O that the resulting spectrum of each sub-carrier satisfies the orthogonality condition with the minimum frequency separation. Figure 5.6 shows the MC-DS-CDMA transmitter of the jth user, the power spectrum of the transmitted signal and receiver respectively where GMD denotes the processing gain, Nc the number of sub-carriers and C'(t) = [Ci 1, Ci 2 C'GMD] represents the spreading code of the jth user. The spread for each sub-carrier is smaller than in single carrier CDMA systems and BER can be improved by using RAKE receiver for each sub-carrier [6-8]. 5.3.2 Enhancements of Rev. B Some of the key enhancements of Rev. B are as follows: Higher Data Rates: It supports peak data rate of 4.915 Mbps per single 1.25 MHz carrier with 64-QAM in downlink. The peak data rates increase linearly with the number of carriers with a maximum of 73.5 Mbps in a 20 MHz spectrum chunk. The uplink peak data rate is 1.8 Mbps per 1.25 MHz carrier, same as Revision A. So, in a 20 MHz spectrum the uplink can support a peak data rate of 27 Mbps. The most common configuration is an aggregation of three carriers within a 5 MHz channel. In this implementation, the Multicarrier EV-DO software upgrade delivers a peak data rate of 9.3 Mbps in the downlink and 5.4 Mbps in the uplink, and with a Rev. B hardware upgrade, the peak data rate in the downlink increases to 14.7 Mbps. The revision supports three new higher data rates (3.7, 4.3, and 4.9 Mbps) with 64-QAM modulation in the downlink. Higher Spectral Efficiency: Rev. B provides higher spectral efficiency due to finer data rate quantization and enhanced granularity for Hybrid ARQ. Finer quantization is achieved with the use of additional packet data formats (27 VS. 14 in Rev. A). All the payloads have at least one, two, and four slots which enhance H-ARQ. It also enables multi-user diversity in both time and frequency domains which help Rev. B systems to achieve higher spectral efficiency than Rev. A. Scalable Bandwidth (Multi-Carrier Operation): Rev. B aggregates multiple 1.25 MHz carriers 