Lecture 9: Spread Spectrum Modulation Techniques

Transcription

1 Lecture 9: Spread Spectrum Modulation Techniques Spread spectrum (SS) modulation techniques employ a transmission bandwidth which is several orders of magnitude greater than the minimum required bandwidth of the signal carried information (i.e., message). Therefore the advantage of SS is that many subscribers can simultaneously use the same bandwidth without significant interference between them. In multi-user interference wireless communication channels, SS modulation becomes very bandwidth efficient. Apart from occupying very wide bandwidth, SS signals are pseudorandom and have noise-like properties when compared with the digital information data. The spreading waveform is controlled by a pseudo-noise (PN) sequence or pseudo-noise code, which is a binary sequence that appears random, but can be reproduced in a deterministic manner by corresponding receivers (about such properties of PN code we will talk below in Lecture 0). Since each subscriber use a unique PN code which is approximately orthogonal to the codes of other subscribers located in the are of service, the receivers can separate each user based on their own unique codes, even though they occupy the same spectral bandwidth at all time of service. So, we can summarize SS modulation technique as: Principles: Each user is assigned a unique code sequence to encode outgoing information. The receiver can decode the signal after reception and recover the original data (the code is known in advance). Band Width (BW) of the code signal is much wider than BW of the information carried signal. The encoding process enlarges (spreads) the spectrum of the transmitted signal carried information. Criteria: ) Transmission BW is much larger than information BW. ) Radio frequency (RF) BW is determined by a function other than the information being sent (i.e., exclusion of modulation techniques such as frequency modulation, phase modulation). All mentioned above implies that, up to a certain number of users, interference between SS signals using the same frequency is negligible. Since all users are able to share the same frequency spectrum, SS modulation can eliminate frequency planning, because all cells can use the same frequency channels. Other main goal to use SS systems for wireless communication is resistance to multipath fading occurs in such channels. As we know from previous course, wideband signals are frequency selective, that is, can be affected by frequency selective fading. Using SS modulation with signals, having uniform energy over a

2 very large bandwidth, at any given time only a small portion of the spectrum will be affected by such kind of fading. Moreover, the multipath resistance properties occur due to the fact that the delayed multipath components of the transmitted PN modulated signal will have close-to-zero correlation with the original PN code, and will thus appear as another uncorrelated user, which is ignored by the receiver. SS systems are not only resistant to multipath fading (flat or frequency selective), but they can also exploit the multipath components of the total signal to improve the performance of the wireless communication networks. About these features of SS modulation we will talk below. 9.. Main Definitions and Characteristics of Spread Spectrum Modulation Processing Gain (PG): During spreading process the original signal power over a broad BW is defined by socalled the processing gain, which can be determined as Bt PG = B where Bi is info BW This is clearly seen from Fig. 9.. i B is transmit BW t B i Fig. 9. B t Why we can eliminate interference between users using the same frequency band is clear seen in Fig. 9. for the case of two users to not be interfering with one another. & Fig. 9. In fact, from the left and middle pictures follows that two users generate a spreadspectrum (SS) signal from their narrowband data signals. From the right pictures

3 follows that both users transmit their SS signals at the same time (upper picture). But at the receiver only the signal of user is coherently summed with its own PN code by the user despreader (demodulator) and the user data only recovered. The SS signal of user contains at the level of spread signal with low energy over whole spectrum of frequencies. The same situation will be with any noise or multipath fading or with interference between users (see Fig. 9.3). Narrowband Interferer Filtering Data with DS-SS I i-th SS Original Data i-th SS I Modulator Receiver Demodulator Fig. 9.3 We will summarize the SS modulation spectral capability as follows: - Protection against multipath interference. - Privacy. - Interference rejection. - Anti-jamming capabilities, especially narrowband jamming. - Low probability of interception (LPI). Types of SS Modulation Technique: There are number of SS modulation techniques that generate spread-spectrum signals: Direct Sequence Spread Spectrum (DSSS) Modulation. It is based on direct multiplication of the data-bearing signal with a high chip rate spreading PN code. Frequency Hopping (FH). It based on jump in carrier frequency, at which the data-bearing signal is transmitted. This jump (change) in frequency occurs according to the corresponding spreading PN code. Time Hopping (TH). Here, the data-bearing signal is not transmitted continuously. Conversely, it is transmitted in form of short bursts, where the times of the bursts are decided by the spreading PN code, that is, jump in time is noted. Chirp Modulation (CM). Used in military radar applications by use lowpower frequency sweeping. Hybrid Modulation (HM). It based on combination of the up methods to combat with their disadvantages. For all techniques of SS modulation, a following concept is used: Averaging Avoidance Accordance of the concept and the modulation technique is shown in Table 9.. Table 9. DS FH TH Chirp Hybrid Averaging x x Avoidance x x x x 3

4 9.. Direct Sequence Spread Spectrum (DS-SS) Binary data modulates an RF carrier, and then the data carrier is modulated by the PN code signal which consists code bits called chips, which can be either + or. Higher BW of chips allows spreading. To obtain the desired spreading of the databearing signal, the chip rate of the PN code signal must be much higher that the chip rate of the data-bearing (information) signal. The coded narrowband signal before spreading is modulated using BPSK, DPSK, QPSK or MSK. The received spread spectrum signal for a single user can be represented as Es S ss ( t) = m( t) p( t)cos(π f ct + θ ) (9.) Ts where m(t) is a data sequence, p(t) is the PN spreading sequence, f c is a carrier frequency, and θ is the carrier phase angle at t=0. The data waveform is a time sequence of non-overlapping rectangular pulses, each of which has amplitude equal to + or. Each symbol in a data sequence represents a data symbol with duration T s and energy E s. Each pulse in PN spreading sequence represents a chip, as a rectangular pulse with amplitude equal to + or and durationt c. If B ss is the bandwidth of spread spectrum signal S ss (t) and B is the bandwidth of modulated signal m( t)cos(π f c t + θ ), the spreading due to p(t) gives B ss ( t) >> B. Therefore, the SS-modulation is usually called the wideband modulation and the data modulation is called the narrowband modulation. In Fig. 9.4, modified block diagram of DS-SS transmitter is presented. data Data Modulator Wideband modulation Carrier Fig. 9.4 Today, to omit the data modulation and use BPSK for the code narrowband modulation a new improved technique is used, the block-diagram of which is shown by Fig. 9.5: data X Wideband modulation code carrier 4

5 The DS-SS signal resulting from this transmitter is shown in Fig. 9.6: Data Signal Signal Data Signal x Signal BPSK-modulated Signal The rate of the code signal is called the chip rate. One chip denotes one symbol when referring to spreading PN code signals. In this figure, 0 code chips per information (data) symbol are transmitted, i.e., the code chip rate is 0 times the data rate. So, the processing gain PG=0. A block-scheme of the receiver is shown in Fig Demodulator Data Demodulator Synchr./ tracking Generator Carrier Generator Properties of DS-SS Modulation Fig. 9.7 a) Multiple Access Capability. If multiple subscribers use the channel at the same time, there will be multiple DS signals overlapping in time and frequency. At the receiver, dispreading is used to remove the spreading code. Only desired user will be despreaded at the receiver in the information bandwidth (narrowband) as is shown for the case of two users in Fig b) Multipath Interference Rejection. This means that if the desired signal and its versions that are delayed due to multipath phenomena (fading) for the time more than double chip duration within the PN code, dispreading will treat the delayed signals as the interfering signals, putting only a small part of the power in the data bandwidth, as shown in Fig

6 c) Narrowband Interference Rejection. At the transmitter multiplying the narrowband signal with wideband code sequence, the spectrum of the narrowband signal is spreaded (see Fig. 9.3), so that its power in the information bandwidth decreases by a factor equal to the processing gain (in Fig. 9.5 PG=0). d) Low Probability of Interception (LPI). Because of its low power density per hertz due to overlapping the whole signal spectrum all the time, the DS-SS signal is difficult to detect and intercept by a hostile listener. This makes it very difficult to detect a DS-SS signal. Advantages of DS-SS Technique: - easy code generation; - simple frequency synthesizer (single carrier frequency); - coherent demodulation of the SS signal is possible; - no synchronization between users is necessary. Disadvatages of DS-SS Technique: - difficult to acquire and maintain local synchronization between the local code and the received signal; - for correct reception, locally generated code and received code sequence must be synchronize in a fraction of chip time. BW is limited to 0-0 MHz (no available contiguous frequency band). 9.. Frequency Hopping In such a SS modulation technique the carrier frequency of the modulated information signal changes periodically; the hopping pattern is decided by the code signal generated by the PN sequence During time interval T, the carrier frequency remains the same, but after each time interval the carrier hops to another or possibly the same frequency according to the corresponding PN sequence individual for each user. The set of available frequencies the carrier attain is called the hop-set. The frequency occupation of FH-SS modulation technique differs considerably from DS-SS modulation technique. A latter occupies the whole frequency band when information is transmitted, whereas an FH-SS uses only a small part of bandwidth when transmits, but the location of this part differs in time. So, during whole time of user service, the same average bandwidth finally can be used. This difference is clearly seen from illustrations presented in Fig frequency frequency FH time DS time Fig

7 The second difference between DS-SS and FH-SS modulation is of how them reduce the narrowband interference: in DS-SS it is reduced by a factor of PG and strong interference may overload the DS receiver; in FH-SS it will cause an error in the system and therefore may be handled by FEC (Forward Error Correction). The block-scheme of an FH system is given in Fig Baseband Modulator Up Converter Down Converter Baseband Modulator Generator Frequency Synthesizer Frequency Synthesizer Synchr. tracking Generator Fig. 9.9 At the transmitter (located at the left-side of the block scheme), the data-bearing signal is baseband modulated. Using a fast frequency synthesizer, which is controlled by a PN code signal, the carrier frequency is converted up to the transmission frequency. The inverse process takes place at the receiver located at the right-side of the block scheme. Using a locally generated code PN sequence, the received signal is converted down to the baseband. The data is recovered after baseband signal demodulation. Due to synchronization according to hoping pattern the correct dispreading of the signal at the demodulator is possible. Within frequency hoping there is a distinction which is based on hoping rate of the carrier. If the hopping rate is much faster than the data (symbol) rate, the modulation is considered to be fast frequency hopping (F-FH). Here, one bit/symbol is transmitted in different frequencies, that is, its time duration is much larger than hopping time of frequencies change. In this case, spectrum of a single hopping frequency is dictated mainly by the pulse shape. Therefore, abrupt changes broadens the spectrum, limiting the number of hop frequencies. At the same time, in F-FH regime, smooth changes may be achieved by decreasing the transmitting power before the frequency hop and increase it again after the frequency hop. If now hopping process occurs slower than the data (symbol) rate, the modulation is considered to be slow frequency hopping (S-FH). Therefore in such systems, multiple bits/symbols are transmitted at the same frequency. Properties of FH-SS Modulation As has done above for DS-SS modulation, let us discuss the properties of FH-SS modulation with respect to multiple access capability, multipath interference rejection, narrowband interference rejection, and probability of interception. a) Multiple Access Capability. In the F-FH modulation technique, one symbol is transmitted in different frequency bands. At the receiver, dispreading is used to remove the spreading code. If the desired user is the only one to transmit in most frequency bands, the received power 7

8 of the desired signal will be much greater than the interfering power and the signal will be received correctly. In the S-FH modulation technique, multiple symbols are transmitted at one frequency. If the probability of other users transmitting in the same frequency band is low enough, the desired user will be received correctly most of the time. For those times that interfering users transmit in the same frequency band, errorcorrecting codes are used to recover the data transmitted during that period. b) Multipath Interference Rejection. In the F-FH modulation, the carrier frequency changes a number of times during the transmission of one symbol. Thus, a particular signal frequency will be modulated and transmitted on a number of carrier frequencies. It is known, that the multipath effect is different at the different carrier frequencies (due to frequency selective fading). As a result, signal frequencies, which are amplified at one carrier frequency, will be attenuated at another carrier frequency and vice versa. At the receiver the responses at the different frequencies are averaged, thus reducing the multipath interference. c) Narrowband Interference Rejection. If a narrowband data signal is interfering on one of the hopping frequencies, a number of which is equal to PG (processing gain), the desired user will use (on the average) the hopping frequency where the interferer is located /PG percent of the time. The interference is therefore reduced by factor PG>>. d) Low Probability of Interception (LPI). The difficulty in intercepting an FH-SS signal lies not in its low transmission power. During a transmission, it uses as much power per hertz as a continuous transmission. But the frequency at which the signal is going to be transmitted is unknown, and the duration of the transmission at a particular frequency is quite small. Therefore, although the signal is more readily intercepted than a DS-SS signal, it is still a difficult task to perform. Advantages of FH-SS Technique: - synchronization is much easier for FH-SS technique than for DS-SS technique, it is within a fraction of the hop time. By use a large hop-set, the hop time will be much longer than the chip time of a DS-SS technique. - higher spread-spectrum bandwidths; - the probability of multiple users transmitting in the same frequency band at the same time is small. A far-located user cannot be interfered by closelocated user, so the near-far performance here is much better than by use DS-SS technique; - FH-SS offers higher possible reduction of narrowband interference than a DS-SS since the larger possible bandwidth a FH modulation can employ. Disadvatages of FH-SS Technique: - a highly sophisticated frequency synthesizer is necessary; - an abrupt change of the signal when changing frequency bands will lead to an increase in the frequency band occupied. To avoid this, the signal has to be turned off and on when changing frequency; - coherent demodulation is difficult because of the problems in maintaining phase relationships during hopping. 8

9 9.3. Time Hopping In time hopping (TH-SS) the data signal is transmitted in rapid bursts at time intervals determined by the code assigned to the user. For this purpose the time axis is divided to frames and frames are divided to M time slots. During each frame in the process of data transmission the user will transmit all its data in only one of the M time slots. Therefore, the frequency of data transmission is increased by a factor of M. The time slot is determined by the PN code used for desired subscriber. A block scheme of TH- SS modulation technique is given in Fig data Buffer slow in fast out Data modulator Data demodulator Buffer fast in slow out data Generator Carrier Carrier Fig. 9.0 Figure 9. shows the time-frequency plot of the TH-SS modulation technique. Comparing this figure with Fig. 9.8, it can be clear seen that the TH-SS uses the whole wideband spectrum for short periods instead of parts of the spectrum all of the time. Frequency Properties of TH-SS Modulation Fig. 9. Time Following the same procedure as for the previous modulation techniques, let us discuss the properties of TH-SS modulation with respect to multiple access capability, multipath interference rejection, narrowband interference rejection, and probability of interception. a) Multiple Access Capability. The multiple access capability of TH-SS signals is occurred in the same manner as that of the FH-SS signals: by making the probability of users transmissions in the same frequency band at the same time small. In the case of time hopping, all 9

10 transmissions are in the same frequency band, so the probability of more than one transmission at the same time must be small. This is again achieved by assigning different codes to different users. If multiple transmissions occur, error-correcting codes that the desired signal can still be recovered. b) Multipath Interference Rejection. A TH-SS signal is transmitted in reduced time. The signaling rate, therefore, increases and dispersion of the signal will now lead to overlap of adjacent bits. Therefore, no advantage is to be gained with respect to multipath interference rejection. c) Narrowband Interference Rejection. A TH-SS signal is transmitted in reduced time. This reduction is equal to /PG, where PG is the processing gain. Because the desired subscriber only receive the interfering signal /PG percent of the time, the interference is therefore reduced by factor PG>>. d) Low Probability of Interception (LPI). With TH-SS technique, the frequency at which a user transmits is constant, but the times of transmission are unknown, and the duration of the transmission is very short. When multiple users are transmitting, this makes difficult for an intercepting receiver to distinguish the beginning and the end of a transmission and to decide which transmissions belong to which user. Advantages of TH-SS Technique: - implementation is easier than FS-CDMA; - useful when the transmitter is average-power limited but not peak-power limited; - near-far performance is better than DS-SS technique. Disadvatages of TH-SS Technique: - it takes a long time to synchronize the code, and the receiver has a short time to perform it; - if multiple transmissions occur, a lot of data bits are lost. A good correcting code and data interleaving are necessary Other Spread Spectrum Techniques Chirp SS This technique bases on spread the spectrum by a linear modulation of the carrier frequency. The processing gain of such modulation is defined as PG = B T Figure 9. shows the frequency-time plot of the Chirp-SS modulation technique. 0

11 f B f T t t Fig Hybrid Techniques Include all wireless systems that employs a combination of two or more SS modulation techniques, as CDMA, TDMA, FDMA, GSM etc (see Lectures -) As examples we can note: DS/FH, DS/TH, FH/TH, DS/FH/TH. The main goal of such combinations is to combine advantages of each separate technique. For example, the combined DS/FH technique allow us to use in a new system both the anti-multipath advanced property of the DS-SS modulation and the favorable near-far operation of the FH-SS modulation. In such a system, the data signal is first spread using a DS code signal. Then, the spread signal is modulated on a carrier whose frequency hops according to another PN code sequence. A code clock ensures a fixed relation between the two codes. A disadvantage of such combined SS modulation techniques is that it increases complexity of transmitter and receiver Performance of Main Spread Spectrum Modulation Techniques Performance of DS-SS Modulation Let us consider a DS-SS system with K multiple access users, each of which has a PN sequence with N chips (with duration T c ) per message symbol period T such that T = N T c. If we assume that the average probability of the bit error can be described by the error function Q (which is simply related to Gaussian distribution function), we have for K- users, which are served as identically distributed interferers, a convenient expression for the average probability of bit error (BER) given by

12 ] where / K N 0 P = + e Q (9.) 3N Eb Eb is a bit energy and N 0 is a white (Gaussian) noise RF power spectral density (i.e., the white noise power normalized on RF signal bandwidth B). For a single user, K=, this expression reduces to the BER expression for BPSK modulation of the data signal. In the case of K- interferers for the interference limited situation in the system, when thermal noise is not so significant, the BER expression can be simplified to E b N 0 and 3N P e = Q (9.3) K This is the irreducible error floor due to multiple access interference and is obtained by the assumption that all interferers provide equal power, the same as the desired user, at the DS-SS receiver. In practice, however, the near-far problem presents difficulties for DS-SS system (see above), because one close-in user may dominate the received signal energy at the base station, making the Gaussian assumption inaccurate. For large number of users (K>>), the BER is limited more by the multiple access interference than by thermal white noise Performance of DS-SS Modulation In FH-SS systems, several users independently hop their carrier frequency while using BFSK (binary frequency shift keying) modulation of data signal. If two users are not simultaneously utilizing the same frequency band, the BER for BFSK can be expressed as E = b P exp e (9.4) N 0 If there are K- interfering users in the system and M possible hopping channels (called slots), there is a /M probability that a given interferer will be presented in the desired user s slot and, finally, the overall probability of BER can be modeled as P e E = b K K exp + N M M 0 (9.5) For K=, (9.5) reduces to (9.4), the standard probability of error for BFSK data signal modulation. If in general case of K- interferers, as above E b N 0, then the K probability of BER approaches P e = M which illustrates the irreducible error rate due to multiple access interference. Formula (9.5) was obtained by assuming that all users hop their frequencies synchronously, which is called slotted frequency hopping. For asynchronous FH-SS system, where due to the various propagation

13 3 delays radio signals will not arrive synchronously to each user, the probability of BER is more complicated and described by the following formula: = 0 exp K b K b b e N M N M N E P (9.6) where b N is the number of bits per hop. FH-SS modulation technique has an advantage over DS-SS technique in that it is not so depend on the near-far problem, because signals are generally not utilizing the same frequency simultaneously and the relative power levels of signals are not as critical as in DS-SS system. This problem is also not fully resolved in FH-SS system, but significant improvement compared with DS-SS has been obtained. Some special examples and exercises will be given for students.