Method of and means for spread-spectrum transmission

An analog signal sent from one telephone subscriber to another is periodically sampled at a transmitting terminal and each sample is held for a storage period equal to a sampling interval as well as to a basic pulse period of a random or pseudorandom sequence of bipolar binary pulses. A stepped wave formed from these samples is mixed with that pulse sequence before being conveyed, e.g. by frequency modulation, to a receiving terminal where the wave so distorted is mixed with a like pulse sequence to re-establish the undistorted wave which may be contaminated by interferences from other users of the same signal link. The steps of the re-established wave are integrated over periods equal to the sampling interval and the integration products are stored for like periods to produce a purged stepped wave from which high-frequency components are subsequently filtered out to restore the original analog signal. The signal link utilized may be an emergency channel made available, e.g. for communication with a mobile station, when regular channels are busy.

FIELD OF THE INVENTION 
My present invention relates to a method of and means for sending signals 
from a plurality of transmitting stations to respective receiving stations 
over a common channel, e.g. a radio link, forming part of a so-called 
spread-spectrum telecommunication system. 
BACKGROUND OF THE INVENTION 
Spread-spectrum modulation has long been used in the transmission of 
digital signals from different sources over a common channel, its purpose 
being to insure the privacy of the transmitted messages and to minimize 
mutual interference among simultaneously transmitted signals. At a 
transmitting station, outgoing digital signals (which could be analog 
signals converted into binary form) are multiplied with a random or 
pseudorandom sequence of binary pulses whose period is a submultiple of 
the symbol duration; the sequences generated at the transmitting stations 
are essentially uncorrelated among one another. Such sequences may 
conform, for example, to the well-known Walsh function. At an associated 
receiving station, the incoming digital signal distorted by the 
aforementioned pulse sequence--and possibly encumbered by interfering 
signals from other users of the same channel--is multiplied with a pulse 
sequence which is a precise replica of the one used at the transmitting 
end and is properly synchronized therewith. This procedure re-establishes 
the original binary signal which is then integrated over limited 
intervals, equaling the duration of the symbols, to eliminate accompanying 
interferences. The digital signal thus purged can be reconverted, if 
desired, into analog form. 
Since digital signals require a considerable bandwith for their 
transmission, spread-spectrum modulation has heretofore been generally 
limited to military uses. Moreover, the analog/digital conversion at the 
transmitting end and the digital/analog reconversion at the receiving end 
are cumbersome procedures which limit the utility of such systems for 
civilian purposes, e.g. for intercommunication among telephone 
subscribers. 
OBJECTS OF THE INVENTION 
The general object of my present invention is to provide a method of and 
means for enabling the transmission of analog signals by the 
spread-spectrum technique without the need for intervening conversions 
into and from digital form. 
A more particular object is to adapt the spread-spectrum technique for use 
with mobile stations carried, for example, aboard automotive vehicles. 
SUMMARY OF THE INVENTION 
I have found that the direct transmission of analog signals by the 
aforedescribed technique is possible if the outgoing signal is 
preprocessed before being multiplied with the binary pulse sequence 
individual to the station considered and if the incoming signal at the 
associated receiving station is subjected to an after treatment 
reconstituting the original analog signal by eliminating high-frequency 
components due to the preceding short-term integration, all with 
maintenance of the essentially analog character of the transmitted signal. 
In accordance with my present invention, the preprocessing involves a 
sampling of the outgoing analog signal at intervals not exceeding the 
reciprocal of twice the bandwidth of that signal, with storage of the 
resulting samples for periods equaling the sampling intervals. This 
produces a stepped wave which is multiplied at the transmitting and 
receiving ends with identical and synchronized random or pseudorandom 
binary pulse sequences of bipolar character so that the first 
multiplication inverts the polarity of certain amplitude steps while the 
second multiplication restores that polarity; the two identical binary 
sequences have a basic pulse period which advantageously equals the length 
of the sampling intervals and storage periods referred to above but could 
also be a whole multiple thereof. At the associated receiving station, the 
integration (following the second multiplication) is performed over 
intervals also equaling the sampling intervals to provide discrete signals 
with amplitudes proportional to those of the steps of the re-established 
wave which, upon storage for periods again equal to these sampling 
intervals, yield a delayed counterpart of that wave substantially purged 
of interfering signals. The delayed wave has a low-frequency component 
conforming to the outgoing analog signal which can readily be isolated by 
low-pass filtering. 
Thus, a system for the transmission of analog signals in accordance with my 
present invention comprises first and second arithmetic means at a 
transmitting station and at an associated receiving station connected to 
respective generators of synchronized binary pulse sequences for 
performing the two multiplication steps set forth above, together with 
circuitry including integrating means connected to the second arithmetic 
means for substantially reconstituting the outgoing signal. Pursuant to my 
present improvement, I further provide sample-and-hold means upstream of 
the first arithmetic means for periodically taking samples of an outgoing 
analog signal and holding the samples for uniform storage periods equaling 
the sampling intervals, thereby giving rise to a stepped wave undergoing 
distortion in the first arithmetic means; this distorted wave is 
reconverted by the second arithmetic means into the original stepped wave 
which may be encumbered by interfering signals to be suppressed upon 
subsequent integration. The aforementioned circuitry also includes holding 
means connected to the integrating means for receiving a series of 
discrete signals with amplitudes proportional to those of respective steps 
of the reconstituted original stepped wave, as discussed above, followed 
by filter means for extracting the low-frequency component of that wave 
conforming to the original analog signal.

SPECIFIC DESCRIPTION 
In FIG. 1 I have shown a transmitting station ST.sub.i communicating with a 
receiving station SR.sub.i via a common signal channel CA which could be a 
cable or a radio link. Station ST.sub.i is representative of a group of 
transmitting stations connected to the input end of channel CA via 
respective lines L.sub.l -L.sub.m ; similar lines l'.sub.l -L'.sub.m 
extend from the output end of that channel to a group of receiving 
stations of which station SR.sub.i is representative. 
Transmitting station ST.sub.i serves a subscriber's telephone set UT 
symbolized by a microphone. A subscriber set served by receiving station 
SR.sub.i is symbolized by a speaker UR. The equipment shown in FIG. 1 and 
described hereinafter is designed only for one-way transmission from 
subscriber set UT to subscriber set UR; it will be understood, however, 
that similar equipment is to be provided for transmission in the reverse 
direction. 
The equipment of station ST.sub.i consists essentially of a spread-spectrum 
modulator MSD working into a signal transmitter TR which is connected by a 
line L.sub.i to channel CA and which may operate, for example, by 
frequency modulation of a carrier. A signal receiver RR of station 
SR.sub.i, complementing transmitter TR, recovers the baseband signal from 
the arriving carrier and feeds it to a spread-spectrum demodulator DSD. 
Modulator MSD comprises a sample-and-hold circuit CT connected to one 
input of a first multiplier M1 having another input connected to a pulse 
generator G1 which is synchronized with circuit CT and emits a random or 
pseudorandom binary pulse sequence alternating between a positive and a 
negative voltage of the same absolute magnitude. Such a pulse generator 
may include, for example, a shift register with a multimode feedback 
varying in a programmed manner. The readout rate could also be subject to 
a programmed variation as long as the basic pulse period is not greater 
than twice the reciprocal of the bandwidth of the outgoing signal, i.e. 
does not extend over more than a half-cycle of the highest voice frequency 
to be transmitted. Receiver RR is fed by a line L'.sub.i. 
Demodulator DSD comprises a second multiplier M2 having one input connected 
to receiver RR and another input connected to the output of a pulse 
generator G2 which is identical with pulse generator G1 and is 
synchronized therewith in a manner symbolized by a connection SY; such 
synchronization could be achieved, for instance, by a pilot frequency 
outside the voice band. Multiplier M2 works into an integration circuit IN 
synchronized with generators G1 and G2 so that its output voltage returns 
to zero at the end of each pulse period. Thus, circuit IN is of the 
integration-and-dump type as shown, for example, in commonly owned U.S. 
Pat. No. 4,201,909. This integrator feeds a holding circuit TE, also 
synchronized with generators G1 and G2, from which a connection extends 
via a low-pass filter FPB to speaker UR. 
The holding operation of circuit CT discriminates against the 
higher-frequency components of the outgoing voice signal by introducing an 
attenuation g(f) proportional to (.pi.fT/sin.pi.fT).sup.2 where f is any 
constituent frequency of that signal and T is the holding period, or 
duration of a sampling interval, of circuit CT; this holding period is the 
same as that of circuit TE and also corresponds to the basic pulse period 
of generators G1 and G2. If that pulse period is variable as noted above, 
the two holding periods would have to be automatically readjusted through 
a suitable coupling. 
The relative attenuation is particularly marked for signal components whose 
half-cycles have a length 1/2f not much greater than period T. I therefore 
prefer, especially in situations where period T exceeds one-third of the 
reciprocal of the bandwidth of the voice signal, to provide means for 
compensating this frequency-dependent attenuation either at the 
transmitting or at the receiving end. Thus, FIG. 2 shows a modified 
spread-spectrum modulator MSD' in which the sample-and-hold circuit CT is 
preceded by a pre-emphasis circuit PE having a gain proportional to 
attenuation g(f). Alternatively, as illustrated in FIG. 3, a modified 
spread-spectrum demodulator DSD' can be provided downstream of filter FPB 
(or possibly ahead of same) with a shaping filter FSF whose frequency 
characteristic is complementary to that of circuit CT. 
In FIG. 4 I have shown a subscriber set UT' which may be carried aboard a 
vehicle together with a transmitting station ST'.sub.i whose signal 
transmitter TR' has an antenna TA for sending out radio-frequency carriers 
frequency-modulated by the outgoing signals. A signal receiver RR' of an 
associated receiving station SR'.sub.i, serving a subscriber set UR', has 
an antenna RA for intercepting a transmitted carrier to be 
frequency-demodulated if that carrier is intended for station SR'.sub.i, 
as determined by conventional coding. (Here, again, equipment needed for 
signal transmission in the opposite direction has not been illustrated.) 
The carriers which can be sent out via antenna TA and received by antenna 
RA constitute a plurality of signal channels including several normal 
channels (carrying only one message each) and one spread-spectrum channel 
adapted to be used simultaneously by several subscribers, some of which 
could also be served by simpler stations such as those shown in FIG. 1. 
Station ST'.sub.i differs from station ST.sub.i by the provision of a 
channel tester CLT controlling a switching circuit OST which alternatively 
connects subscriber set UT' to transmitter TR' via a direct path P or via 
spread-spectrum modulator MSD. Similarly, a switching circuit OSR 
controlled by a channel tester CLR in station SR'.sub.i alternatively 
connects receiver RR' to subscriber set UR' via a direct path P' or via 
spread-spectrum demodulator DSD. Thus, tester CLT determines whether 
transmitter TR' has access to an available normal channel, in which case 
the path P is used to modulate the voice signals from subscriber set UT' 
directly upon an outgoing carrier; if all normal channels are busy and the 
subscriber wishes to communicate with station SR'.sub.i, e.g. because of 
an emergency situation, circuit OST in station ST'.sub.i switches the 
microphone UT' to modulator MSD whose output signal is then modulated by 
transmitter TR' upon a carrier reserved for spread-spectrum communication. 
Similarly, tester CLR in station SR'.sub.i identifies the channel on which 
incoming signals are received and causes these signals to be switched by 
circuit OSR to path P' for direct delivery to speaker UR' if a normal 
channel is being used; otherwise, the incoming baseband signal passes 
through demodulator DSD for restoration of the original analog signal. 
Modulator MSD or demodulator DSD could be replaced by circuitry MSD' (FIG. 
2) or DSD' (FIG. 3), respectively. 
I shall now describe, with reference to FIG. 5, the operation of modulator 
MSD and demodulator DSD in the system of FIG. 1 or FIG. 4. 
Graph (a) of FIG. 5 shows an outgoing analog message signal s.sub.i (t), 
simply illustrated as a sine wave, emitted by the microphone UT (or UT') 
and sampled in circuit CT at intervals T. The resulting samples, 
designated s.sub.i (nT) hereinafter, are stored for respective periods T 
so as to give rise to a stepped wave w(t) as shown in graph (b). The pulse 
sequence p.sub.i (t) emitted by generator G1, shown in graph (c), has a 
basic pulse period T whose length is a fraction of a half-cycle of sine 
wave s.sub.i (t). The actual pulses of the sequence p.sub.i (t) extend 
each over one or more periods T and are either positive or negative, with 
a magnitude assigned the value .+-.1. When wave w(t) and sequence p.sub.i 
(t) are multiplied in device M1, with their pulse and storage periods T 
coinciding, a distorted wave x(t) as shown in graph (d) results. Depending 
on the nature of channel CA, the latter wave can be sent directly or by 
way of a carrier to demodulator DSD; such a carrier, of course, will 
always be required when the channel is a radio link as shown in FIG. 4. A 
signal y(t) appearing in the output of receiver RR (or RR') may differ 
from wave x(t) by accompanying interfering signals which have not been 
illustrated. These interferences are substantially suppressed by 
integrator IN whose output signal s.sub.r (t), shown in graph (e), is a 
succession of sawtooth pulses of width T and of a slope proportional to 
the amplitude of the purged output signal of multiplier M2 in the 
corresponding pulse period. The peak of each sawtooth, reached at the end 
of a pulse period generically designated nT, constitutes a discrete signal 
s.sub.r (nT) which is also proportional to the amplitude of that output 
original and is preserved in holding circuit TE for a further period T 
whereby the stepped wave w(t) of graph (b) is reconstituted, with a phase 
lag equal to T, as shown in graph (f). A low-frequency component of the 
output signal of circuit TE conforms, upon being extracted by filter FPB, 
to the original message signal s.sub.i (t). 
Signal y(t) fed at an instant .+-. to multiplier M2, with nT&lt;t&lt;(n+1)T, is 
given by the following equation: 
##EQU1## 
where a.sub.j represents the amplitude and s.sub.j (nT) represents the 
shape of a signal, coinciding with a step s.sub.i (nT) of wave w(t), 
concurrently sent over the common channel CA from any other transmitting 
station to a respective receiving station; symbol p.sub.j (t) designates 
the associated pulse sequence. Another term of equation (1), omitted for 
the sake of simplicity, represents the contribution of thermal noise. 
An integrated sample s.sub.r (nT) of signal s.sub.r (t) in the output of 
circuit IN, appearing during a pulse period nT, is given by: 
##EQU2## 
The factor p.sub.i.sup.2 in the first integral will always have unity value 
while the integrals of the second term will average out to substantially 
zero. Thus, equation (2) can be rewritten in the form: 
##EQU3## 
Apart from the time factor T and other amplitude changes to which the 
incoming signal is subjected during processing, each step of the delayed 
wave shown in graph (f) of FIG. 5 will therefore rather faithfully conform 
to a respective message sample s.sub.i (nT). 
Structurally, the components lying between sample-and-hold circuit CT and 
holding circuit TE in the system of FIG. 1 correspond to those of 
conventional spread-spectrum systems. 
The structure of FIG. 4 is identical with that disclosed in my copending 
application Ser. No. 215,794 of even date relating to a somewhat different 
method of and device for the transmission of analog signals by the 
spread-spectrum technique.