Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1998-10-22
2004-08-24
Nguyen, Steven H. D (Department: 2665)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S320000, C375S131000
Reexamination Certificate
active
06781980
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to communications, and, in particular, to spread-spectrum wireless communications systems.
2. Description of the Related Art
The IS-95 standard, an interim standard published by the Telecommunications Industry Association, is an existing wireless communications standard that is based on spread spectrum communication techniques, also known as code division multiple access (CDMA) techniques. As is known in the art, CDMA techniques employ channels distinguished by different spreading codes. By combining each signal with the spreading code for the channel, each signal is spread over a much wider frequency band than the frequency band occupied by the signal prior to combining with the spreading code. This differs from traditional time division multiple access in which each channel transmits during a unique time frame and frequency division multiple access systems in which each channel is designated a unique portion of an available frequency band and/or modulates a unique carrier.
FIG. 1
is a block diagram showing a typical wireless IS-95 network
100
. Network
100
includes a group of remote users
112
-
115
generally in communication with base stations
109
-
110
through an air interface. Base stations
109
-
110
are, in turn, connected to a land line network
102
through a switching center
104
, which tracks the positions of remote users
112
-
115
in the network and allocates capacity of base stations
109
-
110
to remote users
112
-
115
.
In
FIG. 2
, there is shown a block diagram of a transmit portion, or forward link, of a base station
109
-
110
of the network
100
. The forward link in a communications network is the communication path through a CDMA communication channel of the air interface from a base station
110
, for example, to one or more of remote users
112
-
115
(e.g., wireless telephones). The reverse link is defined for each remote user and is the communication path from one of the remote users
112
-
115
to the base station
110
. For the forward link, digital signal processing block
202
performs processing of voice, voiceband data, or digital data signals from the land line network
102
. Radio Frequency (RF) modulation section
204
typically receives the processed signals from digital signal processing block
202
, and modulates an RF carrier signal with the processed signals in multiplier
208
. The optional D/A converter
206
converts a digital bit stream of the processed signals to analog signals used to amplitude or frequency modulate the RF carrier signal. The D/A converter
206
is shown as an option since, in alternative systems, digital bit values of the processed signals may be used to directly modulate the phase of the RF carrier signal. The modulated RF carrier signal is typically a low-power signal, which is then amplified to a high power signal in RF amplifier
210
. The high power signal is filtered in transmit filter
212
, and provided to air interface by antenna
214
.
FIG. 2
illustrates a single modulation path for a processed IS-95 signal modulating a single RF carrier signal and occupying, for example, a 1.25-MHz bandwidth. However, as is known in the art, multiple processed IS-95 signals may be transmitted in different frequency bands, each having a 1.25-Mhz bandwidth. An IS-95 transmit portion having several IS-95 signals modulating M carriers and transmitted in M different frequency bands is shown in FIG.
3
.
Referring now to
FIG. 3
, there is shown a block diagram of a transmit portion of a base station (e.g.
110
) of IS-95 wireless network
100
. Base station
110
of wireless network
100
comprises M IS-95 signal generators
302
(where M is an integer greater than 0), combiner
304
, and RF circuitry
306
and antenna
308
. Each signal generator
302
receives low-rate (narrowband) data streams for up to 64 different users and processes that low-rate data to generate a communications signal conforming to the IS-95 standard. Each signal generator
302
of a base station
110
in network
100
generates an IS-95 signal at a different carrier frequency. The signals from the different signal generators are combined by combiner
304
, which may typically be an analog RF combiner. The combined signal is processed by high-power RF circuitry
306
for transmission by antenna
308
to any number of the wireless unit remote users
112
-
115
.
According to the IS-95 standard, the narrowband data stream for each user is multiplied by a particular code sequence and then modulated at a particular carrier frequency. For a given signal generator
302
, the narrowband data stream for each user is encoded with a different code sequence, but modulated at the same carrier frequency. The effect of modulating the narrowband data for multiple users at the same carrier frequency is to spread all of the narrowband data for each user over the entire carrier-frequency band. In order to ensure that the modulated signals for different users do not interfere with one another, the code sequences are selected to ensure that the modulated signal for each user is orthogonal to the modulated signals for all other users in the same carrier-frequency band.
The IS-95 standard employs an RF signal for a carrier-frequency band that has a 1.25-MHz bandwidth, and which contains the encoded samples of several (up to 64) user conversations (voice or data sessions). Each user conversation comprises a baseband user signal of up to 9.6 Kbps, or possibly 14.4 Kbps, that is spread in bandwidth by a 1.228-MHz direct sequence digital encoding signal. The spreading rate, also known as the chip-rate, is therefore 1.228-MHz in the IS-95 standard. The encoding is achieved by using, for each user conversation, one of a set of 64 orthogonal Walsh codes, also known as Walsh functions or Walsh sequences. The Walsh codes of a given set are orthogonal in that the receiver reproduces the original user signal only if the received signal is demodulated with the same Walsh code used at the transmitter. Otherwise, uncorrelated noise is produced in the receiver. The digital signals of each user can simply be added together before being applied to the modulation part of the RF subsystem, as shown in FIG.
3
.
Referring now to
FIG. 4
, there is shown a block diagram of a portion of each signal generator
302
of
FIG. 3
of base station
110
of wireless network
100
. According to the IS-95 standard, each signal generator
302
is capable of supporting low-rate (narrowband) data streams for up to 64 different users using a single carrier frequency. Each user is assigned a different one of 64 orthogonal IS-95 forward-link Walsh codes.
FIG. 4
shows the processing performed on the data stream for one of the users supported by an exemplary implementation of signal generator
302
. That is, blocks
402
,
404
,
406
,
408
,
410
and
412
shown in
FIG. 4
would be repeated within signal generator
302
for each user with its own data.
In particular, for a single user data stream, convolutional encoder
402
provides a degree of error protection by applying convolutional encoding to the user's data stream to generate encoded signals. Block interleaver
404
applies block interleaving to the encoded signals to generate interleaved signals. Block interleaver
404
provides further error protection by scrambling data in time. In a parallel path, long pseudo-noise (PN) code generator
406
generates code sequences that are then decimated by an integer value in decimator
408
to reduce the length of the sequence so as to prevent identification of the sequence. The sequences provided by the long PN code generator
406
and decimator
408
perform encryption to provide a degree of security to the communications process. Multiplier
410
combines the interleaved signals from block interleaver
404
with the decimated code signals from decimator
408
.
The resulting signals from multiplier
410
are then combined with one of the 64 different Walsh sequences W
N
by Walsh-code multiplier
412
.
Dajer Miguel
LaConte Peter K.
Rubin Harvey
Lucent Technologies - Inc.
Nguyen Steven H. D
Tran Thien
LandOfFree
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