Complexity Comparison of OFDM and CDMA for Wideband Communication Systems

Complexity Comparison of OFDM and CDMA for Wideband Communication Systems

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Complexity Comparison of OFDM and CDMA for Wideband Communication Systems Laurence Mailaender Lucent Technologies, Bell Laboratories Holmdel, NJ USA lm@lucent.com Abstract : It is commonly believed that OFDM systems have a complexity advantage over CDMA. We show that there are two ways for CDMA complexity to remain competitive with OFDM: a multi-channel approach, in which a wideband system is composed of multiple narrow-band systems; and a frequency-domain equalization approach, where the CDMA receiver is based on the same efficient FFT technique used in OFDM. By making some assumptions on receiver design and wireless channel characteristics, we find simple formulas for the required number of complex multiplications per second. MIMO and broadcast systems are considered as special cases. I. Introduction In recent years, Orthogonal Frequency Division Multiple access (OFDM) has become the basis for several standardized wideband wireless systems, including WiFi, WiMax, and IEEE 802.20. It is also widely anticipated that OFDM will become the basis for future “4G” cellular telephony. OFDM offers the benefits of: “near-orthogonality” among same-cell users in moderate delay/frequency spread channels for both up- and down-links; the ability to intelligently assign users to tones; and the ability to control the transmit power and bit loading per tone, amongst others. It is also widely believed that OFDM offers a complexity advantage (measured in complex multiplications per second, or cmults/sec) over CDMA [1,2]. In this paper, we focus on comparing the complexity of the required baseband detection stages in OFDM and CDMA as the signaling bandwidth increases. Wireless receivers operating over wideband channels generally require equalization to correct for frequency-selective fading. For 3G CDMA with 1.25 MHz of bandwidth, the Rake receiver has traditionally been used. In recent years it has been recognized that chip-level equalization in the CDMA downlink yields near-orthogonality of the codes [3], and this is expected to make CDMA performance competitive with OFDM. However, traditional equalization techniques impose high computational complexity, especially as the signal bandwidth increases. Increasing the bandwidth increases the effective span (in symbols) of the channel impulse response. Indeed, we will see below that the traditional LMS-based time-domain equalization approach has complexity () 2 N Ο as bandwidth increases 1 , whereas OFDM is well known to have complexity log N N Ο . We find that there are two techniques by which CDMA can break out of this “complexity trap.” In the multi-channel (MC) option, multiple contiguous narrowband channels are used to synthesize a wider bandwidth channel. This obviously requires multiple narrowband detectors in parallel. The second option is a single carrier (SC) system using frequency- domain equalization (FDE), which exploits the same FFT processing used in OFDM and therefore has similar complexity. With either of these approaches,
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Complexity Comparison of OFDM and CDMA for Wideband Communication Systems

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