Multi-user acquisition procedure for multipoint-to-point synchronous CDMA systems

DETAILED DESCRIPTION The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention, wherein a subscriber unit is provided with circuitry and methods enabling synchronization with a central point, e.
g.
, a radio base unit, of a CDMA communications system in the presence of a plurality of interfering other subscriber units.
In a first aspect this invention teaches a method for use in a synchronous CDMA communications system that employs orthogonal pn spreading codes.
The method is intended for synchronizing transmissions from a subscriber unit (SU) to a radio base unit (RBU), and includes the steps of: (a) transmitting individual ones of a plurality of bursts from the SU to the RBU, each burst being transmitted with a different pn spreading code timing alignment; (b) receiving individual ones of the plurality of bursts with the RBU and determining a power estimate of each received burst; and (c) in response to a determined power estimate of one of the bursts exceeding a threshold, sending a message from the RBU to the SU.
The message indicates that the SU is to use for subsequent transmissions the pn spreading code timing alignment that was employed when transmitting the burst that exceeded the threshold.
In a preferred embodiment of this invention the step of transmitting transmits an indication of the pn spreading code timing alignment used when transmitting a given one of the bursts.
Each burst is transmitted with a pn spreading code timing alignment that differs by 1/2 chip from a previous pn spreading code timing alignment.
The RBU executes the following steps when receiving the SU burst transmissions: summing M samples per chip to obtain a first result; despreading the first result; summing L chips per symbol from the first result to obtain a second result; summing a plurality of symbols from the second result to obtain a power estimate of a current pn spreading code timing alignment that was transmitted by the SU; obtaining a scaled averaged power estimate that includes a power estimate from a previous pn spreading code alignment for forming the threshold; and comparing the power estimate of the current pn spreading code timing alignment to the threshold