Spread-spectrum communication techniques

by T.S.D. Tsui and T.G. Clarkson

Spread-spectrum modulation can be used in a radio system to reduce the likelihood of intercept, as well as providing some protection against jamming and interference. These anti- jam, anti-interference and low- probability-of-intercept properties are highly desirable in secured communication systems. In addition, spread-spectrum techniques have been proposed to combat spectral congestion by improving the efficiency of spectrum utilisation. Spread-spectrum techniques are also widely used in ranging systems and in local-area networks (LANs) or other multiple-access systems. In this article, the three main spread-spectrum techniques are outlined and then applications of frequency-hopping systems and some of the techniques used are described. 1 Introduction

In a spread-spectrum radio system, the information to be transmitted is spread over a wide bandwidth such that it cannot be decoded by a conventional radio receiver. Only those spread-spectrum receivers using the same code sequence as the transmitter can despread the wide- bandwidth signal back into the baseband, where the desired information can be retrieved. Although a spread- spectrum transmission is more difficult to detect and to receive, privacy is not guaranteed by this modulation process unless the codes used for spreading are crytographically secure. Even more security can be provided if further encryption coding is applied to the message.

Spread-spectrum communications may be defined’ ‘-‘as systems that possess these two characteristics:

the transmitted signal is spread over a frequency band much wider than the minimum bandwidth required for the information to be sent, and

0 the spreading of the signal is achieved by encoding it with a pseudo-random code sequence, otherwise known as pseudo noise (PN), which is independent of the information itself.

In spread-spectrum systems, the notions of ‘processing gain’ and ‘jamming margin’ are of importance as these are performance measures of the system. Processing gain can be described in any spread-spectrum system as the improvement gained by the system process, i.e. by bandwidth spreading, over conventional modulation techniques. Therefore at the receiver, the processing gain can be expressed as the ratio between the signal-to-noise ratio (SNR) at the receiver output. where the signal has been despread, to the SNR at the receiver input where spread-spectrum signal appears. Thus.

(1) SNR at receiver output SNR at receiver input

processing gain = ~

In practical systems, a more convenient measure of processing gain is the ratio of the spread RF bandwidth to the information rate of the system,’ i.e.

However, this does not imply that the system can operate in the presence of interfering or jamming signals whose energy levels are in the order of the intended transmission energy multiplied by the processing gain.

The figure of merit, or jamming margin, J is used to describe the capability of a spread-spectrum system to operate in such an unfriendly environment:

jamming margin = J = C,-SNR,,,,,, (3

where SNR,,, is the minimum signal-to-noise ratio acceptable at the receiver input.

For example, consider a system with a processing gain of 10 dB and a receiver demodulator requiring a minimum input SNRof 10 dB so that an acceptable data error rate can be obtained. The maximum interference that can be tolerated is therefore 10 dB over the message signal. Any noise or interference levels higher than 10 dB will disrupt the communication link.

2 Time hopping

In the time-hopping (TH) spread-spectrum technique. signals are transmitted in short pulses whose intervals are determined by a pseudo-random code sequence. This technique seeks to accomplish uncertainty in the communication channel for any unintended receivcrs by varying the time intervals between transmissions.

ELECTRONICS Pr COMMUNICATIOY EUGINEERING JOUKNM FERRITARY 1994 3

s(r) = spreading pulses I t

1 -

0 I , n I I I I

I Fig. 1 Time-hopping signals in the time domain

Fig. 1 shows a signal with a bit rate of 1/T. In each period of T seconds, there are T / r subdivisions. During each bit duration, a subdivision (a ‘chip’) is selected at random by the spreading code, which is independent of the information, so that the intervals between successive pulses are vaned accordingly. The product signalx(t) is the desired spread-spectrum representation which is modulated for RF transmission. The transmission bandwidth is thus T / r times larger than the baseband signal bandwidth since the PN code has a pulse rate of 1 / ~ .

TH in its simplest form employs a carrier with a constant frequency. The transmission link is therefore easily disrupted if a jamming signal is transmitted at that particular carrier frequency. In this respect, TH used as a stand-alone system does not really offer the anti-jamming property usually associated with spread-spectrum techniques. This is because of the dilficulties in transmitting a signal with an extremely low duty cycle when the average power must be comparable with that of a jammer; the peak transmitted power must be P / s where P is the average power of the transmitter. If very narrow pulses are to be transmitted, then not only is the peak power very high, but adjacent channel interference will be high and ringing of tuned circuits will cause severe intersymbol interference.

3 Direct-sequence spread-spectrum

Direct-sequence spread-spectrum (DS/SS) is achieved by

modulating the message signal by a pseudo-random spreading code whose ‘chip’ rate is much higher than the bit rate of the intended message.

The duration of a chip is therefore equal to the period of the clock which generates the pseudo- random spreading code (Fig. 2). The pattern of the chips is determined by an information- independent pseudo-random spreading code. The chip rate is a function of the length of this pseudo- random sequence. For a code ol 16 chips per bit, the chip duration becomes T/l6 seconds and the bandwidth for transmission is thus 16 times larger than the original data bit rate.

At the receiver, signal despreading is attained by multiplication with. an identical pseudo-random code, reversing the process of encoding at the transmitter, thereby reproducing the baseband information. Undesired signals which are not synchronous with the reference code at the receiver will also be multiplied with the local code

replica and therefore spread to a bandwidth equal to that of the unwanted signal plus the bandwidth of the I” code. An IF bandpass filter with a passband equal to the message bandwidth will reject asynchronous signals in the band. Some techniques for achieving correlation of the receiver with the transmitted signal are discussed in Reference 6.

The signal-to-noise ratio improvement gained by spreading the message for transmission is the advantage of using DS/SS over conventional modulation techniques.

Processinggain in DS/SS The power spectral density function of a RPSK (binary

phase-shift keyed) direct sequence signal modulated by a carrier of frequencyf, has an envelope the form [ (sinr)/x12 with nulls at integer multiples off, wheref, is the frequency of the pseudo-random code. In general, the main lobe, i.e. null-to-null bandwidth, is considered the RF bandwidth, SW,,, of the DS system and is equal to twice the pseudo- random code rate, i.e. SW, = 2 Xf,. An IF bandpass filter with a passband bandwidth of 2 xf is used to reject the unwanted signals.

Using Eqn. 2, the processing gain is therefore

BW 2xR, Go = XI.= - ‘b

where R, is the PN code rate and Rh is the data bit rate.

4 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1994

- - I- _ _ ~

A multi-user link can be established using LWSS, provided that the users sharing the same bandwidth use pseudo-random codes that have low crosscorrelation properties to minimise inter-user interference. Unknown users will appear as noise in the link and only if the exact replica of the PN code is available will their receivers be able to demodulate the incoming signal. The process by which DS/SS transmissions can co-exist in the same bandwidth is called code division multiple access (CDMA).

4 Frequency-hopping spread spectrum

In the frequency-hopping spread-spectrum (FH/SS) technique, the message signal is modulated onto a carrier with a hopping frequency. The hopping pattern is

determined by an independent pseudo-random code. At any instant, a carrier frequency is selected from a range of predetermined frequencies known only to the system users. The communication link is therefore difficult to disrupt if a jammer does not have any knowledge of the frequencies used or their hopping pattern. The usual method of baseband modulation is binary or M-ary frequency shift keying (BSFK or MFSK).

In an FH system, the range of frequencies used can be large and is dependent on the pseudo-random code used. For a 13-stage m-sequence code, the number of possible discrete frequencies is 2'" -1 = 8191. The total bandwidth required is the product of the number of frequency channels and the frequency deviation of the modulating process. For a 8191 frequency channel FH system employing BFSK, with a frequency deviation of 25 kHz, the

V

0 f

-v - ,

PN sequence = p(f)

t

1- - - - -- - ......... I

I I

I , I

- 1

-1 - - -

' 1 1 0 0 1 1 1 0 0 1 0 1 0 0 0 0 1 0 0 0 0 1 1 0 0 1 0 0 1 1 1 0 1 1

Fig. 2 information bit divided into chips in DS/SS

ELECTRONICS & COMMUNICATION ESGI N ICE KIN G .I OUKNAI. I:EBRYART 1 99-I >

I 1 1 1 1 1 1 1 1 1 1 1

frequency

Fig. 3 4 kHz and six channels are shown at 4 kHz spacing. The centre frequency is 10 MHz and the horizontal scale 8 kHz per division. The vertical scale is 10 dB per division

Spectrum of an FH transmitter. The hop rate is

I l l l l l l l l l J

frequency

Fig. 4 4 kHz and a six channel group at 8 kHz spacing is shown with the third channel suppressed. The centre frequency is 10 MHz and the horizontal scale 8 kHz per division. The vertical scale is 10 dB per division

minimum bandwidth required would be over 200 MHz for non-overlapping frequency channels. Fig. 3 shows the spectrum of a frequency-hopping transmitter using six channels at 4 kHz channel spacing and a hop rate of 4 kHz. The horizontal scale is 8 kHz per division and each vertical division represents 10 dB. There is some frequency modulation on the transmitted signal as can be seen from the spectral lines spaced at approximately 0.8 kHz. Such a system, where the hop rate and channel spacing are similar, may be expected to suffer from greater adjacent channel interference than would an FH transmitter using wider channel spacing. Fig. 4 shows the spectrum of a transmitter using the same hop rate but with 8 kHz channel spacing.

At the receiver, an identical PN sequence to the one at the transmitter is used by the local frequency synthesiser to reproduce the carrier frequencies for down-conversion. Additional offsets are used to translate the synchronous

Spectrum of an FH transmitter. The hop rate is

signals to an IF band so that asynchronous signals can be rejected using an IF filter. These frequencies are matched to the hopping pattern so as to down-convert the incoming message to the appropriate IF band. As with DS/SS, any unwanted signal not correlated with the frequency- hopping pattern will be despread to a much wider bandwidth. The use of an IF bandpass filter rejects these undesired signals before baseband demodukation takes place.

Processinggain in FH/SS For conventional modulation techniques using one fxed

frequency channel, an interferer need only transmit at that particular frequency to disrupt the communication link completely. In FH/SS, if there are N frequency channels, the jamming of a single channel will only cause a degradation of data error rate by a factor 1/N. Thus from eqn. 2, the processing gain in a FH/SS system is equal to N, i.e.

transmission bandwidth - N x baseband bandwidth - - baseband bandwidth baseband bandwidth

Jammingsignals in FH Consider an FH system with N = 100 frequency

channels. For an interferer to jam the transmission link completely, its bandwidth will have to cover a significant proportion of the frequency channels in use. By doing so, the interferer’s effective power at any single frequency will be decreased proportionately. In other words, a 20 dR increase in power is required by the interferer if it is to maintain the same level of interference at each of the 100 frequency channels. On the other hand, a narrowband jammer in one channel can cause a relatively high error rate of lo-’ for simple BFSK systems. The chip error rate (CER) can be described as:

(4)

where C,is the number of jammed channels, i.e. where the interferer’s power level is higher than the message signal power.

The power spectral density function of a narrowband jammer is shown in Fig. 5. The simplest way to combat this kind of continuous wave (CW) jamming is by using a narrowband filter tuned to the jamming signal band, thereby providing a reasonable amount of noise rejection. Another countermeasure is to remove the jammed frequency channels from the frequency set.

In the light of the above interference scenarios, some other processes for transmission must be used to imprcve the data error rate. One solution is to increase N, the number of discrete frequencies used, so that the degradation due to a narrowband interferer will be reduced but a wider bandwidth for the communication link will be required if the channel spacing or frequency deviation remains the same. In practice, noncontiguous, overlapping frequency spacing can be used to increase the number of channels within a limited bandwidth.

6 EIECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1994

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Another way to improve the error rate is to increase the number of hops per bit. For example, a bit of information can be conveyed over three hop frequencies so that a decision on the bit is only taken on the majority being correct, i.e. 2 out of 3 bits are correct. Using this method, the chip rate will have to be increased if the same information rate is to be maintained. The limitation here is the switching speed of the frequency synthesiser.

Another jamming scenario is that of an intelligent jammer following an FH transmission by first detecting which frequency is being used and then transmitting interference at that frequency. The follower frequency detector needs to cover a relatively wide band so as not to miss out on the spread signal. Having found which frequency is in use, the interferer amplifies the received signal, mixes it with noise and retransmits it to the intended receiver. This is an effective jamming strategy in slow FH/SS systems. To combat this kind of jamming, a fast FH/SS system can be employed so that the frequency carrier will have hopped to another channel before the jammer follower manages to detect and retransmit the signal.

For example, consider a FH system operating at 10 000 hops per second the distance covered by the transmission in each hop period is therefore approximately 30 km. If the follower jammer is situated 30 km farther than the intended receiver from the transmitter then, even if the jammer is able to detect and respond to the transmission instantly, the carrier used will have hopped to another frequency before the interference arrives. From this viewpoint, the faster the FH/SS system can operate, the more difficult it is for the transmission to be disrupted.

5 Hybrid systems

Hybrid systems are used to overcome the shortcomings of a single SS technique in certain applications.

In frequency hopping and direct-sequence hybrid systems, the direct-sequence signals are modulated onto a

I 1 I I I 1 I I I l l frequency

Fig. 6 hop rate is 4 kHz with channels spaced 100 kHz apart and the DS chip rate is 200 kbivs. The centre frequency is 10 MHz and the horizontal scale 100 kHz per division. The vertical scale is 10 dB per division

Power spectrum of a hybrid FH/DS system. The

Fig. 5

hopping frequency carrier. As bandwidth spreading in a DS system is limited by the rate of the PN code, further spectrum spreading can be accomplished by use of FH so that when the DS signals are modulated onto the hopping carrier, the signal energy would appear across a very wide band. The spectrum of a DS transmission appears to be a wideband pseudo-noise signal. The frequency spectrum of a hybrid FH/DS system is shown in Figs. 6 and 7, where the horizontal divisions represent 100 kHz per division. In Fig. 6, a hop rate of 4 kHz is chosen over three channels with a channel spacing of 100 kHz; the DS chip rate is 200 kbit/s. For comparison, Fig. 7 shows the same transmitter, but with 200 kHz channel spacing for the FH carrier.

One application of a hybrid system is to vastly increase the spread bandwidth. Suppose a SS system intends to spread the bandwidth to 500 MHz. Employing a single SS technique, it would re,quire either a 250 Mbit/s DS system or a FH system with 50 000 frequency channels at 10 kHz spacing, for example. By incorporating both DS and FH spread-spectrum techniques, the same bandwidth spreading can be achieved by using a 50 Mbit/s code sequence and ten frequency channels at 50 MHz spacing. It is clear from these figures that the hybrid designs do not place the same demands on circuit speed as does a single SS system of similar performance.

Time and frequency hopping hybrids tend to be used

Narrowband jamming in an FH system

1 i ' i i i i i i i i 1 frequency

I Fig 7 Power spectrum of a hybrid FH/DS system. The hop rate is 4 kHz with channels spaced 200 kHz apart and the DS chip rate is 200 kbiVs. The centre frequency is 10 MHz and the horizontal scale 100 kHz per division. The vertical scale is 10 dB per division

ELECTRONICS & COMMUNICATION ENGINEEKING JOL'KUAL FEKRI'ARY 1994 I

Fig. 8 Maximal length sequence generators

I most widely in situations where a large number of users with widely variable distances or transmitter power are to operate simultaneously using a single link. Simple coding is usually used in these circumstances for its addressing capability rather than for spectrum spreading. Although plain TH is easily disrupted, when used in conjunction with other spread-spectrum modulation techniques in a hybrid system the effectiveness of the overall system is significantly increased.

When using direct-sequence modulation in a code division multiple access (CDMA) environment as described in Section 3, there may not be sufficient access for each user due to the crosscorrelation properties of the codes used. Time hopping may then be used to create extra channel slots by time division multiplexing.

6 Pseudo-noise spreading code and synchronisation

APN sequence is used to encode data in a spread-spectrum system. An offset in the PN code between a transmitter and a receiver will dislocate the system completely until synchronisation is acquired. The receiver will not be able to despread the incoming signals into the correct bandwidth, therefore they will be rejected by the IF filters as noise. Therefore all spread-spectrum systems require an initial synchronisation period to align the code sequences. The pseudo-random code sequence used should therefore have low autocorrelation to minimise

false synchronisation and, if required, a long repeat period to make it difficult for non-authorised users to acquire synchronisation.

The term pseudo-random is used to describe codes that appear to be random but can be reproduced by deterministic means. Pseudo-random sequences can be broadly divided into two main categories: secure and nonsecure.

Maximal length sequences, more often called m- sequences, are nonsecure codes and these have been shown to be decipherable when a number of contiguous bits (at least twice the number of stages in the shift register) in a sequence are known. Although m-sequences are nonsecure, they are often utilised for their linearity and bandwidth spreading properties in spread-spectrum communication and ranging systems. Transmission security can be provided only if the message is also encoded with a cryptographically strong code.

Fig. 8 shows a simple configuration of a binary m- sequence generator implemented using shift registers and modulo '2' adders. The outputs from selected delay elements are added before being fed back to the first stage. A '0' or a '1' at the multiplier determines which outputs are added. Since the generator relies on modulo '2'additions, the sequence will stay at zero if all the outputs are zeros, so this is a condition that should not be allowed to happen. An 'OR gate at the first stage input can be used to inject a one to the sequence if required. The outputs taken from each delay element therefore form an n-tuple

2"- 1 peak value = 2"- 1 Y L

I j I 1 bit 0 + 1 bit

' A

sequence offset

Fig. 9 Autocorrelation function of an ideal rn-sequence

number which can be used, for cxample, in channel selection of a frequency synthesiser. For an n-stage sequence generator, the number of n-tuple combinations is therefore 2"-1, excluding the all-zeros output.

In Fig. 8, the output of the sequence generator forms a series of '0's and '1's. For an m-sequence, the longest code produced at this output is 2"-1 bits, thus referred to as a maximal length sequence. This code is also periodicwith aperiod of (2"-1)xT, where Tis the clock period of the shift register.

The autocorrelation function describes the similarity between the sequence being tested

8 ELECTRONICS & COMMIJNICATION ENGINEERING JOURNAL FEBRUARY 1994

I - ~

and its phase-shifted version. It can be expressed as the number of bits that agree minus the number of bits that disagree between the two sequences being compared. Fig. 9 shows the autocorrelation function for an n-stage ni- sequence. The triangular form represents linrarity in the function up to +1 bit shifts. The peak magnitude at zero shift is 2”-1 because all 2“-1 bits of the two m-sequences agree. Because of the periodicity of the m-sequence, the autocorrelation will peak again at 2”-1 shifts when the sequence repeats itself.

Since the autocorrelation function of an m-sequence reaches a maximum at zero shift, not only can a spread- spectrum receiver easily detect whether the incoming signal is genuine, but also the chances of talse synchronisation are minimal.

7 Synchronisation of FH systems

It is generally undesirable for there to be any special preamble in the modulation or the hopping pattern otherwise system security may be compromised. ‘Therefore acquisition techniques for the synchronisation of a frequency-hopping transmitter and receiver have been derived.

One technique anticipates the transmitted hopping pattern at the receiver, which waits for a frequency in the set which is expected in the near future. In this way the receiver is preset to a future point in the pseudo-random frequency sequence. When a signal is detected on this channel. the receiver starts to hop, continuing the sequence from the preset point. If this received signal coincides with the transmitter being at the same point in

the scqucncc, then the receivcr continues to detect a signal each hop period.

This technique has some drawbacks, however. The signal detected on the waiting frequency might be derived from a jammer. be random noise, or it might mean that the transmitter has used this frequency -hut at some other point in the pseudo-random sequence. In these cases, the receiver starts to hop. bur there is no correlation with the transmitter. This is soon detected as the receiver will find that it is receiving no signal during many of the hop periods. In this case, the receiver will select another waiting frequency and attempt to obtain synchronisation again. A more severe drawback is that the synchronisation of the transmitter’s and receiver’s codes might be missed if a fade occurred at the critical moment on the waiting frequency.

To improve the acquisition performance of the system in the presence offading. another approach is used where the receiver anticipates the transmitted sequence but, instead of waiting for a single frequency, the receiver hops slowly through the sequence until a signal is detected (a ‘hit’), at which point the receiver hops at the rcgular hopping rate. If a number of ‘hits’ are detected in succession then the probability that the receiver is synchronised with the transmitter is high.’ If the correlation between incoming signals does not hold, then it is likely that the signals received were spurious and the receiver reverts to slow hopping in order to acquire synchronisation.

FH system iwzplemepitatioii The latter acquisition technique above has been

irnplemenled in a 16 khop/s system (Fig. 10). This is illustrative of many of the problems which need to be

Y

t I I 1

ARM

data out

1 mq..; of day

data gale

Fig. 10 Controller and Sequence Generator, the latter two using RlSC (reduced instruction set computer) processor boards (ARM = Acorn RlSC machine)

A fast frequency-hopping radio control system. Three functional control systems are shown (- - -): DSP,

overcome in building such a fast frequency-hopping system. One major difficulty in a fast hopping system is the potential intersymbol interference created in the receiver by narrowband filters. A narrow-bandwidth filter will tend to ring when the receiver's local oscillator changes frequency. Signal shaping takes place at both the transmitter and receiver to minimise this effect. The filters used in the IF stages have to possess a carefully selected bandwidth in order to trade adjacent channel performance against intersymbol interference. In the 16 khop/s system, the filter bandwidth selected was approximately three times the channel spacing of 25 kHz. In spite ol using such a bandwidth, the delay imposed on the signal passing through this IF filter represents about one half a hop period and this reduces the time available for processing the received pulse.

To filter and detect the IF signal, digital signal processing (DSP) techniques were used.".y Since the signal received is of finite duration (one hop period) it is treated as a causal system. Multiple samples of the signal are taken during one hop period of the receiver. These are processed to determine whether a signal has been received and, if so, whether that signal is early or late with respect to the receiver's timing. In this way, not only can the output of the filter be used to control the acqu synchronisation, but it can also be used to generate an early/late signal for fine synchronisation and to maintain synchronisation once the receiver is correlated with the transmitter

In the system shown in Fig. 10, dual-processor filters (for I and Q signals) based on the AT&T DSPl6A devices sample the IF signal at a rate of 125 kHz. A2-bit result from the DSP section describes the signal received as hit, early, late or miss.

to the controller which algorithm. This controller

determines the state of the receiver, which is either in-lock or unsynchroniscd. If thc rcceivcr is in-lock, then only small timing adjustments at the receiver are allowed to keep the received signal centred in the receiver's timing window. These adjustments are necessary because of clock drift and relative movcment of cithcr the receiver or the transmitter. If the receiver is unsynchronised, then the controller scts the hop rate to be 20% slower than normal whilst no signals are received. Should a signal be received which exceeds a thrcshold, then the controller sets the hop rate to the nominal value. The control of the rcceivcr's hopping rate is governed by a ladder alg~rithrn.~ If a signal is received, the receiver's state is moved one place up the ladder, otherwise the state moves one place down the ladder. Thus, when a signal is received in successive hop periods, the lop of the ladder is quickly reached and the receiver is said to be in-lock. It can be seen that a small number of signal misses can be tolerated whilst the ladder is being climbed and this improves the performance of the acquisition algorithm in the presence of fading, noise or jamming. Should the receiver encounter a number of hop periods in which no signal has been received, thc receiver's state descends the ladder. The ladder is of auficierit length to allow for long fades. Should the bottom

of the ladder be reached, the receiver is deemed to be out of lock and therefore is unsynchronised with the transmitter. In this case the acquisition cycle is rcstarted.

A separate system generates the hopping sequence and this also implements the timing adjustments in the receiver. The sequence generator generates a pseud@ random sequence which, in this case, will not repeat for many days, It is possible to instruct the sequencegenerator to pass channel numbers to the frequency synthesiser starting from any chosen time. In this way the receiver may be made to anticipate the transmitter, by being set to a point in the sequence which is ahead of that in the transmitter. Small timing adjustments are enabled by a special timing circuit which allows the modulus of a counter to be changed during its operation. This timer Eenerates a strobe to control the time at which the synthesiser changes frequency and the timer also clocks a hardware first-in first-out store (FIFO) which is filled with the sequence numbers output from the sequence generator. The use of a hardware FIFO is cssential in order to achieve accurate control of the synthesiser; interrupt latencies in a software solution would not allow the system to funclion correctly.

It is highly desirable that the signal received during one hop period be used to determine the length of the next hop period. In this way the maximum acquisition performance is obtained, otherwise timing adjustments will have a latency of one hop period or more. It is difficult to update the receiver's timing each hop but Fig. 11 shows the relationship between the input and output signals of the processors which enables this to be performed.

The IF filter imposes a delay of approximately half of a hop period. Thus the processing time available to determine the length of the next hop is reduced as shown in Fig. 11. Not all the IF samples have been received by the DSP processor when the next hop period starts. Therefore the signal from the next hop period is already being received by the RF stages at the point when its duration is being decided. The shaded area in Fig. 11 shows that the hop length may be decreased by 1% or increased by 1% or 20% from its nominal value for early/late and acquisition modes, respectively. All timing adjustments must be completed before the shaded area is entered. The software in each subsystem and the hardware interfaces between subsystems have been organised so that the time between converting the last DSP sample and changing the modulus of the hoplength counter is minimised. In this system, that interval has been reduced to around 20 ps.

8 Other spread-spectrum applications

GPS One application of DS/SS is in ranging systems, for

example the NAVSTAR Global Positioning System (GPS).'' In the GPS system, anumber olsatellites are used to achieve global coverage. Each satellite transmits a signal which gives the time of transmission (GPS time) and the satellite's position. The receiver is passive and receives a signal from four satellites, from which three co-ordinates of position and one of time difference are computed. The

10 ELECUIRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1994

~ ~ I -

RF stage

I

output of IF filler

'- filter delay -' sampled IF signal

(CONT)

Fig. 11 functional systems shown in Fig. 10 are responsible for the signals shown. CONT = Controller; SEQ = Sequence

The critical timing relationships between the control signals in the fast frequency-hopping radio. The three

Generator

satellites transmit on two frequencies: 1575.420 MHz (L,) and 1227.600 MHz (LJ. L, carries a civil 'clear acquisition' (C/A) spread-spectrum code and also a military 'precise' (P) code modulated onto quadrature carriers; & carries only the P code. The C/A code is a 1023 bit Gold code clocked at 1.023 Mbit/s and repeats every 1 ms. The P code is clocked at 10.23 Mbit/s and is formed from the product of two relatively-prime Gold codes each of which has a sequence length of over 15 million, so that the resulting code does not repeat for 38 weeks. However, the P code is restarted at midnight GMT each Saturday-Sunday.

The correlation of the transmitted pseudo-random code with an identical copy at the receiver allows an extremely accurate measure of range to be obtained in terms of the relative time offset required to achieve correlation. With its higher chip rate, the P code is capable of ten-fold greater accuracy than the C/Acode. The repeat time of 1 ms for the C/A code means that an unambiguous range can be measured to the nearest 300 km, but since the code needs to be aligned to within one chip period, the accuracy is better than 300 m. Because the autocorrelation function decreases linearly to t1 bit, a receiver can measure distances to better than t l / ~ bit accuracy. Using postprocessing techniques, accuracies of better than 10 m are routinely claimed" using a C/Acode.

Frequency hopping Frequency-hopping systems are mainly used for military

applications owing to the complexity involved in synthesiser design when a fast hopping rate is used. A hop rate of up to 2000 hops/s can be used in the Jaguarl'system but higher hop rates can be achieved in practical systems, as shown above. Such radios usually operate in the VHF or UHF bands, although HF versions are sometimes used. A development of Jaguar is Racal's Panther Radio System. In its VHF version, Panther hops at 100 Hz within a narrow subband of nine outer bands, each of which is 6.4 kHz in width. Each outer band contains 256 channels. Alternatively, the outer bands may be 58 MHz in width, each containing 2320 channels. For HF use, a hop rate of

10 Hz is used with up to 100 channels, which may be pre- programmed in 100 Hz steps. Upper sideband, lower sideband and CW modulation modes are supported.

Frequency hopping has also been proposed as a means of making more efficient use of the frequency spectra'"-'' by re-using existing fixed-channel allocations. For example, if a fast frequency-hopping (FFH) transmitter operates at 16000 hops per second and uses 1000 frequency channels, then a receiver tuned to one of these futed frequencies will receive interference on average 16 times a second, but the interference lasts only 1/16 000th second each time. Only a sparse network of FFH transmitters is likely to be possible in such circumstances so that the service provided by the primary, fixed service is not disrupted. Using time division multiplexing in addition to FFH, allows the dwell time on each channel to be reduced; alternatively, the hop rate may be reduced for the same dwell time as an FFH-only system.

CDMA Qualcomm Inc. of California have pioneered the use of

CDMA techniques in cellular radio systems and set up trials in 1991 involving 70 mobiles in order to test fully- loaded cells. Conflicting claims for cell capacity have frequently been made for time, frequency and code division multiple access (TDMA, FDMA and CDMA) systems. It should be noted that it is the total Qualcomm system which gives the required performance and not simply the use of CDMA alone. For example, a major advantage for DS/SS is that the same spectrum is reused in every single cell; users are separated by the CDMA operation. It is also possible with CDMA to exploit the voice activity gain whereby the transmitter is muted during periods of silence. It is not possible to exploit this in FDMA or TDMA systems because the necessary channel reassignments cannot be performed asynchronously. A typicalvoice duty cycle is35%, so for CDMAthe transmitter is only active for a fraction of the time and this reduces the average mutual interference between users and increases the CDMA efficiency by a factor of 2. Cell sectorisation

ELECTRONICS & COMMIJNICATION EKGINEERIKG JOLlRN.41. FEKKL'AKY 1994 11

(achieved by base antenna design) allows a further increase in the number of users that can be accommodated. T h e pilot CDMA system operated in the 70 MHz region, and used a PN chip rate of around

In order to take advantage of new and proposed spread- spectrum radio systems, chips with programmable data and chip rates from 100 bit/s to 64 Mbit/s are now available which support BPSK, QPSK (quadrature phase-shift keying), MSK (minimum shift keying) and other modulation types.

9 Summary

In TH/SS systems, the transmission link can be easily disrupted if interference occurs a t o r around the c a m e r frequency; therefore TH is ineffective as an anti-jam technique. T h e benefit of a’l“ system is in the reduction of transmitted power. It is easy to implement and provides a multi-user capability in TDMA applications.

DS/SS systems are lairly straightforward to implement because the direct modulation process only requires multiplication of a signal by the spreading code. A DS/SS transmission can be well-hidden d u e to its energy and bandwidth spreading properties, making it difficult to detect and intercept. DS/SS h a s good anti-jam properties and can b e used in code division multiple access (CDMA) systems. Fast PN code generators are possible and the PN code rate dictates how wide the bandwidth of a DS signal is.

T h e main disadvantage of DS/SS is its poor near-far performance. This is a problem when there are very strong unwanted signals near to the receiver, which cause blocking of weaker signals since DS signals occupy a continuous spectrum around the carrier. In a cellular network, mutual interference between use r s is minimised by carelul power control a t each transmitter.

Frequency-hopping spread-spectrum h a s some properties that are complementary to DS/SS. Of all the SS techniques described, the greatest amount of bandwidth spreading is normally achieved by FH/SS. Unlike DS, which h a s a continuous spectrum, FH can b e designed to reside in a noncontinuous spectrum as shown in Fig. 4 This is an advantage because it means that o ther transmissions in the same part of the spectrum can b e accommodated, for example an FH transmitter can be made to avoid known television transmissions. FH/SS h a s a good near-far performance. FH can b e used quite effectively a s an anti-jamming technique and, if operated at a high speed, it is difficult for afollowerjammer to interfere with the transmission. T h e r e a rc difficulties in implementing highly portable FH/SS systems with fast hop rates since the switching speed required of the frequency synthesiser normally implies a complex design. Careful shaping of the transmitted pulse is required if sidebands and spurious responses are to be minimised.

T h e availability of direct digital synthesisers @DS)

1.25 MHz.

purity of DDS devices is not as good a s for conventional direct synthesis.

References

1 NICHOLSON, D.L.: ‘Spread spectrum signal design’ (Computer Science Press, LJSA, 1988)

2 PICKHOLTZ, R.L., SCHILLING, D.L., and MILSTEIN, L.B.: Theory of spread-spectrum communications - a tutorial’, IEEE Trans., COM-30, May 1982, pp.855-884

3 ZIEMER, RE., and PETERSON, R.L.: ‘Digital communications and spread spectrum systems’ (MacMillan, IJSA. 1985)

4 DMON, R.C.: ‘Spread spectrum systems’. Uohn Wilry, Ncw York, 1984)

5 COOPER G.R. and McGILLEM, C.G.: ‘Modem communications and spread spectrum’ (McGraw Hill Int., Singapore. 1986)

6 POWY, G.J.R, and GRANT, P.M.: ’Simplified matched filter receiver designs for spread spectrum communications applications’, Electron. & Commun. Eng. J , 1993, 5, (3, pp.5y-64

7 ZEIN, N.F., and CHAMBERS, W.G.: ‘Serial acquisition in a 16 khops/s fast-frequency-hop system’, IEE P r o d , 1YY1, 138, (3), pp.141-147

8 ZEIN, N.F., CHAMBERS, W.G., and CLARKSON, T.G.: ‘Use of a matched filter for serial acqu hopping system’. IEEE MILCOM ‘90 Conference, Monterey CA, Vol. 1,5.5.1-5.5.5.lst-3rd October 1990

9 ZEIN, N.F., CL4RKSON. T.G., TSUI, T.S.U.. and HARRIS, RM.: ‘DSP design for a Lst-frequency-hoppiiig radio’. Proc. Int. Symp. on DSP for Communication Systems, Coventry, 7th-9th September 1992

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11 ‘Receiver survey’, GPS World, (GPS World Corporation, Eugene, Oregon, January 1992)

12 MUNDAY, P.J., and PINCHES, M.C.: ‘Jaguar-V frequency- hopping radio system’, IEE Proc.-F, 1982, 129, (31, pp.2 13-222

13 SCHILLING, D.L.. PICKHOLTZ, RL., and MIISTEIN, L.B.: ‘Spread spectrum goes commercial’, IEEE Spectrum, August 1990, pp.40-45

14 COOK, C.E., and MARSH, H.S.: ‘An introduction to spread spectrum’, IEEE Commun. Mug,, March 1983, pp .g l6

15 TAYLQR J.T., and OMURA. J.K.: ‘Spread sprctrum technology: a solution to the personal communications services frequency allocation dilemma’, IEEE Commun. Mug., February 1Y91,29, (2). pp.48-51

16 SCHILLING, U.L., MILSTEIN, L.B., PICKHOLTZ. RL, KULLBACK, M., and MILLEK, F.: ‘Spread spectrum for commercial communications’, IEEE Commun. Mag., April 1991,29, (41, pp.66-79

17 KOHNO, R.: ‘Pseudo-noise sequences and inlerference cancellation techniques for spread spectrum systems - spread spectrum theory and techniques in Japan, IEICE Trans., May 1991. E74, (5). pp.108.7-1092

0 IEE:1994 First received 20th Novrmber 1992 and in final form 13th December 1993

~

makes fast frequency hopping feasible SinCC switching speeds of well below 1 PS are possible, but Power consumption is high at fast clock speeds and the output

me authors with the Department oIElectronic and Electrical Engineering, King’s College London, Strand, London WC2R 2L’, UK.

12 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL FEBRUARY 1994