Wireless direct sequence spread spectrum digital cellular telephone

Abstract

The present invention is directed to novel implementation techniques which makes orthogonal CDMA practical in an a short range mobile telephone environment where significant multipath fading exists. Specifically, novel techniques for establishing the time base, frequency, and power control necessary to achieve orthogonality. Use of a high power sounding burst on the outbound link permits: 1) antenna diversity selection to minimize the probability of a faded condition, 2) local frequency locking at the subscriber terminal which avoids the requirement for a costly precision frequency standard, and 3) essentially instantaneous inbound power control based on the outbound receive signal level. This is effective since time division duplexing is used and both transmission and reception take place on the same frequency. With the short frame structure and unique placement of the sounding burst the correlation between the outbound and inbound path losses is very high. Thus, the signal structure and control algorithms result in a greatly reduced signal level range at the base station/PBX to achieve high efficiency in a orthogonal CDMA system. Real world effects such as filtering, multipath time spread, and time base error destroy orthogonality and introduce a degree of cross coupling between supposedly orthogonal channels. Thus, the system provides accurately controlled power levels in this highly dynamic environment.