Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-08-28
2001-12-18
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S149000, C375S150000, C375S152000, C370S324000, C370S329000
Reexamination Certificate
active
06331998
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to despreading a transmitted direct sequence spread spectrum (DSSS) signal and particularly relates to despreading a transmitted spread spectrum (SS) signal that is data modulated.
BACKGROUND OF THE INVENTION
Radio frequency (RF) communication links are known for providing connectivity in point-to-point (e.g., walkie talkies) and point-to-multipoint (e.g., television transmission signals) applications. Typically, these applications have used a high power narrow band width transmission to ensure receipt of a data signal. The bandwidth of a typical RF transmission is allocated based on the complexity of the data signal. These systems are designed to concentrate energy close to a center frequency, which results in an increased transmission range. In this way, noise in a transmission environment, such as signal noise due to other RF transmitters, is overcome by tuning a receiver to the frequency of the desired signal. At this frequency, the power of the data signal is maximized and is more likely to be distinguished from background noise.
A DSSS transmission is a form of an RF transmission which differs from the typical RF transmission discussed above. With a DSSS transmission, instead of concentrating a high power transmission within a narrow bandwidth, a relatively low power transmission is spread out over a bandwidth much wider than is required to send the signal.
A DSSS system includes on the transmitting side, spreading the data signal, modulating the spreaded data, and up converting of the transmitted signal frequency to produce a DSSS signal. On the receiver side, the DSSS signal is down converted, demodulated, and despread to recover the data.
In operation, a data signal is “spread” over a wide bandwidth and then transmitted. In some cases, background transmission noise may have a higher peak power than the spread signal transmission. Yet, since the noise typically has a narrow bandwidth, only a small portion of the DSSS signal transmission may be obscured. Consequently, the SS signal can be reliably reconstructed at the receiver.
At the receiver, a received DSSS signal is collapsed (“despread”) by removing the effect of the spreading. In this way, the signal may be regenerated. Despreading a DSSS signal requires knowledge of the spreading technique utilized at the transmitter since the same spreading technique must be utilized to despread the DSSS signal. It is for this reason that a DSSS signal is difficult to intercept or jam since to effectively do either, requires specific knowledge of the spreading technique utilized.
There are numerous ways to spread a signal. Typically, the spreading technique is selected to be independent of the signal. In DSSS, the data signal is combined with a pseudo noise binary bit stream (“PN code”). The PN code is utilized to modulate the carrier of the data signal. Typically, the PN code is synchronous with the data signal but has a much faster bit rate. By combining the data signal and the PN code, a spread data signal is produced.
The PN code is selected to appear random having approximately an equal number of 0's and 1's or −1's and 1's. The clock period of the PN code is called the chip time of the PN code and each PN code bit is called a chip.
In the following described figures, like reference numbers are utilized to describe functional blocks that perform the same or similar functions.
FIG. 1
shows a direct sequence transmitter wherein a binary adder
110
combines a PN code
125
from a PN code generator
120
with a data signal
100
to produce a spread data signal
115
. The spread data signal
115
is modulated to a carrier frequency through the use of a modulator
130
. The modulated signal is then in-phase, quadrature (I-Q) channel frequency up converted by an up converter
140
to produce a DSSS signal suitable for transmission.
FIG. 2
illustrates the operation of the binary adder
110
shown in FIG.
1
. As shown, the data signal
100
is combined with the PN code
125
to produce the spread data signal
115
. The modulation technique shown is a binary phase shift key (BPSK) technique wherein the data signal is modulated such that a change in phase of 180° corresponds to a 0 or 1 binary state. As shown, combining the data signal with the PN code results in the PN code being either unchanged for a data bit of 1, or inverted for data bit of 0. In a similar modulation technique, the opposite approach may be utilized wherein the PN code is inverted for a data bit of 1, and left unchanged for a data bit of 0. This modulation process is called bit-inversion modulation.
For a transmitter wherein BPSK is the modulation scheme, the signal observed at the receiver through an additive white Gaussian noise (AWGN) channel is:
r
(
t
)={square root over (2
S
+L )}
d
(
t+&xgr;T
0
+&zgr;T
c
)
c
(
t+&zgr;T
c
)*cos(&ohgr;
0
t+&thgr;
)+
n
(
t
) (1)
where:
S is the signal power,
T
c
is the chip time of the pseudo noise (PN) code,
T
0
is the data bit time, which is assumed to be a multiple of T
c
,
c(t) is the PN code waveform,
d(t) is the data signal,
&ohgr;
0
and &thgr; are the carrier frequency and waveform phase,
&zgr;T
c
is the received PN code phase,
&xgr;T
0
is the received data-bit-phase offset (assuming that the data stream timing is synchronous to the received chip time,
n(t) is an AWGN component (e.g., background noise) with one sided power spectral density N
0
.
In equation (1), AWGN is assumed for ease of discussion. The multiplication of c(t) with d(t) is the spreading of the data signal. In the receiver, the received signal is I-Q phase down converted. A coherent receiver (e.g., a receiver synchronized to the SS signal) will remove cos(&ohgr;
0
t+&thgr;) during the demodulation process. Thereafter, the demodulated signal is multiplied by the PN code c(t) to despread the data d(t).
The problem with despreading a received signal is that the PN code receiver must be synchronized with the PN code utilized by the transmitter. In other words, the receiver has to acquire the transmitted PN code. Yet, due to the effect of the data signal d(t) at any given time, the demodulated signal may represent the PN code or the inverted PN code, as shown in FIG.
2
. Correlation between the transmitted PN code and the PN code at the receiver is essential for recovering the data signal. A typical circuit used to recover a data signal is a correlator.
FIG. 3A
shows a correlator
300
in which a demodulated signal
310
is combined with a PN code wave form signal
320
having an estimated PN code phase. The combined signal is received by an integrate and dump function
340
discussed in more detail below.
To exclude the effect of the data signal d(t) when the correlator is acquiring the PN code, many communication systems have a pilot channel (e.g., a channel separate from the spread data signal) with the data signal d(t) all equal to 1 or −1. In this way, the effects of the data signal on the PN code is known and therefore may be negated for purposes of acquiring the PN code.
The receiver PN code clock can be synchronized to the transmitted PN code transitions. This process though does not ensure that the transmitted PN code is acquired since the phase of the receiver PN code may still differ from the phase of the transmitted PN code. Yet, since the PN code has approximately an equal number of −1's and 1's (see discussion above), multiplying a transmitted PN code by a synchronous, but out-of-phase, receiver PN code results in a small output signal. In other words, the signals are destructively combined. When the transmitted PN code and receiver PN code are in-phase, the signals are constructively combined.
Specifically, the PN code has the property:
R
(
τ
)
=
∫
0
MT
c
c
(
t
)
c
(
t
+
τ
)
ⅆ
t
{
≈
0
,
&LeftBracketingBar;
τ
&RightBracketingBar;
>
T
c
=
(
1
-
&LeftBracketingBar;
τ
&RightBracketingBar;
T
c
)
MT
Lin Wen-Chang
Su Yu T.
Chin Stephen
Ha Dac V.
Industrial Technology Research Institute
Proskauer Rose LLP
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