A Study On The New Spread Spectrum Receiver

Communication systems have lived through the transformation from analog communication to digital communication,from narrowband communication to wideband communication since it came into being. Wideband, intelligence and IP are several main development trends. Finally fusion is the need of communication development. On the other hand, spread spectrum communication is one of the main methods of wideband communication. Since it has the special features of anti-interference, anti-noise, anti-multipath fading, low power spectrum and desired privacy, it has been more and more noticed closely. The key technologies contain the constitution of spread spectrum system, middle frequency receiving AGC, match filtering, seizure and tracking of PN code, differential demodulation and error control, etc. The constitution of spread spectrum system is the reflection of its function and main features. Spread spectrum receiver that has the function of narrowband interference suppression is one of them. The main issue, which is thought about, is the ability of narrowband interference suppression of spread spectrum system, while the spread factor may not be increased. The spread spectrum receiver this paper present is the one that has the ability of narrowband interference suppression. At the same time, noise and multiple access interference suppression function are also thought about. It has been well known that spread spectrum system itself has the ability of narrowband interference suppression. In the course of correlation despreading of the receiving signals, the desired spread spectrum signal is despread. At the same time the band of narrowband interference signal is spread to the band that spread spectrum code occupies. So the amplitude of narrowband interference signal lower, and the influence on useful signals decline. But this ability depends on the spread factor of spread spectrum system. The more spread factor is large, the more the ability of narrowband interference suppression is strengthened. On the other hand, since it is difficult to be performed, the spread factor of spread spectrum system cannot be too large. So the ability of narrowband interference rejection is limited, especially for the rejection of strong narrowband interference. Whereas study indicates that by adding preprocessing unit before correlation despreading unit in receivers, the narrowband interference rejection ability may be improved greatly. At the same time, spread factor spread spectrum system itself has may not be added. On the basis of this idea, many scholars present their own narrowband interference rejection methods, respectively. Based upon the study on the previous block transform domain narrowband interference suppression methods, by deduction, this paper derives a new spread spectrum receiver structure. This structure is got on the basis of the linear transform done being orthogonal transform. Based upon this condition, not only the previous correlation despreading unit and decision&detection unit are all moved into the orthogonal transform, but also the previous invert transform unit is omitted. Thus processing speed for signal is improved. Besides that, noise and multiple access interference suppression function are added. Then the deliberate property analysis is performed for the presented spread spectrum receiver. This analysis mainly contains the one of convergence speed, misadjustment and bit error rate. It is noticed that on the single tone interference and real orthogonal transform condition, the energy of base band bit signal, the power of interference and the average power of white Gaussian background noise decide the scope of convergence factor μ. Convergence speed depends upon the type of the selected orthogonal transform. It is unitedly decided by convergence factor μand the minimum eigenvalues of autocorrelation matrix of the input signal vector of the narrowband interference suppression unit. Misadjustment is independent of the selected orthogonal transform. It depends upon convergence factor μand the trace of the autocorrelation matrix of the received signal vector. And the bit error rate property is also independent of the selected orthogonal transform. It is decided by misadjustment, the autocorrelation matrix of the received signal vector and the designed parameter of noise and multiple access interference rejection unit. Besides that, the comparison with the property of the previous block transform spread spectrum receiver is carried out. It is noticed that since the presented spread spectrum receiver first despread the received signal in transform and then suppress narrowband interference, the adaptive unit only needs to learn the feature of narrowband interference. So convergence speed is superior to the one of the previous block transform spread spectrum receiver. For the bit error rate property, if only the band that the main energy of narrowband interference occupies contain smaller proportion of useful signal energy corresponding to the band to its total signal energy, after convergence speed is analogous with the one of the previous block transform spread spectrum receiver,narrowband interference suppression ability is superior to the one of the previous block transform spread spectrum receiver. By deduction, lapped transform domain adaptive spread spectrum receiver is derived. Previously, lapped transform is mainly applied to multirate coding of speech or image signal. The purpose is to avoid the blocking effects in the course of multirate coding. Since lapped transform is different from ordinary block transform, on the condition that the subband number of transform domain is equal to each other, the length of the base function of lapped transform is longer. And between the two or more adjoined blocks, part overlap exists. So when it is applied to adaptive narrowband interference suppression, the structure will be different from ordinary block transform receiver. And the property will be different, either. So the special study is carried out. Lapped transform adaptive spread spectrum receiver is presented. Here the noise and multiple access interference suppression function are also thought about. And the property analysis of the presented spread spectrum receiver is performed. It is noticed that its convergence speed also depends upon the selected orthogonal transform. It is also decided by convergence factor μand the minimum eigenvalues of autocorrelation matrix of the input signal vector of the narrowband interference rejection unit. The misadjustment property is different from the one of the forward spread spectrum receiver presented. It cannot be independent of the selected transform. It depends upon convergence factor μand the trace of autocorrelation matrix of the input signal vector of the narrowband interference suppression unit. The bit error rate property is also different from the one of the forward spread spectrum receiver. It cannot be independent of the selected orthogonal transform. It depends upon the steady state weight vector of narrowband interference rejection unit, misadjustment and the designed parameter of noise and multiple access interference suppression unit. The property of the presented modulated lapped transform and extended lapped transform spread spectrum receiver is compared with the one of the forward spread spectrum receiver. It is noticed that when the subband number of transform domain, signal to interference ratio, the band position of narrowband interference and signal to noise ratio are all the same, convergence speed is slower than the one of the forward spread spectrum receiver. For the bit error rate property, it is noticed that since the presented spread spectrum receiver (especially the extended lapped transform adaptive spread spectrum receiver) is influenced more seriously by noise, in the distance of small signal to noise ratio, when the convergence speed of the narrowband interference suppression unit is analogouswith the one of the forward presented spread spectrum receiver, bit error rate property is inferior to the one of the forward spread spectrum receiver. But in the distance of large signal to noise ratio, since the influence of noise has become unimportant, and since on the condition that the subband number of transform domain is equal to each other, the length of the base function of lapped transform is longer, the contaminated subband number is less in lapped transform domain. So when convergence speed of narrowband interference suppression unit is analogous with the one of the forward spread spectrum receiver, bit error rate property is superior to the one of the forward spread spectrum receiver too much. Thus the presented modulated lapped transform and extended lapped transform domain adaptive spread spectrum receiver fit for the larger signal to noise ratio situation. The new method of suppressing multi-tone interference is presented. Since the study on multi-tone interference is less performed before, and since not all single tone interference suppression methods fit for multi-tone interference rejection, the special study on multi-tone interference rejection is carried out. The method of utilizing wavelet packet excision is presented to suppress multi-tone interference. And it also fits for single tone interference rejection, for single tone interference is only a special case of multi-tone interference. The feature of this method is adaptive iteration process is not needed. So it fits for tracking rapid interference. And the calculation burden is not too much. Bit error rate is lower than the one of the previous methods. The presented method utilizes the feature of wavelet packet. That is, the energy leakage between the subbands is less, the isolation between the subbands is better, and the frequency resolution can reach too high. On the basis of these features, every frequency of multi-tone interference is positioned. The method of analyzing power spectrum is used to decide whether the element of signal in subspace is contaminated by interference. The existence of multi-tone interference is decided by inspecting whether the power spectrum of part of signal in every subspace changes abruptly. If interference does exist, the decomposition of subspace continues. Else, decomposition stops. So calculation burden is cut down. If the frequency resolution of the contaminated subspaces is equal to the preset value, decomposition stops. Then the part of signal in contaminated subspaces is cut off. By controlling the selection of the preset frequency resolution, we suppress multi-tone interference, meanwhile the energy loss of the useful signal is not too much. It ensures that the bit error rate property may not decline too much. During the course of simulation, Daubechies wavelet packet and larger N are adopted in order to improve the outcome of positioning interference. It is noticed that forboth single tone rejection and multi-tone rejection, the bit error rate property of the presented method is all superior to the one of the method in document [56]. The spread spectrum receiver whose spread factor is not equal to the blocking length is presented. The forward presented spread spectrum receivers are all derived on the condition that spread factor is equal to the length of a block. Based upon the study in forward chapters, spread spectrum receiver whose spread factor is not equal to the blocking length is derived. For the case that spread factor is less than the blocking length, the method of zero-pad in the end of spreading code block corresponding to every base band bit signal is adopted. And for the case that spread factor is integer times of the blocking length, by deduction, the method that adding buffering in transform, then summing, the outcome of summing is sent to suppress narrowband interference is derived. If spread factor is not just integer times of the blocking length, the method of combining the upper two methods is adopted. It is noticed that convergence speed and misadjustment of the presented spread spectrum receiver are elementarily in accordance with the ones of the spread spectrum receiver whose spread factor is equal to the blocking length. When convergence speed is analogous with each other, bit error rate property is not much different from the one of the spread spectrum receiver whose spread factor is equal to the blocking length. For the presented modulated lapped transform and extended lapped transform adaptive spread spectrum receiver, convergence speed is still slower. And they still fit for larger signal to noise ratio situation.