5.8 Conclusion 227 5.7.2 CDMA Systems Pilot channel design for CDMA systems has been studied by many authors. The optimal power ratio of pilot and data channels was obtained in Schramm (1998). Under the optimal power ratio, the loss resulting from imperfect channel estimation was calculated. In Schramm & Muller (1998), optimal pilot symbol spacing for pilot symbol­assisted BPSK over Rayleigh fading channels with L d diver- sity paths was obtained. The loss resulting from imperfect channel estimation was also calculated. In Ling (1999), the author provided a comprehensive analysis and performance comparison, in terms of channel estimation, detection, and cut-off rate, under two training schemes, i.e., pilot channel-assisted and pilot symbol-assisted schemes, in CDMA systems. Training parameters were optimized based on Bhattacharyya bounds and cutoff rates. The optimal spacing of periodically inserted pilot symbols for an LMMSE channel and data estimator over flat Rayleigh fading channels for multiuser CDMA sys- tems was discussed in Evans (2002). The estimation of both time- and frequency-selective channels for short-code CDMA systems with single-user receiver was addressed in Baissas & Sayeed (2002), where a pilot channel was used in parallel to the data channel. 5.7.3 Ultra Wideband