Tapped delay line RAKE receivers are commonly used to achieve path diversity gain in DS-CDMA systems: Here, a bank of correlators is used to separate the multipath components and a combiner coherently recombines the multipath energies. In this paper, the optimal combining techniques for DS-CDMA RAKE receivers (under different criteria) are introduced. Specifically, based on the maximum likelihood criteria, optimal combining techniques are derived and discussed for the following scenarios: (1) channel-fade-information is not available at the receiver; (2) a statistical characterization of the channel fades is available at the receiver, but the actual, instantaneous fade information is not; and (3) the instantaneous channel fade information is available at the receiver. This paper shows that maximum ratio combining (MRC), typically considered the optimal combining scheme when fade parameters are perfectly tracked, is sub-optimal. We present the optimal alternative to traditional MRC.

In the author's earlier work, we demonstrated how performance degradation and PAPR concerns in OFDM can be overcome by application of carrier interferometry (CI) spreading codes. In this work, the authors employ RF test equipment and analyze the practical performance of OFDM (orthogonal frequency division multiplexing) based IEEE 802.11a WLAN vs. their proposed carrier interferometry (CI) OFDM based WLAN. Specifically, RF test results (in a typical indoor office environment) are used to analyze the proposed CI/OFDM and CI/COFDM technologies. It is shown that in a typical office environment, at a bit error rate of 10/sup -3/, the CI technology gains 5-7 dB over current OFDM.

The paper introduces to orthogonal frequency-division multiplexing (OFDM) systems a novel pseudo-orthogonal carrier interferometry spreading code which spreads each parallel data stream over all the OFDM carriers. Pseudo-orthogonal carrier interferometry (PO-CI) spreading codes are carefully selected to introduce the following benefits to OFDM: up to 2N parallel data streams can be coded onto N carriers, with little degradation in performance; when rate 1/2 channel coding is applied in addition to PO-CI spreading codes, the resulting binary phase-shift keying OFDM systems demonstrate the performance of coded OFDM and the throughput of uncoded OFDM; PO-CI codes are carefully selected to spread in a manner which eliminates the peak-to-average power ratio problems characteristic of traditional OFDM.

WLANs (wireless local area networks) have emerged as a powerful architecture capable of supporting the requirements of broadband wireless communications. The IEEE 802.11a 5 GHz WLAN standard employs OFDM in its physical layer. This paper introduces carrier interferometry OFDM to the current 802.11a WLAN, and demonstrates that the corresponding small changes to the traditional OFDM scheme lead to notable improvements in probability of error performances and a doubling in throughput. Calling the modified WLAN system pseudo-orthogonal carrier-interferometry WLAN (or PO-CI-WLAN for short), we show that in office buildings, at a bit error rate of 10/sup -3/, PO-CI-WLAN offers a 3 dB performance gain over the IEEE 802.11a 5 GHz WLAN and simultaneously doubles the throughput.

OFDM (orthogonal frequency division multiplexing) is susceptible to poor probability of error performance in fading channels. To enhance OFDM's performance, many architectures utilize channel coding. The addition of coding, adds both redundancy and frequency diversity, but comes at a cost of reduced overall throughput (typically by a factor of 2). This paper introduces a novel carrier interferometry phase coding to enhance the performance in OFDM systems without bandwidth expansion or decreased throughput. It is shown that at a bit error rate of 10/sup 3/, this method gains 14 dB over OFDM, equaling the performance of COFDM. A system is now available demonstrating the benefits of coded OFDM, which maintains the throughput of OFDM. The cost is one of increased receiver complexity.

Abstract Multi‐carrier technologies in general, and OFDM and MC‐CDMA in particular, are an integral part of the wireless landscape. In this second part of a two‐part survey, the authors present an innovative set of spreading codes known as CI codes, and demonstrate how these significantly increase performance and capacity in OFDM and MC‐CDMA systems, all the while eliminating PAPR concerns. Regarding OFDM: the spreading of each symbol over all N carriers using CI spreading codes (replacing the current one symbol per carrier strategy) are presented. CI codes are ideally suited for spreading OFDM since, when compared to traditional OFDM, CI‐based OFDM systems achieve the performance of coded OFDM (COFDM) while maintaining the throughput of uncoded OFDM, and, at the same time, eliminate PAPR concerns. When applied to MC‐CDMA, CI codes provide a simple means of supporting 2N users on N carriers while maintaining the performance of an N‐user Hadamard Walsh code MC‐CDMA system, i.e., CI codes double MC‐CDMA network capacity without loss in performance. The CI codes used in OFDM and MC‐CDMA systems are directly related to the CI pulse (chip) shapes used to enhance TDMA and DS‐CDMA (see part 1): hence, the CI approach provides a common hardware platform for today's multi‐carrier/multiple‐access technologies, enabling software radio applications. Copyright © 2002 John Wiley & Sons, Ltd.

OFDM (orthogonal frequency division multiplexing) is susceptible to high peak-to-average power due to an unstable envelope. Many solutions have been utilized in order to decrease the high peaks that are possible, but in these cases complexity is also added to the system architecture. In our earlier work, we introduced CI codes as a powerful tool to increase OFDM performance. This paper shows how carrier interferometry phase coding eliminates peaks in the signal envelope and in effect the problems associated with large PAPR.