Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-03-11
2001-09-18
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S130000, C375S227000, C455S063300, C455S069000
Reexamination Certificate
active
06292519
ABSTRACT:
The present invention relates to spread spectrum communications, and in particular, to accurate mobile transmit power control in a spread spectrum communications system.
A direct sequence spread spectrum system (DSSS) is a wideband system in which the entire frequency bandwidth of the system is available to each user all the time. A DSSS system employs a spreading signal that considerably expands or “spreads” the bandwidth of the transmitted information (baseband data) much more than the minimum bandwidth required to transmit the baseband data. The spreading of the data is performed using a spreading signal sometimes called a spreading sequence, or a spreading code, or a pseudo-noise (PN) code. Different users are distinguished by using different spreading codes. That is why DSSS systems are also referred to as Direct Sequence-Code Division Multiple Access (DS-CDMA) systems. The two terms are used interchangeably throughout the following description. The code signal is independent of the data and is of much higher rate (the chip rate) than the data signal (the bit or symbol rate).
At a CDMA receiver, the inverse operation of compressing or “de-spreading” the received signal bandwidth is performed in order to recover the original data signal and at the same time to suppress the interference from other users. De-spreading is accomplished by cross correlation of the received spread signal with a synchronized replica of the same code signal used to spread the data. Different users are provided with different PN codes or PN codes that are offset in time that allows their transmission to be separately decoded at a receiving station.
Spread spectrum systems have a number of advantages. For instance, contrary to other types of mobile radio access systems, CDMA base station receivers diversity combine separate multipaths, (e.g., a first line of sight path and a second path reflecting off a building), and achieve enhanced signal reception and performance. A RAKE receiver is used to handle multipath propagation. A RAKE receiver captures most of the received signal energy by allocating one of a number of parallel demodulators (referred to as “fingers”) to each of the strongest components of the received multipath signal. The outputs of all the RAKE fingers are combined (taking the best from each finger) after a corresponding delay compensation to construct an optimum received signal.
Another advantage is CDMA systems tolerate interference—but only up to a certain threshold limit. The introduction of additional active mobile radio transmissions to the CDMA system increases the overall level of interference at the cell site receivers (base stations) receiving CDMA signals from the mobile radio transmitters. The particular level of interference introduced by each mobile's transmission depends on its received power level at the cell site, its timing synchronization relative to other sites at the cell site, and its specific cross-correlation with other transmitted CDMA signals.
Because of this multi-user interference limit, power control is very important in CDMA systems. Typically, power control attempts to achieve a constant mean power level for each mobile user received at a base station irrespective of how close or far each user is to the base station. This goal is readily understood by an analogy between a CDMA system and a party where a number of people are socializing in a room. As more and more people join the party, the room becomes more crowded, and the general noise level increases. As the noise level increases, it becomes more difficult for a listener to decipher a conversation. That difficulty is generally compensated for by the speaker (transmitter) speaking louder (higher transmit power). But speaking louder also exacerbates the existing noise level problem because it makes it even more difficult for others to hear so those others speak louder. Very soon the situation escalates to the point where no one at the party can comfortably or effectively communicate.
Accordingly, an important task of CDMA base stations is to control mobile transmit power of each mobile user, and they typically do so using a fast Transmit Power Control (TPC) algorithm. One efficient algorithm for the fast power control in DS-CDMA systems is based on Signal-to-Interference Ratio (SIR) measurements of mobile transmission received at the base station. The SIR is defined as the ratio of the data bit energy (E
b
) to the interference (including noise) power spectral density (I
o
). If the interference is assumed to be white noise, the power spectral density is equivalent to the interference power.
Two kinds of SIR measurements may be made in a base station. The first is a short term SIR measurement used for the generation of an uplink TPC message transmitted to the mobile. The short term SIR measurement value is compared with a reference E
b
/I
o
value, and depending on the result, the mobile is ordered to increase or decrease its transmit power by some predetermined amount, (e.g., 1 dB). The second kind of SIR measurement is a long term SIR measurement used to adjust the reference E
b
/I
o
level in order to achieve a specified frame error rate at the base station. Because the average power level received from the mobile varies depending on the terrain features, such as buildings and hills, the average mobile power is adjusted to achieve the specified bit error rate or frame error rate at the base station.
SIR measurement also finds application in mobile stations. For example, SIR measurement may be used for mobile-assisted soft-handover where the mobile measures SIR using the pilot signal transmitted from different base stations in order to establish one or more concurrent connections with the one or more base station(s) providing the best received signal quality. Another application of SIR measurements in a mobile station is Forward Loop Power Control where the base station adjusts the amount of power allocated to each mobile user so that the measured SIR at each mobile achieves a specified error rate.
In any case, the estimation of data bit energy E
b
is performed after de-spreading and RAKE combining in the receiver. Depending on the SIR measurement application, it may be performed using a short or a long averaging period. A short averaging period is used when the E
b
value is measured using only pilot preamble symbols transmitted at the beginning of each time slot within a data frame. For example, in third generation mobile systems, a typical signaling format may consist of 10 msec frames, each divided into 16 time slots of a 0.625 msec duration, where one time slot period corresponds to one power control period. Due to a relatively small number of pilot symbols, (e.g., 4), for practical spreading factors, (e.g., 16-256), the short term SIR measurements experience large fluctuations at low input SIR values, (e.g., less than 5 dB). Thus, for short term SIR measurements, the number of samples available for coherent E
b
estimation may be insufficient to eliminate the influence of noise and interference.
For long-term SIR measurements, it is possible to non-coherently average E
b
measurements over an entire frame. The E
b
values obtained for the pilot preamble and for each individual data symbol are averaged over the time slot period, and E
b
values obtained from all slots are averaged at the end of the frame to produce the final long term E
b
measurement value.
In both short and long term SIR measurement, the interference power I
o
may be averaged over a number of frames. More specifically, I
o
may be obtained by correlating the input signal multiple times with an uncorrelated (in the ideal case, orthogonal) PN code or with time-shifted versions of the original PN code used at the transmitter and averaging the multiple, squared, absolute correlation values over the number of frames.
An example of the relation between actual and estimated SIR in a DS-CDMA system is shown in
FIG. 1
for a range of SIRs between −5 and 30 dB. The spreading factor (SF), equal to the number of PN chips use
Chin Stephen
Liu Shuwang
Nixon & Vanderhye P.C.
Telefonaktiebolaget LM Ericsson (publ)
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