Fast forward power control during soft handoff

Details Fast forward power control during soft handoff Fast forward power control during soft handoff

1999-04-22

2003-02-04

Trost, William (Department: 2683)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C370S318000, C370S320000, C370S330000, C370S331000, C370S335000, C370S350000, C370S503000, C455S522000, C455S442000, C455S069000

Reexamination Certificate

active

06515975

ABSTRACT:

TECHNICAL FIELD
The present invention relates in general to wireless communication systems and in particular to an efficient method of and apparatus for improving the implementation of fast forward power control during soft handoff.
BACKGROUND
In a CDMA (code division multiple access) cellular system complying with an industry standard specification as set forth in IS-95A, the repetition rate of the forward link power control signals is slow, on the order of 50 Hz. The MS (mobile station) reports the status of the forward link frames to the SBS (selector bank subsystem) of a BSC (base station controller) via the entire set of BTSs (base station transceiver subsystems) in soft handoff with the MS. Based on the frame quality, the power control algorithm, as implemented by the SBS, determines the new transmit power levels for all the BTSs in soft handoff with the MS. The SBS relays the new transmit power levels to the BTSs. Hence, the power level used by each BTS in soft handoff with the MS is always equal. That is, the forward link power allocated to the MS at each of the BTSs is always synchronized to the same power level.
When a forward link frame is transmitted to the MS from the BTSS, it takes a finite amount of time before the MS receives it. If a communication mode designated as Rate Set
2
is employed, then the MS encodes an EIB (Erasure Indicator Bit) into the next outgoing reverse link frame, signifying the status of the forward link frame. After additional propagation delays, each BTS in soft handoff with the MS relays the demodulated reverse link frame and associated frame quality metrics to the SBS. The SBS then updates the forward transmit power levels of the forward link based on the EIB and instructs the BTSs to use the new level. Therefore, the time between the transmission of a forward link frame and the corresponding increase/decrease in transmit power is a constant delay. The delay is based upon the architecture (propagation/processing delays of the various interconnecting blocks) of the system. In rate set
1
, on the other hand, the power control is accomplished through the use of the reverse link “Power Measurement Report Message” (PMRM), which is triggered by a count of the number of bad forward link frames.
The fact that the power control process is slow implies that under certain channel conditions, specifically low speed movement of the MS and single multipath environments where long deep fades are expected, a high average forward transmit power is required to meet a given GOS (grade of service).
An evolution of CDMA, popularly designated as 3G (third generation), includes a fast forward link power control scheme wherein the MS determines whether or not it requires more forward link power to maintain the GOS. The decision is transmitted rapidly to the BTSs via a reverse link dedicated control channel.
With the introduction of fast forward link power control, there was the expectation that forward link capacity would increase by large amounts at low mobile velocities. However, all the published analyses, known to the present inventors, carried out to characterize the actual capacity gain have failed to consider the performance of the algorithm during soft handoff. The term “soft handoff” throughout the remainder of this document is intended to define the situation where an MS is in communication with two or more BTSs preparatory to the potential of being transferred from one cell to another. The independence of the reverse links during soft handoff, in terms of slow/fast fading and distance/antenna related path losses to the mobile, result in different raw bit error rates. Individual BTSs that are in soft handoff, rapidly control their transmit power based on the forward power control bits they demodulate from the reverse link. This may, and often does, result in a deviation in instantaneous transmit power at the different BTSs. Depending on the degree of soft handoff, the average path loss difference between mobile and BTSs, and the power control parameters used, the resulting required forward link transmit power for a user may be quite high due to this mismatch. This translates to capacity degradation.
The main conclusion that may be drawn from the above, is that, to understand the practical effects of fast forward power control on capacity, one must consider the fact that an MS may be in a soft handoff mode for a significant amount of time. During this time, there is substantial opportunity for the transmission power levels of the different BTSs in soft handoff with the MS to become substantially non-synchronized. Additionally, one or more soft handoff forward links may be inadequately controlled due to poor reverse links. Such a situation detrimentally affects the potential capacity of such a system.
A CDMA system using fast forward link power control is more fully disclosed and discussed in one of several co-pending patent applications, such as “FAST FORWARD LINK POWER CONTROL IN A CODE DIVISION MULTIPLE ACCESS SYSTEM,” filed Sept. 17, 1997, having application Ser. No. 08/932,093, to Chheda et al, and assigned to the same assignee as the present invention. This application is hereby incorporated into this document in its entirety by reference.
To realize the fast forward power control technique, the MS estimates the SNR (signal to noise ratio) per power control group after maximally ratio combining the received signal energy per multipath. The estimation is compared to a threshold and, based on the comparison, a power control, up or down, command is generated. The MS adjusts this SNR threshold every frame, or at 50 Hz, based on the quality of forward link frames. Each time a forward link frame is received in error, the threshold is increased. This represents situations where the SNR may be insufficient for the current mobility conditions. If the forward link frame is good, the SNR threshold is reduced. This is based on the assumption that the SNR is sufficient for the given GOS levels. The increase and decrease of the threshold are related by a FER (frame error rate) requirement. The up/down decisions are the control mechanism that ensures that the received SNR or the forward link is equal to the threshold.
During soft handoff, the different BTSs, of a 3G fast power control system, must now independently demodulate the MS power control decisions. Based on the demodulated decision, the BTS will increase/decrease its transmitted power. It is quite likely that the reverse links between the MS and the different BTSs will have different instantaneous power control bit error rates. Consequently, the actual decision that each BTS makes may not be the same. In other words, some BTSs may demodulate the power control bit decision in error. With the existing prior art network architecture, the BSC cannot synchronize the BTSs to the same power level after each power control decision because of the inherent processing and queuing delay of the central processing unit and interconnecting communication links.
If the reverse links from MS to BTSs were error free, then the transmitted power at each BTS per power control group time segment would be equal and perfectly synchronized. However, as mentioned above, each BTS to MS reverse link is likely to have a different instantaneous error rate due to the independent slow/fast fading, and different distance/antenna related path losses. Thus, if each link has a different error rate, then the actual transmitted powers from each BTS to MS during soft handoff deviate. This may result in loss of diversity. This is further explained via a possible scenario discussed below.
It may be assumed that the MS is in two-way soft handoff with the network. If the BTSs could be perfectly synchronized, then they would power up/down together. Hence, the diversity gain is optimized; when one path fades, the other is used and the reverse scenario also holds. In practice, even when the BTSs start out at the same level transmitted power level, due to different power control bit error rates per reverse link, their transmit power levels and up

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