for jointly addressing these issues. We propose signal processing in canonical multipath-Doppler coordinates for attacking physical layer impairments in an integrated fashion. The canonical coordinates are derived from a fundamental characterization of channel propagation dynamics in terms of discrete multipath-delayed and Doppler-shifted copies of the spread-spectrum signaling waveforms. The multipath-Doppler shifted waveforms constitute an approximately orthogonal basis and the corresponding signal representation naturally connects the various channel e ects. First, all processing relating to multipath propagation can be directly performed in the canonical coordinates. Second, the same coordinates provide a canonical subspace-based representation of the desired signal and interference which fully incorporates channel dispersion e ects. Finally, the maximally parsimonious nature of the coordinates and their simple computation a ord a direct handle on transceiver complexity. Various facets of the integrated framework are illustrated in the context of interference suppression, channel estimation, and diversity processing.