Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Utility Patent
1998-03-10
2001-01-02
Ton, Dang (Department: 2732)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S333000, C370S334000
Utility Patent
active
06169731
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wireless communication systems, and more particularly, to a method for controlling the transmit power of a communication device operating in a wireless communication system.
BACKGROUND OF THE INVENTION
Wireless communication systems, such as analog and digital cellular communication systems, personal communication systems (PCS) and other similar wireless communication systems, provide a great deal of freedom to their users. A wireless communication system user is always in touch. And, in spite of the complexity underlying the wireless communication system, to the user using the system is as easy as dialing a phone number.
Sometimes, a user will be unable to place or receive a call, or an ongoing call will be unexpectedly disconnected. One has to remember that at least a portion of the wireless communication system is a radio frequency (RF) link between the remote user and the system. There are a number of factors which influence how and why a call may not be completed or is disconnected. For example, system capacity, i.e., the number of available radio frequency links, may be exhausted. The user might be out of range, or interference may render the radio link unsuitable for maintaining the call. In any event, the user is inconvenienced.
Of course, in wireless communication systems, such as cellular and PCS communication systems, high call completion and high system capacity are requirements. In a type of cellular and/or PCS communication system known as code division multiple access (CDMA) digital cellular system, call setup and system capacity are related to the call processing algorithm implemented during the call setup process, and to the transmitting power of mobile stations (MS) operating in the system.
In a CDMA communication system implemented in accordance with the IS-95-A standard, land- and mobile-originated calls are prone to failure during the signaling protocol required to establish or complete a call. That is, the communications between the MS and the base station (BS) to place the MS on a Traffic Channel (TCH, i.e., a communication link used for transmitting and receiving data, typically coded speech, between the system and the mobile station). In certain systems, call failure rates may be as unacceptably high as 2-4% of all call attempts. These failures are attributable to several weaknesses in conventional implementations of the BS signal processing executed during call set-up, and particularly, that portion associated with the BS acquiring the MS preamble transmission leading to these call failures.
The conventional preamble acquisition search algorithm is a single-dwell serial search technique applied to one or both receiving antennas at the BS. If a search statistic associated with any multiple signal component exceeds a threshold, acquisition is declared with the multiple signal component generating that statistic being used for subsequent demodulation of the received signals. In order to discriminate between genuine signal multipath components and noise, and to meet a tolerable specification for false preamble acquisition (e.g., 1 in 1000 call attempts at 7 decibels (dB) information bit energy
oise density (Eb/No) signal level with
3
seconds delay prior to preamble transmission) the search integration period “dwell time” must be large. This increases the time required to search cells within the communication system with large radii when the MS location information is not available to the receiver, or is unreliable.
Following declaration of acquisition, rake fingers are assigned to the delays at which the preamble acquisition search algorithm indicated valid multiple signal components to be present, and demodulation begins. After a correct or false preamble acquisition, however, the BS will signal to the MS that acquisition has occurred.
Upon receipt of the BS Ack. Order indicating valid preamble acquisition, the MS ceases preamble transmission. In the case of false acquisitions, this deprives the BS receiver of the optimal signal to use to attempt proper acquisition. The BS receiver then has to attempt to re-acquire the MS using the modulated null traffic frame sequence that—according to the IS-95-A specification—follows the preamble phase. However, this frame sequence is an ⅛-rate transmission, and the efficiency of the re-acquisition attempt is highly reduced since signal is not present on the channel ⅞'s of the time.
If the BS receiver has falsely acquired the preamble signal the first MS Ack. Order transmitted by the MS in response to the BS Ack. Order declaring acquisition may be lost. This is because the BS receiver may be unable to successfully demodulate the MS Ack. Order, which depends on whether the BS modulated data search has successfully reacquired the MS before transmission of this message. Retransmission of the MS Ack. Order unnecessarily extends the call set-up time.
Another undesirable side effect of false preamble acquisition is the possibility of instability in the MS transmit power control loop. In one possible implementation, the idle pattern initially transmitted to the MS via the traffic channel power control sub-channel is replaced with power control decisions based on the demodulated power control metric upon detection that at least one of the four available rake fingers is in the “lock” state. The initial finger assignments that drive this decision may be made prior to the demodulation phase at the same time that preamble acquisition is declared.
The filters controlling the lock status of these assignments are updated, on average, every 10 ms at the rate of the ⅛-rate power control groups (PCGs). Depending on how the lock filter state variables are initialized, and the delay between the finger assignments and the first ⅛-rate PCG, it is possible for fingers that have been falsely assigned during the preamble acquisition search algorithm to delays at which no signal components exist to remain in lock for several 10's of milliseconds before lock filtering changes their status to the “unlocked” state. At this point, the idle pattern is restored.
Since the receiver power control metric computed every PCG from such false assignments will almost always result in the +1 dB “power-up” decision, and since the MS continues to transmit preamble and hence act on every received power control bit until the BS Ack. Order acknowledging preamble acquisition is received, the MS may receive several consecutive +1 dB power control decisions until a) all the falsely assigned fingers autonomously exit lock under lock filter control thus restoring the power control idle pattern, or b) a subsequent modulated data or “multipath” search detects the true signal delay and assigns an available finger, thus correcting the power control metric estimate.
Since these processes may last for 10's of milliseconds, erroneous power control decisions may well continue sufficiently long for large “spikes” in MS transmitted power of several 10's of dB to be observed at the MS transmitter. Field and laboratory testing has verified evidence of these problematic transients or “spikes” (which may be as high as 30 decibels (dB)) in mobile station transmit power during call origination. As is known, in a CDMA communication system, system capacity, as related to cell size, is closely effected by MS transmit power. So not only do these spikes effect the MS attempting to originate or terminate a call, they may also effect other MSs operating in the system.
Yet another potential weakness in the existing implementations is the use—immediately after the acquisition procedure terminates - of the rate-determination scheme intended to discriminate between transmission rates used during variable-rate transmissions (Service Option 1 in an IS-95-A implemented CDMA communication system). This can lead to erroneous frame classifications and poor power control loop performance.
The response of the rate determination algorithm to frames comprising the preamble seque
Garg Sanjeev
Stewart Kenneth A.
Haas Kenneth A.
Motorola Inc.
Ton Anthony
Ton Dang
LandOfFree
Method and apparatus for signal acquisition and power control does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for signal acquisition and power control, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for signal acquisition and power control will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2553607