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1、外文资料原文 DADS with short spreading sequences for high data rate communications or improved BER performance Vincent Le Nir and Bart Scheers Department Communications,Information Systems and Sensors (CISS) Royal Military Academy 30, Avenue de la Renaissance B-1000 Brussels, Belgium E-mail: vincent.lenir
2、bart.scheersrma.ac.be Abstract In this paper, a method is proposed to improve the performance of the delay and add direct sequence (DADS) modulation scheme. On one hand, the selection of a short pseudo-noise (PN) sequence is used to improve the data rate performance. On the other hand, when high dat
3、a rates are not required, the same short PN sequence is replicated to form a long PN sequence. In this case, noise reduction by averaging is used to improve the bit error rate (BER) for high spreading factors.Theoretical BER formulas are derived and verified by simulations in additive white Gaussian
4、 noise (AWGN) and frequency selective Rayleigh channels. The modulation proposed DADS scheme is implemented using the CogWave software to allow the exchange of video, audio and text between two USRPs through a graphical user interface (GUI) developed in Qt4. Spread spectrum modulation scheme, transm
5、it reference, non-coherent detection I. INTRODUCTION Delay and add direct sequence (DADS) is a digital modulation scheme in which a pseudo-noise (PN) sequence is used on one hand as an embedded reference signal, and on the other hand for modulating the data information 1, 2. This modulation scheme p
6、rovides a processing gain and therefore inherits the advantages of conventional spread-spectrum communications such as multipath mitigation, anti-jamming, and multiple access capabilities. Moreover, the DADS modulation scheme has a very simple receiver structure with no carrier recovery which can ex
7、ploit the multipath diversity of frequency selective channels, contrary to conventional spread-spectrum receivers in which several Rake fingers are needed along with carrier recovery 3. However, it can be observed that the bit error rate (BER) performance degrades as the length of the PN sequence in
8、creases, making DADS less attractive for high spreading factors. In this paper, a method is proposed to improve the performance of the DADS modulation scheme. On one hand, the selection of a short pseudo-noise (PN) sequence is used to improve the data rate performance. The use of short PN sequences
9、reduces the advantages of DADS for anti-jamming and multiple access capabilities but keeps the advantages for multipath mitigation and receiver simplicity. On the other hand, when high data rates are not required, the same short PN sequence is replicated to form a long PN sequence. In this case, noi
10、se reduction by averaging is used to improve the bit error rate (BER). Theoretical BER formulas are derived for additive white Gaussian noise (AWGN) and frequency selective Rayleigh channels. It is shown that with noise reduction by averaging, the BER performance of DADS no longer degrades as the le
11、ngth of the PN sequence increases. It is also shown that the multipath diversity of frequency selective channels is exploited without any modification in the receiver structure. Theoretical BER formulas are verified by simulations in AWGN and frequency selective Rayleigh channels. The proposed DADS
12、modulation scheme is implemented using the CogWave software 4. CogWave is a free and open-source software platform aiming at developing cognitive radio waveforms. The CogWave software allows the exchange of video, audio and text between two USRPs through a graphical user interface (GUI) developed in
13、 Qt4/Gstreamer. The USRP hardware driver (UHD) C+ application programming interface (API) allows to receive and transmit IQ samples. Combining CogWave with USRP gives a rapid prototyping platform for physical layer design and algorithm validation through a real-time video, audio and text transmissio
14、n. The remainder of this paper is organized as follows. In Section II, we derive the theoretical BER formulas with the selection of a short PN sequence and the selection of a long PN sequence using noise reduction by averaging in AWGN channels. The theoretical BER formulas are compared with simulati
15、ons. In Section III, we derive the theoretical BER formulas with the selection of a short PN sequence and the selection of a long PN sequence using noise reduction by averaging in frequency selective channels. A comparison between theoretical and simulated BER performance is also performed. In secti
16、on IV, some details are provided about the implementation of the proposed DADS modulation scheme using the CogWave software to allow the exchange of video, audio and text between two USRPs through a graphical user interface (GUI) developed in Qt4/Gstreamer. Finally, Section V concludes this paper. I
17、I. THEORETICAL VS SIMULATED BER PERFORMANCE OF DADS IN AWGN CHANNELS A. Selection of a Short Pseudo Noise Sequence for DADSThe transmission chain of the DADS modulation scheme with the selection of a short PN sequence is shown in Figure 1. Assuming that K bits have to be transmitted, the PN sequence
18、 of length M is repeated K times to form the reference signal. The transmitted signal is the sum of two signals, namely the reference signal and its delayed version multiplied by the information signal. Considering an AWGN channel, the received signal can be modeled as (1) with D the delay (in chips
19、), the information bits taking values in -1,1 with data rate , the transmitted chip of the PN sequence and in the AWGN with variance per dimension. The selection of a short PN sequence whose length M is twice the delay D used in the modulation scheme is given by 2 (2) A PN sequence satisfying this c
20、riterion can be easily generated from the possible codes. The ratio between the number of codes satisfying the auto-correlation criterion and the total number of codes for delays D= 2,4,6,8 are 0.5, 0.375, 0.3125, and 0.2734 respectively. The reception chain of the DADS modulation scheme is shown in
21、 Figure 2. The correlator output is given by (3) With (4)Figure 1: Transmission chain of the DADS modulation scheme with the selection of a short PN sequence Figure 2: Reception chain of the DADS modulation scheme The correlator output can be divided into a useful, interference and noise parts The c
22、orrelator output can be divided into a useful, interference and noise parts (5) Assuming that the correlator output approaches a Gaussian distribution, the bit error rate (BER) performance can be expressed analytically as 5 (6) with erfc(.) the complementary error function.andare deterministic value
23、s. For large M , the correlator output approaches a Gaussian distribution with mean and variance (7) The integrated useful part, interference partand the mean of the noise part are given by (8) with the energy per chip and the variance of the noise part is given by (9) Knowing that a transmitted dat
24、a bit is the sum of two sequences of length M , the energy per bitcan be written as=2M , the derivation of the BER formula leads to the following expression 2 (10)Figure 3 shows the BER performance of the DADS modulation scheme with the selection of a short PN sequence for different values of M in A
25、WGN channels. For small values of M (4 and 16), theoretical and simulated curves do not match since the Gaussian approximation is not satisfied. In this case, simulated curves perform better than theoretical ones. For large values of M (64 and 256), theoretical and simulated curves are similar. Figu
26、re 3: Comparison between theoretical and simulated DADS modulation scheme with the selection of a short PN sequence in AWGN channels Figure 4: Transmission chain of the DADS modulation scheme with selection of a long PN sequence B. Selection of a Long Pseudo Noise Sequence for DADS and noise reducti
27、on by averaging The transmission chain of the DADS modulation scheme with selection of a long PN sequence is shown in Figure 4. The selection of a long PN sequence can be easily generated from the repetition of the short PN sequence selected with criterion (2) whose length N is twice the delay D use
28、d in the modulation scheme ( N =2D ). The selected short PN sequence is repeated T times to form the long PN sequence of length M . The demodulation algorithm can exploit the redundancy of the PN sequence by averaging the received noisy chips in the same bit as shown in Figure 5. The idea is then to
29、 correlate the delayed version of the received signal with its enhanced version. Assuming that the PN sequence of length N has been repeated T times, the enhanced received signal can be generated by averaging the T chips . (11) With j = i mod N,Knowing that and N=2D,this can be rewritten as (12) The
30、 correlator output becomes (13)With Figure 5: Generic receiver exploiting noise reduction by averaging in DADS (14)We assume that D =M . The integrated useful part , interference part and the mean of the noise part are given by (15)with the energy per chip and the variance of the noise part given by
31、 (16) leading to the following BER (17) Figure 6 shows the BER performance of the DADS modulation scheme with the selection of a long PN sequence for M= 4096 and different values of N in AWGN channels. The difference between theoretical and simulated BER curves is very small. Simulations show that t
32、he same performance is obtained for any value of M = NT. With noise averaging, the BER performance no longer degrades as M increases. As the BER performance still degrades as N increases, the delay D should be kept as small as possible. Figure 6: Comparison between theoretical and simulated DADS mod
33、ulation scheme with the selection of a long PN sequence in AWGN channels III. THEORETICAL VS SIMULATED BER PERFORMANCE OF DADS IN FREQUENCY SELECTIVE RAYLEIGH CHANNELS A. Short Pseudo Noise Sequence for DADS We consider a frequency selective channel with AWGN. The received signal can be modeled as (
34、18) with L the number of taps and the complex-valued channel attenuation for the l th tap. (4) can be re-written as (19) is a deterministic value. We assume that the cross-correlation between the interference part and the noise part is zero and we assume that with the maximum delay spread. The corre
35、lator output approaches a Gaussian distribution for large M with mean and variance (20)The integrated useful part , the mean of the interference part and the mean of the noise part are given by (21) with the energy per chip and the variance of the interference part and the variance of the noise part
36、 are given by (22) The derivations of the BER formula give the following expression (23）With （24）Figure 7 shows the BER performance of the DADS odulation scheme with the selection of a short PN sequence M=64 for different values of L in frequency selective ayleigh channels. The maximum delay spread
37、is set to the number of taps L . Theoretical and simulated curves are ery similar. One can observe that the DADS modulation scheme can exploit the multipath diversity of frequency lective channels without any modification in the receiver ructure. The performance of the DADS modulation scheme with th
38、e selection of a short PN sequence in frequency lective Rayleigh channels approaches the AWGN erformance as L increases. Figure 7: Comparison between theoretical and simulated DADS modulation scheme with the selection of a short PN sequence in frequency selective Rayleigh channels ( M=64 ) B. Long P
39、seudo Noise Sequence for DADS The enhanced received signal can be generated by averaging the T chips leading to the following formula (25)The enhanced received signal multiplied by the conjugate delayed version of the received signal gives + + (26) + + We assume that the cross-correlation between th
40、e interference part and the noise part is zero and we assume that . The integrated useful part, the mean of the interference part and the mean of the noise part are given by (27) with the energy per chip and the variance of the interference part and the mean of the noise part are given by (28) leadi
41、ng to the following BER (29)With （30） Figure 8 shows the BER performance of the DADS modulation scheme with the selection of a long PN sequence with N=64 and M=4096 for different values of L in frequency selective Rayleigh channels. Theoretical and simulated BER curves are very similar. Simulations
42、show that the same performance is obtained for any value of M =NT . The multipath diversity of frequency selective channels is also exploited without any modification in the receiver structure. Moreover, with noise averaging, the BER performance no longer degrades as M increases. As the BER performa
43、nce still degrades as N increases, the delay D should be kept as small as possible but larger than the maximum delay spread . The performance of the DADS modulation scheme with the selection of a long PN sequence in frequency selective Rayleigh channels approaches the AWGN performance as L increases
44、. Figure 8: Comparison between theoretical and simulated DADS modulation scheme with the selection of a long PN sequence in frequency selective Rayleigh channels ( N=64 ) IV. IMPLEMENTATION OF THE PROPOSED DADS MODULATION SCHEME USING THE COGWAVE SOFTWARE The proposed DADS modulation scheme is implemented using the CogWave software 4. CogWave is a free and open-source softw
