Method and apparatus for allowing soft handoff of a CDMA reverse link utilizing an orthogonal channel structure
View Patent ↗Method and apparatus for base stations and subscriber units allows soft handoff of a CDMA reverse link utilizing an orthogonal channel structure. Subscriber units transmit an orthogonally coded signal over a reverse link to the base stations. A given base station provides timing control of the timing offset of the reverse link signal. Based on at least one criterion, an alignment controller determines that the given base station should hand off timing control to another base station, and a soft handoff process ensues. In response to a command or message for soft handoff of the subscriber unit from the given base station to another base station, the subscriber unit makes a coarse timing adjustment to the timing of the coded signal. The subscriber unit may make fine timing adjustments based on feedback from the base station controlling timing. Multiple base stations may provide power control feedback to the subscriber unit.
1. A method in a base station for timing alignment of a field unit comprising:
receiving a first reverse link signal from the field unit;
determining a gross timing offset with respect to other reverse link signals from other field units sharing the same reverse link logical channel;
transmitting the gross timing offset to the field unit;
following a coarse timing adjustment performed by the field unit based on the gross timing offset, calculating a fine timing adjustment based on a metric of the transmission path between the base station and the field units associated with a second received reverse link signal; and
selectively determining based on the metric whether the base station should control the timing alignment of the field unit during a soft handover.
2. The method of claim 1 further comprising:
transmitting a message to other base stations whether the base station is going to control the alignment of the field unit.
3. The method of claim 2 further comprising:
reporting the timing offset in the form of a timing command.
4. The method of claim 2 further comprising:
causing the reverse link signal to be orthogonally aligned with the signals from at least one other field unit on the reverse link logical channel.
5. The method of claim 4 further comprising:
determining a power level of the reverse link signal; and
providing feedback of the power level to the field unit in the form of a power command or a power message.
6. The method of claim 1 , further comprising initiating timing control handoff based on at least one of the following criteria: a metric of the transmission path between the field unit and at least one of the other base stations exceeds a threshold for a predetermined timespan, a metric of the transmission path between the field unit and at least one of the other base stations exceeds a threshold relative to a metric of a transmission path between the base station and the field unit for a predetermined timespan, a metric of the transmission path between the base station and the field unit drops below an absolute metric, and a metric of the transmission path between at least one of the other base stations and the field unit exceeds an absolute metric.
7. The method of claim 6 , wherein the metric includes at least one of the following: power, signal-to-noise ratio (SNR), variance of the power, variance of the SNR, between the orthogonally aligned path an non-orthogonally aligned paths between the given field unit and the base station and the other base stations, relative ratio of the power, SNR, the variance of the power, or the variance of the SNR, bit error rate, and energy per chip divided by the interference density (Ec/Io).