Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-10-19
2004-04-27
Vo, Don N. (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S144000, C375S143000
Reexamination Certificate
active
06728296
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is direct sequence spread spectrum systems, and more specifically, direct sequence spread spectrum systems in which traffic and forward error correction codes relating thereto are transmitted in parallel over separate channels.
2. Background
Wireless communication systems are an integral component of the ongoing technology revolution. Mobile radio communication systems, such as cellular telephone systems, are evolving at an exponential rate. In a cellular system, a coverage area is divided into a plurality of “cells”. A cell is the coverage area of a base station or transmitter. Low power transmitters are utilized, so that frequencies used in one cell can also be used in cells that are sufficiently distant to avoid interference. Hence, a cellular telephone user, whether mired in traffic gridlock or attending a meeting, can transmit and receive phone calls so long as the user is within a “cell” served by a base station.
Cellular networks provide mobile communications ability for wide areas of coverage. The networks essentially replace the traditional wired networks for users in large areas. But wireless technology can also be used to replace smaller portions of the traditional wired network.
Each home or office in the industrialized world is equipped with at least one phone line. Each line represents a connection to the larger telecommunications network. This final connection is termed the local loop and expenditures on this portion of the telephone network account for nearly half of total expenditures. Wireless technology can greatly reduce the cost of installing this portion of the network in remote rural areas historically lacking telephone service, in existing networks striving to keep up with demand and in emerging economies trying to develop their telecommunications infrastructure.
Another area in which wireless technology is aiding telecommunications is in the home where the traditional telephone handset is being replaced by the cordless phone system. A cordless phone system is in many ways a mini version of a WLL system. Cordless handsets in the home allow for untethered use of the handset enabling the user the freedom to move about as long as they stay in range of the base station.
Wireless systems can be classified according to the method used to provide access to multiple users seeking to utilize the system in parallel. In Frequency Division Multiple Access (FDMA) systems, the available frequency spectrum is divided into multiple narrow bands, each of which defines a separate channel. Different users are allocated different bands. Since the bands are separated by frequency, multiple users can access the system in parallel.
In Time Division Multiple Access Systems (TDMA), the available frequency spectrum is divided into multiple narrow bands, and each band is in turn divided into multiple time slots. A channel is defined as a particular time slot within one of the frequency bands. Again, since the channels are separated in time, or time and frequency as the case may be, multiple users can access the system in parallel.
In Code Division Multiple Access (CDMA) or Direct Sequence. Spread Spectrum (DSSS) systems, channels are defined by complementary, orthogonal or pseudo-random spreading sequences or codes. The spreading sequence has a frequency much higher than that of a user's information signal. Each user is assigned a unique spreading sequence. At the transmitter, the user's information signal is multiplied by the spreading sequence assigned to the user. Since the frequency of the spreading sequence is much higher than that of the user's information signal, the bandwidth of the information signal is effectively spread by this action.
The spread signals for each of the users are then simultaneously or concurrently transmitted over the same wideband frequency spectrum. As the receiver, each user's information signal is retrieved from the received signal by multiplying the received signal by the spreading sequence for the user, and then integrating and sampling the product. Since the spreading sequences are orthogonal or pseudo-random, each user's information signal can be retrieved from the same received signal.
A block diagram of a transmitter
110
in a DSSS system is depicted in
FIG. 7. A
user's analog information signal is input to A/D converter
111
, which digitizes the signal into bits. A block or frame of bits is then input to multiplexor
112
, which adds error detection or check bits, such as Cyclic Redundancy Check (CRC) bits, to the frame. The frame of bits, including the check bits, is then input to Forward Error Correction (FEC) coding block
113
, which encodes group of bits into codewords using one of the many known FEC coding schemes, such as convolutional coding. Typically, a group of bits is translated into one codeword. Thus, the coder
113
typically introduces redundancy into the system. The FEC symbols are used at the receiver to perform error correction.
The symbols from the FEC coder
113
are then input to spreader
115
. A unique spreading sequence for a user is generated by spreading code generator
114
. The sequence, which comprises a series of chips, is input to the spreader
115
. The spreader then spreads in frequency the codeword from FEC coder
113
. Typically, the spreader performs this function by multiplying or XORing the symbols from coder
113
and the spreading sequence from generator
114
.
Since the frequency of the spreading sequence is typically much greater than that of the information sequence, the effect of this process is to convert the information signal from a narrowband signal, depicted in
FIG. 9
with identifying numeral
130
, to a wideband signal, depicted in
FIG. 9
with identifying numeral
131
.
An advantage of a DSSS or CDMA system compared to narrowband systems, such as FDMA or TDMA, is its ability to withstand interference from a jamming signal. This property is illustrated in
FIG. 9
, which illustrates an interfering jammer
132
. As can be seen, the effect on a narrowband signal
130
occupying the same or overlapping spectrum as the jammer is quite severe, whereas the effect on the wideband signal
131
is relatively minor.
The processing gain is a measure of the ability of a DSSS system to withstand interference from a jammer. Mathematically, it is given by W/R
B
, where W is the bandwidth of the spread signal, and R
B
is the bit rate of the incoming information signal. In the case in which the chip rate, R
c
, of the spreading sequence is much greater than the bit rate R
B
, the processing gain is approximately equal to R
c
/R
B
.
The coding gain of a system employing a particular form of error detection or correction coding is the amount (in dB) of reduction of E
b
/N
o
, the energy per bit divided by the noise density, that can be achieved at a given bit error rate (BER) by virtue of the coding.
FIG. 15
illustrates the shift in the plot of E
b
/N
o
which results from implementing a certain error detection or correction code. The amount &Dgr; (CG) of shift at a given BER is the coding gain.
In current DSSS systems, a problem is that there is a tradeoff between processing gain and coding gain such that some processing gain has to be sacrificed in order to achieve an improvement in coding gain, and vice-versa. The reason is that the achievement of coding gain requires the addition of redundancy into the system on the incoming information sequence, and the addition of this redundancy necessitates an increases in the incoming information bit rate, R
B
, in order to keep the throughput of the system the same. This increase in the information bit rate results in a decrease in the processing gain.
Another problem is that, due to limitations imposed by the frame structure, the class of FEC coding schemes which can be employed in such systems is limited.
Another problem with current DSSS systems is that there is a lack of robustness in responding to changing physical channel conditions. The re
Mintz Levin Cohn Ferris Glovsky & Popeo
Skyworks Solutions Inc.
Vo Don N.
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