1、Rec. ITU-R SM.1055 129 RECOMMENDATION ITU-R SM.1055 THE USE OF SPREAD SPECTRUM TECHNIQUES (Question IT-R 71/1) (1994) The ITU Radiocommunication Assembly, considering , a) telecommunications objectives; b) direct sequence systems; cl improved performance under multipath conditions; d) traditional na
2、rrow-band signals requires further study; e) that spread spectrum systems can offer improved sharing factors in certain conditions in achieving that spread spectrum systems include frequency-hopping, direct sequence, and mixed frequency-hopping- that spread spectrum systems can offer operational adv
3、antages such as increased resistance to interference and that the mutual interference between spread spectrum signals, and between spread spectrum signals and more that spread spectrum systems operate differently from more traditional narrow-band communications, recommends 1. that the descriptions o
4、f spread spectrum technologies and signal-to-noise calculations contained in Annex 1, be recognized when describing direct sequence (DS), frequency-hopping (FH), and combination frequency-hoppingdirect- sequence (FWDS) modulations; 2. that the signai-to-interference ratios, the minimum required prop
5、agation losses, and other performance degradation measures between potential interferers as described in Annex 2 should be used when studying the effect of individual frequency-hopping and direct-sequence spread spectrum signals on several common signals on a one-to-one basis, including AM (A3E), FM
6、 (F3E), wideband FDM/FM (F8E), and television; 3. that the procedure described in Annex 3 be used when calculating the effect of direct-sequence and frequency- hopping systems on digital receivers, AM voice receivers, and FM voice receivers. Note I - Additional studies should focus on the effects of
7、 multiple spread spectrum interferers in a crowded environment. ANNEX 1 Spread spectrum techniques 1. Introduction This Annex describes broadband “spread spectrum” techniques and the interference rejection capabilities of these systems. A spread spectrum (SS) system can be defined as one in which th
8、e average energy of the transmitted signal is spread over a bandwidth which is much wider than the information bandwidth (the bandwidth of the transmitted signal is wider than the information bandwidth by at least a factor of two for double sideband Ah4 and typically a factor of four or _oreater for
9、 narrow-band FM, and 100 to 1 for a linear SS system). These systems essentially trade the wider transmission bandwidth for a lower power spectral density and increased rejection to interfering signals operating in the same frequency band. They, therefore, have the potential of sharing the spectrum
10、with conventional narrow-band systems ITU-R RECMNxSM. 3055 74 4855232 0523339 323 . 130 Rec. ITU-R SM.1055 because of the potentially low power that is transmitted in the narrow-band receiver passband. In addition, high levels of interference will be rejected by SS receiving systems. These systems s
11、hould therefore be examined to identify how efficiently they use the spectrum. Two distinct types of bandwidth expansion SS techniques need to be discussed. These are the techniques that provide either linear or non-linear interference signai rejection. The classical FM approach typifies non-linear
12、techniques because there is only an increase in the output SIN ratio (dB) when the input SIN is greater than the first or noise capture ratio. This means that the input SIN must be typically greater than 10 dB in order to obtain a linear enhancement against noise. In contrast to the FM type of syste
13、m, the SS systems described in this Annex are linear so that the improvement remains constant even if the input wanted-to-unwanted signai ratio is negative. The output wanted signal-to-interference signal ratio and is defined as the processing gain (PG) of the system. This PG might typically be 100
14、to 1, or larger. PG is defined by the following: is increased over.the input wanted signal-to-interference ratio A system with a PG of 100 (and no loss due to non-ideal signal processing) and a minimum output SIZ of 1 O dB requires that the input S/Z is, at least; (S/Z)in = 10 dB - 10 log 100 = -10
15、dB A linear SS system that can operate with an input (S/Z) of -10 dB is extremely desirable since with an unwanted signal 10 dB higher than the wanted signal, the system can still be effectively used. For conventional systems with an input /i) of -10 dB, the wanted signal would be suppressed or “cap
16、tured” and no information would be transferred. A second major feature of commonly used SS techniques is that the resulting transmitted signal is a wideband low-power-density signal which resembles noise. Therefore, the transmitted signal is not readily detected by a conventional receiver. Recovery
17、of the baseband information from the wideband transmitted signal can be accomplished only through correlation or matched filter (MF) signal processing. Because of this property, the unintended listener does not detect the baseband information, and because of the low power density, it may not cause a
18、ny significant interference effects to other users of the spectrum. SS inherently provides a degree of message privacy to non-SS systems as well as other SS systems using different codes and no special signal processing. The coding also provides a selective addressing capability. Multiple users usin
19、g different codes (code division multiple access - CDMA) can simultaneously transmit in the same frequency band with a minimum amount of cross interference (codes that are used should have a low cross- correlation function). A third advantage of SS techniques over conventional modulation techniques
20、is increased transmission reliability in the presence of selective fading and multipath effects. This advantage can be significant for typically encountered fading transmission mediums, e.g. in tropospheric scatter systems. Increased resistance to multipath is a direct consequence of spreading the t
21、ransmitted bandwidth. As a first approximation, improvement is directly proportional to the ratio of transmitted bandwidth to information bandwidth. Receivers built to detect SS signals typically generate, prior to their final demodulation, a cross-correlation function between a replica of the trans
22、mitted signal and the signal received from the antenna. The correlation function of the wanted signal is always chosen to be as “good” as possible, i.e. maximum output at the centre and the signal falling to near zero in a time period equal to the reciprocal of the transmitted signal bandwidth and s
23、taying at near zero at all other times. Multipath degrades link performance when it combines with the direct signal in such a manner as to degrade the correlation function of the detected signal by reducing its peak value. The introduction of false trailing peaks into the correlation function due to
24、 simple multipath is typically not a problem. The receiver will detect and process the first peak of adequate amplitude, either the direct signal if it is strong enough or the first reflected signal of adequate amplitude if the direct signal is interfered with. In the latter case, timing becomes syn
25、chronized to the multipath return and it is processed in lieu of the direct signal. Consequently, for multipath to be destructive, it must occur with a differential delay less than the duration 131 Rec. ITU-R SM.1055 of the peak of the correlation function, with a phase that causes destructive inter
26、ference (cancellation rather than enhancement) and with an amplitude adequate to prevent the peak from exceeding detection thresholds. As the transmitted bandwidth is increased, the duration of the correlation function peak is proportionally decreased, and the multipath differential path delay that
27、can affect performance is also proportionally decreased. 2. Spread spectrum signai types Definitions for various types of spread spectrum techniquedsignal structure are as follows: Direct sequence (DS) sprend spectrum: a signal structuring technique utilizing a digital code spreading sequence having
28、 a chip rate UTsin much higher than the information signal bit rate UT,. Each information bit of the digital signal is transmitted as a pseudo-random sequence of chips, which produces a broad noise-like spectrum with a bandwidth (distance between first nulls) of 2Bsj, = 2/Tsin. The receiver correlat
29、es the RF input signal with a local copy of the spreading sequence to recover the narrow-band data information at a rate UTs. Frequency-hopping (FH) spread spectrum: a signal structuring technique employing automatic switching of the transmitted frequency. Selection of the frequency to be transmitte
30、d is typically made in a pseudo- random manner from a set of frequencies covering a band wider than the information bandwidth. The intended receiver frequency-hops in synchronization with the transmitter in order to retrieve the desired information. - Hybrid spread spectrum (FHDS): a combination of
31、frequency-hopping spread spectrum and direct- sequence spread spectrum. Chip rate: the rate at which the successive bits of the spreading sequence is applied to the signal information. - Two other types of spread spectrum modulation exist. The first uses pulsed frequency modulation or “chirped modul
32、ation” in which a carrier is swept over a band of frequencies. Radar systems, in particular, may use a sweep rate that is a linear function of time. The second spread spectrum type employs a non-sinusoidal carrier to provide additional processing gain. The following discussion does not include chirp
33、ed or non-sinusoidal spread spectrum types. 3. Signal-to-noise (SI“) performance of DS and FH systems The (S/N) performance of a linear DS spread spectrum system in the presence of Gaussian noise applies to receiver system noise and external noise with Gaussian characteristics. For this condition, D
34、S performance is given by: where: PG : processing gain of the system (S/N)out : output signal-to-noise ratio (correlator output) (S/N)i, : 2 B, : T, : input signal-to-noise ratio (RF input) bandwidth of RF input signal power density spectrum at first nulls time duration of input signal information.
35、The processing gain (equation (2) is considered the most important parameter of an SS system. The peak of the code rate signal autocorrelation function at I = O will have a duration of the order l/BJtfl = T,. The ratio of the duration of the signal information (T,) to the main peak response is thus
36、given by T,/TSm. Thus the larse BI,“ 7 case affords a “pulse compression” effect whereby the signal energy in a relatively long pulse (duration of Ts) is “compressed” into a high level short pulse (duration of TStn). The result is a high detection probability at the intended receiver with no loss in
37、 time resolution. ITU-R RECMN*SM- 3055 94 m 4855232 0523343 TBL 132 Rec ITU-R SM.1055 The same results can be obtained in the time domain by taking the inverse Fourier transform of the respective functions and using equivalent time operations. The FH system basically consists of a narrow-band filter
38、 matched to the information bandwidth pseudo- randomly shifting in frequency over the SS bandwidth. The noise out of the system is therefore governed by the bandwidth of the narrow filter. When an analysis is made of noise or an unwanted interfering signal which occupies the full hopping bandwidth,
39、a decrease in the output unwanted signal power is obtained which is equal to the ratio of the bandwidths. The PG for this case is, therefore, equal to: BFH PG = - BS (3) where: BFH: FH bandwidth B, : wanted information bandwidth. If the FH system utilized the same RF bandwidth as the RF bandwidth in
40、 the DS system to the first nulls and transmits the same information rate, the BFH and Bs in equation (3) are respectively equal to 2 B, and UT, in equation (2) so that the processing gain of both systems is the same, neglecting second order effects. It should be noted that this is only for the case
41、 of a noise or an unwanted signal spread over the wide bandwidth and not for a narrow-band signal. The sensitivity of the FH system does not contain the PG improvement of the DS system and is simply proportional to the noise temperature of the system and the information bandwidth. 4. Signal-to-inter
42、ference performance The following describes the SIZ performance of the DS and FH SS systems. For the case in which the bandwidth of the input interfering signal (Blin) is much less than the bandwidth of the input wanted signal (Bsin), the PG has been obtained as a function of the off-tuning (Ao) as:
43、 I (4) where: Bi, : input bandwidth of the RF interfering signal and all other terms are as defined in equation (2) Am: the radianfrequency difference between the carrier of the wanted and unwanted signal. The rejection of narrow-band interference by DS receivers may be understood as a process where
44、 the interference is greatly expanded in bandwidth (to approximately 2 BS,) by mixing with the spreading sequence in the receiver. The narrow-band filtering in the correlator removes all of the interference except for the portion left within the narrow bandwidth B, of the desired signal. Although na
45、rrow-band interference is rejected to a degree according to equation (4), the wide bandwidth of 2 BSi, could include a large number of narrow-band interfering signals. If one or more of these interferers is much stronger than the desired signal (possibly because of a near-by interfering transmitter
46、and a far-away desired transmitter), it can overcome the processing gain for the DS signa1 and prevent proper operation of the system. This. is known as the “near-far” problem. DS systems need to be designed such that they do not encounter interfering transmitters within the DS receiver bandwidth th
47、at are much stronger than the desired signal. Thus, the use of a wider B, to obtain higher interference rejection may cause a problem because of the larger number of interfering signals encountered within 2 B,. Rec ITU-R SM.1055 133 When the bandwidth of the interference is greater than the wanted s
48、ignal, the PG has been obtained as: . AJIIB,., 1 This clearly shows that the gain is proportional to the bandwidth filtering ratio and the time bandwidth product as would have been expected. Fromthe point of view of the output power ratios, the wideband SS system overcomes interference to the same d
49、egree that it overcomes noise. In the FH SS system, the frequency hopper re-inseris the correct carrier frequency for the wanted signal and mixes it to the IF centre frequency. Any interfering signal, entering at a fixed frequency relative to the centre frequency of the FH SS system, is converted after the FH to a signal of reduced amplitude and random off-tuned frequency due to the FH mixer and IF filter action. The interfering signal is then filtered so that effectively only those signals that fall within the same frequency channel as the desired signal are trans
