De-assigning signals from the fingers of a rake receiver

Details De-assigning signals from the fingers of a rake receiver De-assigning signals from the fingers of a rake receiver

1997-11-05

2003-02-04

Yao, Kwang Bin (Department: 2662)

Multiplex communications

Communication over free space

Combining or distributing information via code word channels...

C370S335000, C370S332000, C455S135000, C455S226300

Reexamination Certificate

active

06515977

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to telecommunications in general, and, more particularly, to a method and apparatus for de-assigning signals from the fingers of a rake receiver.
BACKGROUND OF THE INVENTION
FIG. 1
depicts a schematic diagram of a portion of a typical wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals
101
-
1
through
101
-
3
) that are situated within a geographic region. The heart of a typical wireless telecommunications system is Wireless Switching Center (“WSC”)
120
, which may be known also as a Mobile Switching Center (“MSC”) or Mobile Telephone Switching Office (“MTSO”). Typically, Wireless Switching Center
120
is connected to a plurality of base stations (e.g., base stations
103
-
1
through
103
-
5
) that are dispersed throughout the geographic area serviced by the system and to local- and long-distance telephone offices (e.g., local-office
130
, local-office
138
and toll-office
140
). Wireless Switching Center
120
is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal, which is connected to the system via the local and/or long-distance networks.
The geographic area serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called “cells.” As depicted in
FIG. 1
, each cell is schematically represented by a hexagon; in practice, however, each cell usually has an irregular shape that depends on the topography of the terrain serviced by the system. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center
120
.
For example, when wireless terminal
101
-
1
desires to communicate with wireless terminal
101
-
2
, wireless terminal
101
-
1
transmits the desired information to base station
103
-
1
, which relays the information to Wireless Switching Center
120
. Upon receipt of the information, and with the knowledge that it is intended for wireless terminal
101
-
2
, Wireless Switching Center
120
then returns the information back to base station
103
-
1
, which relays the information, via radio, to wireless terminal
101
-
2
.
Typically, the signal transmitted by a wireless terminal to a base station is radiated omni-directionally from the wireless terminal. Although some of the signal that is transmitted radiates in the direction of the base station and reaches the base station in a direct path, if one exists, most of the transmitted signal radiates in a direction other than towards the base station and is never received by the base station. Often, however, signals that radiate initially in a direction other than towards the base station strike an object and are reflected towards, and are received by, the base station. Thus, a signal can radiate from the wireless terminal and be received by the base station via multiple signal paths.
FIG. 2
depicts a schematic illustration of wireless terminal
101
-
1
as it transmits to base station
103
-
1
. Signal
107
-
1
is received by base station
103
-
1
directly. Signal
107
-
2
, signal
107
-
3
and signal
107
-
4
arrive at base station
103
-
1
after radiating initially in a direction other than towards base station
103
-
1
and only after reflecting off of an object, such as buildings
105
-
2
through
105
-
4
, respectively. Signals
108
-
1
through
108
-
4
radiate from wireless terminal
101
-
1
but never reach base station
103
-
1
.
Because each of the four signals arrives at base station
103
-
1
after having traveled a different path, each of the four signals arrives phase-shifted with respect to each other. And furthermore, depending on the length of the path traveled and whether the signal is reflected off an object before reaching base station
103
-
1
, the signal quality (e.g., the average power of an amplitude-modulated signal, the signal-to-noise ratio, absolute power in dBm, etc.) of each signal is different when received. This is partially due to the fact that when a signal is reflected off of an object, the degree to which the signal is attenuated is a function of, among other things, the angle at which the signal is incident to the object and the geometric and dielectric properties of the object.
FIG. 3
a
depicts an illustrative graph of the average power of an amplitude-modulated signal as a function of time for the direct path signal
107
-
1
in
FIG. 2
, which typically arrives at base station
103
-
1
in the best condition of all the constituent signals.
FIG. 3
b
,
3
c
and
3
d
depict illustrative graphs of the average power of an amplitude-modulated signal as a function of time for the three reflected signals, signal
107
-
2
through signal
107
-
4
, respectively, as they arrive at base station
103
-
1
after signal
107
-
1
. Typically, signals
107
-
2
through
107
-
4
are phase-shifted with respect to signal
107
-
1
, because they each travel a longer path than signal
107
-
1
, and are attenuated to varying degrees, with respect to signal
107
-
1
. Although only four signals are depicted in
FIG. 2
as reaching base station
103
-
1
, in practice, many signals typically reach base station
103
-
1
, each having traveled a different path, such that they interfere to form a composite signal at base station
103
-
1
. This phenomenon is known as the “multipath” problem. To simplify the illustration, the relative phase-shift of the constituent signals has been confined to an integral number of wavelengths of the carrier frequency. The resulting composite signal is shown in
FIG. 3
e.
In typical analog wireless systems in the prior art, the presence of secondary reflected signals at the receiver interfere with the direct path signal. When the system carries television signals, the reflected multipath signals can appear as “ghosts” on the screen of older television sets.
In a code-division multiple access (“CDMA”) wireless telecommunications system each radio receiver endeavors to identify and isolate the highest-quality constituent signals incident on the receiver and to demodulate and combine them to estimate the transmitted signal. As is well-known in the prior art, this process is conducted with, among other things, a finger-assignor and a rake receiver. The finger-assignor analyzes the incoming composite signal, in well-known fashion, and attempts to identify the strongest constituent signals in the composite signal to the rake receiver. The rake receiver isolates and demodulates each of the identified strongest constituent signals, and then combines the demodulation result from each constituent signal, in well-known fashion, to produce a better estimate of the transmitted signal than could be obtained from any single constituent signal. To accomplish this, a rake receiver comprises a plurality, but finite number, of individual receivers, known as “fingers,” each of which isolates and demodulates one constituent signal.
As the wireless terminal moves, the relative signal quality and phase-shift of the constituent signals changes, sometimes considerably. Received constituent signals can disappear, new constituent signals can appear, and existing constituent signals can merge or diverge. The signal quality of a constituent signal can suffer radical momentary changes, which make it appear for a time that the constituent signal no longer exists, although it quickly reappears. Such changes can be due to, for example, Rayleigh fading, or the transmitter passing behind an obstruction. Furthermore, the current receiver techniques in some CDMA technologies (e.g., IS-95 CDMA Rate Set 1) are such that usable constituent signals can have signal qualities so close to that of the noise floor that often random fluctuations in the noise

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