IMPROVEMENTS IN A SPREAD-SPECTRUM MULTIPLEXED TRANSMISSION SYSTEM BACKGROUND OF THE INVENTION The present invention relates generally to• improvements in spread-spectrum transmission systems and in a preferred embodiment the invention is applied to a vehicle location and tracking system.
A number of information bearing channels can share the same medium and approximately the same frequency band and yet be separated at the receiving end with satisfactory interchannel isolation if suitable pseudo-noise (PN) codes are used asynchronously to direct-sequence modulate the channel carriers at a high rate relative to the data rate. This has the effect of spreading the spectrum of the transmitted energy.
At the receiver, the information in each channel is extracted by cross-correlating the incoming composite stream with the code associated with the desired channel. When the clock rates and the epochs of the in-coming and locally-generated codes match, the spread-spectrum energy is collapsed to the relatively narrow, data bandwidth for that channel whilst all the other channel spectra remain spread.
This method enables a particular medium (eg a coaxial-cable transmission line) to carry a large number of channels, separation being achieved at the receiving end by code-division multiple access (CDMA) . The performance of the scheme in terms of signal- to-noise ratio depends on the relative orthogonality of the codes; that is, on their cross-correlation properties. A unique feature is the smooth degradation of signal-to-noise ratio as more users come into the system compared to the sudden loss of performance which occurs in a conventional frequency division multiple access (FDMA) system once the channel capacity is exceeded.
The capability of a spread-spectrum channel to reject
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interference from other signals in other channels and from noise is called the process gain. Mathematically, process gain is given as:
Gp= 10 log1() B/b (dB) ... (1)
where B = bandwidth of spread-spectrum signal b = data or information bandwidth
and it is assumed that the spectral line spacing of the PN codes is small enough for the spectra to be considered continuous.
Consider now the case of one transmitter, one receiver and no data. According to equation (1) the process gain is infinite because b— •> 0. The zero-data example might be a ranging system where it is necessary only to locate the code epoch and, knowing the propagation delay, the range to the transmitter may be calculated; range ambiguity can be avoided by making the code repetition period much greater than the propagation delay. In practice the process gain can be very large, but not infinite, and is limited mainly by the extent of the loss of coherence of the carrier at the receiver relative to the receiver local oscillator. If the 'coherence time' of the received carrier is 7- then b^ "~' and process gain can be increased only by spreading the spectrum of the transmitted signal still further. This can be done by increasing the chip-rate (code clock rate) of the PN code up to a limit set by the electronics or by the ability of the transmission medium to support the spread-spectrum bandwidth.
Referring to Fig. 1 it may be seen that in a spread- spectrum location and tracking system, the vehicle 10 or object to be located emits a continual direct sequence spread-spectrum radio signal 11. This transmission is
received at a number of well-spaced receiving stations 12 in the coverage area and the differences in the times of arrival of the signals at these receivers are measured. Inverse hyperbolic navigation techniques then may be used to compute the position of the transmitter at the central computer 13 which then sends this information to an operator terminal.
Direct sequence spread-spectrum modulation is employed for a number of reasons, one of which is to minimise multi-path effects. Also, since for location and tracking purposes there is no data transmission requirement, there would appear to be potential for very high process gain. Unfortunately the process gain is severely limited in practice. Firstly transmissions from a vehicle moving in an urban, or suburban, area experience Rayleigh scattering and Doppler frequency-shift. As a result, at each receiving site 12 the received signal spectrum is bandlimited to within ±Δf of the centre frequency where Δf = f Q v/c is the maximum Doppler frequency-shift for a vehicle with speed v transmitting on a frequency f (c is the speed of radio propagation) . The coherence time of the carrier depends roughly inversely on the width of the frequency-modulation spectrum so that this scattering sets a lower limit to b, the post-correlation bandwidth. Secondly, the radio- frequency spread-spectrum bandwidth cannot be made arbitrarily wide because of limitations on the coherence bandwidth caused by different fading in different parts of the spectrum. A rough estimate of the available process gain using urban mobile transmitters may be obtained from published data. For a centre frequency of about 450 MHz the minimum coherence time is about 5 ms and the coherence bandwidth is around 1 MHz giving an available process gain of approximately 37 dB. This figure gives a measure of the
level of signal enhancement, over broadband spectrally- continuous noise and interference, achievable by receiver processing.
For a spread-spectrum multi-vehicle location and tracking system in which M transmitters are operating simultaneously, each transmitter to be located and tracked has (M-l) interferers. If CDMA is used, the cross-correlation properties of the codes of the wanted and unwanted signals will determine the extent of the interference. In the commonly-used binary Gold code family, the cross-correlation between any pair of codes generated using n-bit shift registers is bounded by
/0(r)/≤2(n + x> 2 + 1 (n odd) jθ(r)j 2(n + 2)/2 - 1 (n even)
Since these sequences are of maximal length, the number of bits in the code is:
N = 2n-l
and for n > 1 the ratio of the auto-correlation peak to the maximum cross-correlation bound is
R ^ 2(n"1)/2 (n odd)
«2,(*n-2)"/2 (n even)
The larger n is made, the better the wanted signal can be distinguished from the unwanted ones. In other words, the longer the sequence length (N) the better. However,
where T
R = code repetition period
and, as we have seen already, for an urban vehicle- tracking system, both T- K, and fc have practical upper limits set by the coherence time and coherence bandwidth respectively so there is a practical upper limit set on the choice of N. For the particular example quoted above we have N ^~- 5000. With this value of N we have n '■ 12 and hence R 32 giving a maximum 'process gain' of about 15 dB. Clearly in this case CDMA falls well short when its performance is compared to the available process gain (over an interference continuum) of 37 dB.
It is important to understand that the spectral components of a spread-spectrum signal are spaced by fR = l/TR = fc/ . For a given chip rate, long PN codes have spectral lines very close together and short PN codes have widely-separated lines. A long code may be modelled to have a continuous power spectrum but with a short code the discrete lines must be considered, particularly as they affect the process gain which varies in discrete steps according to the number of spectral lines falling into the passband of the post-correlation filter.
The usefulness of a vehicle-tracking or locating system is enhanced in proportion to the number of vehicles which can be located or tracked at the same time. A high, realisable, process gain is needed in such a spread-spectrum multi-vehicle tracking system because of the necessity of isolating each received transmission from the others; a requirement which is exacerbated by the 'near-far problem' .
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From the above discussion it will be recognised that, in the frequency domain, the resulting spread-spectrum signals comprise a multiplicity of discrete spectral lines located either side of the carrier frequency and equally spaced from one another and the carrier.
The structure of the spread-spectrum signal in the frequency domain lends itself to frequency division multiplexing in a manner which makes very efficient use of the available frequency spectrum by allowing a number of spread-spectrum signals to be interleaved. This arrangement was described in the present applicant's co-pending PCT Application No. PCT/AU87/00020.
This invention exploits the quasi-discrete nature of the mobile transmitters' spectra and employs an improvement to the novel form of frequency division multiple access (FDMA) disclosed in PCT Application No. PCT/AU87/00020 to effect this isolation.
SUMMARY OF THE INVENTION The present invention consists in a spread-spectrum transmission system wherein one or more spread-spectrum signals share a frequency band with other services, wherein the other services occupying the frequency band are arranged in discrete channels separated by guard bands, and each spread-spectrum signal has an information bandwidth which is significantly less than its spectral line or band spacing, each signal being produced by modulating a carrier with a pseudo-noise code and the spread-spectrum signal having a centre or carrier frequency and modulation frequency which are selected to cause its spectral lines or bands to occupy the guard bands separating the other service channels, the guard bands being greater than the spread spectrum information bandwidth. In a preferred embodiment of the invention a
plurality of spread-spectrum signals occupy the frequency band and the centre or carrier frequencies of the spread-spectrum signals are spaced by an increment selected to cause the spectral lines of the respective transmitted signals to be interleaved with each other and with the channels of the other services in the band.
The invention is applicable to all spread-spectrum transmission systems where the information bandwidth is much less than the spectral line or band spacing of the transmitted spectrum. The utility of spread-spectrum systems in which the information bandwidth is essentially zero, such as systems using spread-spectrum signals for ranging purposes, is particularly enhanced.
According to other aspects of the invention, a receiver for a spread-spectrum multiplexed transmission system and a spread-spectrum vehicle tracking system are also provided.
BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:
Fig. 1 generally illustrates a vehicle tracking system in which the spread-spectrum multiplexed transmission system of the present invention might be used; Fig. 2 graphically illustrates the frequency domain representation of a spread-spectrum signal;
Fig. 3 graphically illustrates the frequency domain representation of a spread-spectrum signal occupying the same frequency band as a conventional radio communications system;
Fig. 4 is a block diagram of a spread-spectrum transmitter for use in a vehicle tracking system using the present invention; Fig. 5 is a block diagram of a remote site receiver
installation for use in a vehicle tracking system using the present invention; and
Fig. 6 is a block diagram of the installation of Fig. 5 showing the receiver arrangement in greater detail. DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention exploits the quasi-discrete nature of the transmitters' spectra and employs an improved form of Frequency Division Multiple Access (FDMA) . It was recognised in PCT Patent Application No. PCT/AU87/00020 that a novel form of FDMA could be employed with spread-spectrum signals to achieve a saving of bandwidth by interleaving spread-spectrum signals. It is now recognised that even greater efficiencies can be achieved by interleaving not only spread-spectrum signals, but also other communications channels which tend to be equally spaced across a band of the spectrum. Examples of this are broadcast radio and cellular telephone systems which generally have a number of transmitters using a number of channels which are substantially equally spaced across the band. By appropriate selection of centre and modulation frequencies of one or more spread-spectrum signals, these can be made to occupy the inter-channel guard bands of the other communications signals thereby providing a new service, in a frequency band which is already in use, without detrimental effect to the existing services in that band.
To understand the principles involved we refer to Fig. 2 which shows details of the spectrum emitted by a transmitter using a maximal PN code of length N to direct- sequence bi-phase modulate a carrier on a frequency f .
The diagram shows that the spectral lines are spaced by the code repetition frequency f_ R = f„C/N and that the spectrum of the transmitted signal is symmetrical about the carrier frequency f . When this signal is emitted from a mobile vehicle in an urban area it
undergoes Rayleigh scattering and Doppler frequency-shift as the radio waves propagate by a multitude of paths to the receiver. Each line in the spectrum of the received signal exhibits random frequency modulation (as.described above) with most of the energy of the line being contained within a bandwidth of twice the maximum Doppler frequency-shift. Specifically, if the speed of the vehicle is v and the speed of radio propagation is c, the energy of a spectral line is contained essentially in a bandwidth 2Δf where Δf = f v/c. As an example, if f is 450 MHz and v = 100 km/hr we have 2Δf ^85 Hz. In order to enhance the signal-to-noise ratio of this signal by processing in the receiver, the final local oscillator can be direct- sequence modulated with the same PN code as used in the transmitter, the local epoch of the code being adjusted until it matches that of the incoming code. When this happens, the energy contained in all the spectral lines of the received signal is concentrated essentially into the bandwidth 2 f centered on the final intermediate frequency. In other words, the spectrum is collapsed or 'despread' and the process gain is achieved. From the foregoing it is clear that the bandwidth of the final IF must be wide enough to accommodate the collapsed spectral energy. Allowing for an uncertainty + Sf in the carrier frequency of the transmitter, the final IF bandwidth should not be less than 2( Δf + <£f).
The radiated spread-spectrum signal from each transmitter occupies a relatively wide bandwidth B (typically of the order of 1MHz). When M transmitters are operating simultaneously, as in a multi-vehicle tracking system, the use of FDMA would suggest a bandwidth requirement of at least M x B for the system as a whole. In urban areas particularly, the radio-frequency spectrum is viewed as a scarce resource much in demand. Consequently, the use of a bandwidth M x B is likely to be
considered extravagent. The present invention offers an acceptable answer to these objections without the degradation in process gain associated with CDMA.
The present invention uses the fact that although the bandwidth of the signal received from each transmitter is very wide, it has a quasi-discrete line spectrum arranged symmetrically about the carrier. If the spacing between the 'lines' is made large compared to the frequency band each one occupies and if the same spacing is used for all transmitters, it is possible to interleave these 'lines' (or bands) with conventional communications channels. Several of these spread-spectrum channels may be located in the inter-channel guard bands of the conventional communications channels by interleaving the spread-spectrum signals with each other. This is achieved by offsetting the centre frequencies of all the transmitters by relatively small amounts in the following way:
A typical, conventional, narrowband communications system will have available a multiplicity of discrete channels each of which occupies bandwidth F , , evenly-spaced within the band with separation of centre
The unoccupied region between channels, or guard band, is
which may result from a combination of centre frequency errors and finite filter responses.
In the preferred embodiment of the present invention a single or multi-channel spread-spectrum system is implemented, where the band of frequencies occupied by the spread-spectrum system is shared with conventional services, but the spectral lines of the spread-spectrum signals lie within the guard bands between channels of the conventional system. Interference with the conventional
radio system channels by the spread-spectrum signals can be maintained within acceptable limits by limiting the power in the spectral lines of the spread-spectrum signals to a level comparable with that allowed from the adjacent narrowband channel. The process gain available by the use of spread-spectrum techniques can enable interference to the spread-spectrum system by the conventional system to be maintained within acceptable limits.
Let the potential number of simultaneous transmissions be M for a system comprising only spread-spectrum signal transmissions. If all of the transmitters have centre-frequency offsets which are multiples of fpP/M where P is a non-zero integer such that P and M have no common factors then the i channel has a centre frequency offset
where i = 1,2, ,M
Consider the case where this system is to occupy the same band of frequencies as a conventional radio system with parameters F , , F and F . defined above. If parameter
and i is selected within the range
i = 1 , 2. , Q where 1 $ Q <M and QfR/M < F . ,
then by appropriate selection of centre frequency, the spectral lines of the spread-spectrum system can be made to fall within the guard bands between adjacent channels of the conventional system. This is depicted in Fig. 3 where item 15 denotes a channel of a conventional communications system and item 16 denotes a spectral line of a spread-spectrum signal. It will be appreciated that in such a system the actual number of channels available
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will be significantly less than M because of the bandwidth requirements of the narrowband communications channels.
With this hybrid spectrum usage in mind Equation 3 above can be developed as follows: Let the selected spacing between spectral 'lines ' of the composite spread-spectrum signal be f_ , the code repetition frequency be fR and the channel spacing of the conventional communication system be- F , with a channel occupied bandwidth F .. Firstly, we select fR to be an integer multiple of
Fcs; let this be R where
R = £R/F CS (integer) ... (4)
Typically R will be small.
Secondly, we choose the value of f_ such that
is an integer and R is a factor of M. M represents the maximum number of spread-spectrum channels, given the code repetition frequency f and practical limitations on f_ , and assuming there are no communications channels with which to share the spectrum. Now, allowing for the spectrum to be used for conventional communications channels, we define the maximum available number of spread-spectrum channels to be interleaved as Q, where
therefore Q M - R»F
ob/f
L ••• (6)
Equation (6) is to be interpreted as meaning that Q is selected to be the largest integer value satisfying the
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equation which also has R as a factor.
Finally, we can write the following equation defining the offsets in the spread-spectrum centre frequencies which will permit Q channels to be used. Let tfce offsets be f. where
fi = - 3-1 - fR (K+1/M) ... (7)
(K integer)
and i = 1,2,...Q/R; (1+M/R), (2+M/R), ... (Q/R+M/R);
.... [1+(R-1)M/R]; [2+(R-l)M/R]; [ (Q/R+(R-1)M/R]
By way of illustration but not limitation, consider a typical UHF land mobile radio service overlaid by a multi-channel spread-spectrum communications system used for low data-rate telemetry and/or automatic vehicle location. The essential parameters of such a system are shown in Table 1.
TABLE 1 The parameters of one preferred embodiment are given below by way of example but not limitation.
Nominal centre frequency of spread-spectrum transmissions: (^Q) 470 MHz
Mobile radio service channel spacing: (F„_S_) 25 kHz
Ratio of code repetition frequency to mobile radio service channel spacing: (R) 1
Selected 'line' spacing of composite spread-spectrum signal: (fL) 200 Hz
Maximum number of spread- spectrum signals: (M) 125
Mobile radio service occupied bandwidth: (Fob^ 16 kHz
Available spread-spectrum channels: (Q) 45
Frequency increments of spread- spectrum carrier frequencies:
Pseudo-noise code length:
Pseudo-noise code clock rate:
The transmitter is shown schematically in Fig. 4. crystal-controlled oscillator and divider 21 provide a clock for the pseudo- noise code generator 22 and a reference to which the voltage-controlled radio-frequenc oscillator 23 is locked via a programmable divider 24 an phase comparator 25. The output from the pseudo-noise generator is applied to the modulator 26- which bi-phase modulates (0 or 7T ) the RF carrier. This modulated wave is amplified in the output amplifier 27 and radiated from the antenna 28.
A block diagram of the receiving electronics of a Q-channel system is shown in Fig. 5. In the preferred embodiment of an automatic vehicle location system the timing at all remote sites is synchronised to a received timing signal radiated from a fixed location. This timin transmission is received preferably by means of a high-gain antenna 31 such as a Yagi connected to the radi frequency (RF) section 32 of the timing receiver. The intermediate frequency (IF) stages of both the timing reference and main receivers are housed in the same unit 33 where the local clock is synchronised by control signals from the channel 0 timing reference receiver 37 which also supplies a reference epoch signal 50 (see Fig 6) for distribution through this common unit to the Q mai receiver channels 34.
Spread-spectrum signals from the transmitters are received by means of the vertical antenna-array 35 and main receiver RF section 36, and are converted to intermediate frequencies retaining their offsets in accordance with equation (7) above. This is effected in the IF stages. Fig. 6 shows in more detail how the Q individual transmissions are acquired and their epochs are tracked. With reference to this diagram we
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note that all received spread-spectrum signals are amplified in the first wideband amplifier 41 at an intermediate frequency F. This amplifier has a bandwidth wide enough to pass all the spread-spectrum signals from the mobile transmitters. These amplified signals are split equally and passed to Q identical first mixers 42 each of which is fed by a different local oscillator 43. The frequencies of the local oscillators
F l' F2'""'F 0 are ° set rroπl each other in accordance with equation (7) just as for the transmitter carrier frequencies. Consequently receiver channel i with local oscillator frequency F. locates the centre frequency of the signal received from transmitter i at F where i = 1,2,...Q. The outputs of the first mixers are amplified in second wideband amplifiers 44 and applied to second mixers 45 where the PN code generated in code generators 47 operates on the local oscillator ports. Although the same PN code is used, the epochs in each channel are independently varied in response to epoch control signals 48 produced by microcontrollers contained in the detector and microcontroller blocks 46. Each epoch is adjusted until it matches that of the incoming signal for that channel. When this occurs, the spectrum of this signal collapses to the relatively narrow band of frequencies (determined by the Doppler frequency-shift and transmitter crystal-oscillator uncertainty as discussed above) all centred on F . This narrowband signal appears at the output of the second mixer 45 and passes through the narrowband filter 49 to the detector and microcontroller block 46 which detects the signal and maintains a match between the incoming and locally- generated code epoch by appropriate advance/retard adjustment of the locally-generated code. There are many ways of achieving code epoch tracking which will be familiar to those skilled in the art and need not be
described here. Finally, the time difference between the epoch of the code in a tracking channel and the timing reference 50 is measured in the detector and microcontroller block 46 and the time measured for each channel is passed to the remote site computer 38 and finally via the modem 39 and land line 14 of Fig. 5 to the central computer 13 shown in Fig. 1.