Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1999-09-15
2003-09-30
Nguyen, Lee (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S437000, C455S245100, C370S331000
Reexamination Certificate
active
06628958
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to wireless communication systems and, in particular, to forward link power control during soft handoff in wireless communication systems.
2. Description of the Related Art
Wireless communication systems employ Code Division Multiple Access (“CDMA”) modulation techniques to permit a large number of system users to communicate with one another. Such systems work because each signal is coded with spreading sequences, such as with pseudo-random noise (“PN”) sequences, and orthogonal spreading sequences, such as Walsh codes. This coding permits signal separation and signal reconstruction at the receiver. In typical CDMA systems, communication is achieved by using a different spreading sequence for each channel. This results in a plurality of transmitted signals sharing the same bandwidth. Particular transmitted signals are retrieved from the communication channel by despreading a signal from all of the signals. Despreading is achieved by using a known user despreading sequence related to the spreading sequence implemented at the transmitter.
FIG. 1
illustrates CDMA system
100
. The geographic area serviced by CDMA system
100
is divided into a plurality of spatially distinct areas called “cells.” Although cells
102
,
104
,
106
are illustrated as a hexagon in a honeycomb pattern, each cell is actually of an irregular shape that depends on the topography of the terrain surrounding the cell. Each cell
102
,
104
,
106
contains one base station
112
,
114
, and
116
, respectively. Each base station
112
,
114
, and
116
includes equipment to communicate with Mobile Switching Center (“MSC”)
120
, which is connected to local and/or long-distance transmission network
122
, such as a public switch telephone network (PSTN). Each base station
112
,
114
, and
116
also includes radios and antennas that the base station uses to communicate with mobile terminals
124
,
126
.
When a call is set up in CDMA system
100
, mobile terminal
124
communicates with the base station from which mobile terminal
124
receives the strongest pilot signal, in this case base station
112
. Base station
112
and mobile terminal
124
communicate over a forward link and a reverse link. The forward link includes communication channels for transmitting signals from the base station to the mobile terminal, and the reverse link includes communication channels for transmitting signals from the mobile terminal to the base station. Base station
112
transmits control information to mobile terminal
124
over a communication channel, referred to herein as a forward control channel, and it transmits voice or data over a communication channel, referred to herein as a forward traffic channel. Mobile terminal
124
transmits control information to base station
112
over a communication channel, referred to herein as a reverse control channel, and it transmits voice or data over a communication channel, referred to herein as a reverse traffic channel. The signals on the communication channels are organized in time periods, referred to herein as frames. Frames are typically 20-millisecond (ms) in length. Forward traffic frames are frames transmitted over the forward traffic channel, and reverse traffic frames are frames transmitted over the reverse traffic channel.
The number of signals that can be transmitted simultaneously is limited by each of the transmitted signals' fraction of the total power, referred to herein as the power fraction. Thus, reducing the power fraction of each of the signals increases the capacity of the wireless communication system. However, reducing the power fraction of a signal increases the number of errors in that signal. A goal of power control is to adjust the power level of the signals in such a way as to keep the power fractions as close as possible to a level that allows the system to maximize capacity while keeping the number of errors in the signal at an acceptable level. Forward link power control varies the power output of the base station to maintain a constant frame error rate at the mobile terminal. A frame error occurs when one or more uncorrectable bit errors occur in a frame. The frame error rate is the number of frame errors divided by the total number of frames observed. A targeted frame error rate, typically between 1% and 3%, depending on the desired system performance, is selected to minimize power without compromising signal quality. If the frame error rate exceeds the targeted frame error rate, the usefulness of the signal is reduced and the power level is increased to decrease the number of frame errors. If the frame error rate is below the targeted frame error rate, the power level exceeds the optimum power level, and the power level is reduced.
When the mobile terminal is in a soft handoff, all the base stations involved in the soft handoff are involved in the forward link power control. When mobile terminal
126
receives fairly strong pilot signals from more than one base station, in this case from three base stations
112
,
114
, and
116
, the mobile terminal is in soft handoff. This typically occurs when mobile terminal
126
is close to the edge of a cell. All three base stations
112
,
114
, and
116
transmit control information to mobile terminal
126
over respective forward control channels, and voice or data over respective forward traffic channels. In soft handoff, mobile terminal
126
transmits control information to all three base stations
112
,
114
, and
116
over respective reverse control channels, and it transmits voice or data to all three base stations
112
,
114
, and
116
over respective reverse traffic channels.
Base stations
112
,
114
, and
116
transmit forward traffic frames. Each forward traffic frame includes voice or data and error control information, typically in the form of a cyclical redundancy code (CRC). By contrast, each reverse traffic frame includes voice or data and error indicator bits (EIB) for indicating whether the last received forward traffic frame contained an error. Mobile terminal
126
receives the transmissions from all three base stations
112
,
114
, and
116
and combines the signals from all three to obtain the forward traffic frame. Mobile terminal
126
then checks the CRC of the combined signal to determine whether the forward traffic frame is in error. Mobile terminal
126
indicates this determination to all three base stations
112
,
114
, and
116
using the EIB in the next reverse traffic frame that mobile terminal
126
transmits. For example, a zero error indicator bit indicates that the forward traffic frame is not in error, and a positive error indicator bit indicates the forward traffic frame is in error. Upon receiving reverse traffic frames from the mobile terminal, the base stations sends the EIB to selection distribution unit (SDU)
128
. SDU
128
examines all three EIBs, and determines whether the majority of the EIBs indicate an erred forward traffic frame. SDU
128
then indicates to all three base stations whether, and how, they should adjust the power of their forward links. For example, mobile terminal
126
can send an EIB indicating an erred forward traffic frame. Base station
112
and
116
can receive EIB that indicating that there is an error in the frame. However, due to interference on the reverse traffic link between mobile terminal
126
and base station
114
, base station
114
receives an EIB indicating that the frame is not erred. After receiving and examining all three EIBs, SDU
128
would determine that there is an erred frame and indicate to all three base stations to increase the power of their forward link. Typically, it takes about five frames for the base station to transmit the EIBs to the SDU, and for the SDU to perform the determination and notify the base stations.
Therefore, in a conventional CDMA wireless communications system during soft handoff, there is a five frame, i.e., 100 ms, delay between the reception of the current power
Kamel Raafat Edward
Meyers Martin Howard
Weaver Carl Francis
Wu Xiao Cheng
Craver Charles
Herring, Jr. David W.
Lager Irena
Nguyen Lee
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