Code generator for multiple rake finger and method of use

Details Code generator for multiple rake finger and method of use Code generator for multiple rake finger and method of use

1999-07-21

2002-10-08

Yao, Kwang Bin (Department: 2664)

Multiplex communications

Communication over free space

Combining or distributing information via code word channels...

Reexamination Certificate

active

06463048

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to telecommunications equipment, and particularly to a device and method for use in a Code Division Multiple Access (CDMA) telecommunications system.
2. Description of the Related Art
Code Division Multiple Access (CDMA) transmission schemes have become increasingly popular due to the recent growth of the wireless industry. CDMA is a spread spectrum technique whereby data signals are modulated by a pseudo-random signal, known as a spreading code, before transmission. The modulation of the data signals spreads the spectrum of the signals and makes them appear like noise to an ordinary receiver. However, when the same pseudo-random signal is used to demodulate (despread) the transmitted data signal at the CDMA receiver, the data signal can be easily recovered.
An additional advantage of CDMA is that data signals from multiple users can be transmitted simultaneously on the same frequency band. Users are differentiated from one another at the CDMA receiver by spreading codes. Correlators provided at the receiver of the CDMA system recognize each different spreading code and restore (despread) the original data signal. These correlators are often arranged into units called “rakes” or “rake fingers,” the function of which is to assemble and demodulate one received multipath propagated signal component. Each rake finger typically includes one or more correlators and one or more pseudo-random code generator associated with each correlator, each pseudo-random code generator being associated with one correlator. The multiple rake fingers are used to detect the strongest multipath components as described below. Each correlator detects a time shifted version of the original transmitted data signal.
One problem that CDMA and other similar transmission schemes must deal with is multipath propagation. Multipath propagation is a phenomenon which causes many different versions of a transmitted signal, called multipath components, to be created and propagated to a receiver. The multipath components are created because the transmission antenna radiates the data signal in many directions (omnidirectionally), and therefore creates more than one component. Each of these different multipath components may arrive at the receiver at different times due to delays created by obstacles in the respective transmission paths. In other words, since the data signals are transmitted over the airspace between the transmitters and receivers, the transmitted signals will incur delays due to the surrounding environment (e.g. they will bounce off buildings and other structures in the transmission path). Hence, the same data signal may arrive at the receiver (or receivers) at different times. Depending on the environment, the multipath components may combine with each other constructively or destructively. Destructive combination causes the multipath components to effectively cancel each other out. Thus, if the multipath components combine with each other destructively, portions of, or the entire data signal may lost.
CDMA systems deal with the problem of multipath propagation by providing multiple antennas to receive the signal and multiple rake fingers. A receiver that includes multiple reception antennae is often referred to as a diversity receiver. Typically, both antennas will not experience the destructive combination of multipath components simultaneously and each rake finger can handle one multipath component from one antenna to overcome the time delays. For example, a CDMA receiver may have three rake fingers dedicated to the transmissions of a single user, to accommodate three different multipath components. Each one of the rake fingers receives a different multipath portion of the transmitted signal from one of the receiving antennas, each portion having a different associated time delay. The CDMA receiver is fabricated to recognize the different time delays and coordinate the multipath signals so that the original data signal can be retrieved from the multipath components.
FIG. 1
shows an example of a wireless system utilizing such a CDMA architecture and transmission scheme. There is shown a base station
100
and multiple user stations
110
and
120
(e.g. wireless phones). Each user station
110
,
120
has an associated transmitter
140
,
150
, for transmitting signals to the base station
100
. The base station
100
includes two receiving antennae A and B, connected to a receiver
130
for receiving signals from the multiple user stations
110
,
120
. As previously noted such a receiver, including multiple reception antennae (i.e. A and B), is often referred to as a diversity receiver. Connected to each user transmitter
140
,
150
are transmission antennae C, D respectively. Each user station
110
,
120
transmits data signals (i.e. messages) over their respective transmission antennae C, D, to the reception antennae A, B, and through to receiver
130
. The base station
100
receives the transmitted data signals, and relays the data (e.g. messages) to other users of the system.
As stated previously, each data signal may follow different paths to the base station receiver
130
. For example, these paths are shown as transmission paths P
1A
and P
2A
from user station
110
, and paths P
1B
and P
2B
from user station
120
. Although only two multipath components are shown for each user station, any number of multipath components may exist for each transmitted data signal. In most cases, it should be noted that only a few multipath components are dominant (i.e. only a few multipath components are worth considering due to the weak signal strength of the various other non-dominant components). Further, although the multipaths are only shown for transmission from a user station
110
,
120
to the base station
100
, it should be noted that the same multipath components are present when transmitting data from the base station
100
to the individual user stations
110
,
120
, etc. Therefore, the receivers located at the user stations
110
,
120
must also include rake fingers for handling multipath components.
As stated above, in present CDMA receivers, each rake finger includes at least one correlator and a separate pseudo-random code generator for each correlator. In systems with multiple receiving antennae at the base station, such as antennae A and B in
FIG. 1
, the multiplicity of rake fingers creates a problem in that a large number of code generators are required (i.e., one for each correlator of each antenna).
FIG. 2
shows the traditional rake finger
200
for a dual-antenna diversity receiver. As can be seen, the rake finger
200
includes first and second code generators
210
,
220
, and first and second correlators
230
and
240
. Multiplexers
250
,
260
connect the two antennae (e.g. A and B) to each correlator
230
,
240
. The multiplexers
250
,
260
are controlled by control signals
235
,
245
which select the antenna and sector of the cell from which the rake finger is currently receiving data signals. Although the multiplexers
250
,
260
allow the correlators
230
,
240
to receive signals from either antenna, for purposes of this discussion it will be assumed that correlator
230
receives data signals from antenna A only, and correlator
240
receives signals from antenna B only. Thereby, the rake finger
200
forms a single rake finger unit for reception of one multipath signal on both antennas. The base station receiver
130
shown in
FIG. 1
includes multiple rake fingers
200
. Although the device shown in
FIG. 2
is referred to above as a single rake finger, it may also comprise two separate rake fingers, one for the code generator
210
, correlator
230
, and multiplexer
250
, and one for the code generator
220
, correlator
240
and multiplexer
260
.
FIG. 5
shows another conventional rake finger
400
. Rake finger
400
is similar in many respects to rake finger
200
shown in
FIG. 2
, except that it is configured for a quad-antenna system (i.e. 4 antennae) inst

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