CN1152496C - An Adaptive Method for Code Division Multiple Access Wireless Communication System - Google Patents

Abstract

The present invention relates to a self-adapting method for a code division multiple access wireless communication system. At least two kinds of alternative address codes are arranged between an addresser and an addressee; address codes used by communication are dynamically selected on the basis of the real-time measurement of delay spread and Doppler frequency shift; under the condition that the delay spread is larger than a preset threshold value X1, the addresser and the addressee use address codes, and intervals between code pieces of the address codes are smaller than the delay spread corresponding to the threshold value X1; under the condition that the Doppler frequency shift is larger than the preset threshold value X2, the addresser and the addressee use address codes, and the width of zero correlation windows or minimum correlation windows of the address codes is larger than correlation time corresponding to the threshold value X2.

Description

The adaptive approach of code division multi-address radio communication system

Technical field

The present invention relates to the CDMA radio communication field, particularly relate to a kind of adaptive approach of code division multi-address radio communication system.

Background technology

The technology of existing mobile communications system has adopted direct sequence spread spectrum skill (DirectSequence), the narrow-band symbol data of low rate is spread on the very high broadband of spreading rate, therefore under the bigger situation of delay expansion that the multipath of channel causes, the delay expansion time that the spreading code chip-spaced time causes much smaller than the multipath of channel, on frequency, promptly coherence bandwidth transmits shared bandwidth much smaller than symbol.What coherence bandwidth and delay expansion were approximate here is inverse relation.Therefore, under the situation that receiver employing tapped delay is mated, the scope of the multipath that can search for is just bigger, and therefore the effect of inherent frequency diversity is just bigger, the better performances of system.But under the less situation of spreading factor, SF=8,16 etc. for example, the effect of inherent frequency diversity is less, so performance is just relatively poor.Special under the big situation of Doppler frequency-shift, for example when travelling carriage moves with higher speed, channel is along with the variation of time, variation very fast.Even when spreading factor is big, also be difficult to overcome the frequency displacement that travelling carriage moves and causes with 500 kilometers speed.

On the other hand, also have another kind of spectrum-spreading address code, the code word that different user adopts in the system has following characteristic: promptly the auto-correlation of code word and cross-correlation correlation in a window is zero, perhaps all less than a thresholding v.With the cross-correlation is example, i.e. the translation of a code word and another code word is carried out relevant, when translation chip number between [m, m] time, correlation is zero, perhaps less than thresholding v.This window [m, m] is zero correlation window, perhaps minimum associated window.

Be ZL97116324.3 for example in China Patent No., name is called in the Chinese patent of " a kind of coding method of spectrum-spreading address code ", a kind of technology based on time diversity is proposed, promptly adopt a kind of special channel code LA sign indicating number, the characteristics of this channel code are that correlation is very little between the translation of chip arbitrarily of channel code itself and channel code, so receiver can be comparatively strict differentiates different multipath signals.Therefore, based on the system of this sign indicating number track channel change fast.Can effectively overcome Doppler frequency-shift, but the empty accounting of this code book body is very high, promptly to reach same propagation rate, need more band resource, in order to improve band efficiency, adopt the method for multicarrier among the ZL97116324.3, promptly nonzero element takies a carrier wave in the LA sign indicating number, promptly narrower bandwidth, other carrier wave uses for other user.Therefore, inherent frequency diversity is less, but time diversity is bigger, and the interval between promptly adjacent two non-zero chips is bigger.Although this spectrum-spreading address code can overcome high Doppler frequency-shift, for overcoming still existing problems of delay expansion that multipath causes.

τ is defined as the delay scope of multipath, and D is defined as Doppler (multispectral reining in) frequency.About realizing system environments is monitored judgement in the prior art, according to JG.Proakis " DigitalCommunication " third edition Chapter 14, multidiameter scope and Doppler frequency are had been described in detail and define, so multidiameter scope and Doppler frequency are the general knowledge that digital communicating field is known altogether.According to the definition in the JG.Proakis book, can directly calculate.Delay can be adopted the method for the matched filtering of the tapped delay in the JG.Proakis book, and the Doppler frequency can adopt the method for Fourier (Fourier) conversion according to definition.In fact can adopt a lot of methods to estimate.For example a kind of delay method of estimation commonly used is seen paper [1] Satoru Fukumoto, etc, Matched filter based rake combiner forwideband ds-cdma mobile radio, IEICE Trans.Commun.No 7.July 1998.A kind of method of Doppler Frequency Estimation commonly used is seen paper [2] Performance Of PilotCarrier Based Doppler Compensation For CDMA Mobile Satellite Links (1996) Jaakko Talvitie, Gordon Povey; [3] Glisic S.G., TalVitie J., Kumpum  ki T., La tva-aho M., Iinatti J., " Design Study for a CDMA BasedLEO Satellite Network:Downlink System Level Parameters " in BiglieriE., Luise M. (eds.), Signal Processing for Telecommunications, Springer-Verlag, London, 1996.

Summary of the invention

The objective of the invention is to propose a kind of adaptive approach of code division multi-address radio communication system, utilize this method effectively to overcome and postpone expansion and Doppler frequency-shift, thereby significantly improve the speech quality of system.

The object of the present invention is achieved like this: a kind of adaptive approach of code division multi-address radio communication system, wherein set at least two kinds of alternative address codes, between the originator and destination of system, based on the delay expansion in time that multipath signal is caused and the real-time measurement of Doppler frequency-shift, dynamically select the employed address code of communication; And postponing under the situation of expansion greater than predefined thresholding X1, the address code that is adopted between originator and the destination, the time delay expansion that the time interval between its chip causes less than multipath, under the situation of Doppler frequency-shift greater than predefined thresholding X2, the address code that adopts between originator and the destination, it has zero correlation window or minimum associated window, and the width of this zero correlation window or minimum associated window greater than with thresholding X2 corresponding coherence time.

Core concept of the present invention is dynamically to select different address codes at different Channel Transmission conditions.The invention enables when the multidiameter expansion is big, adopt the communication mode of frequency diversity to communicate; For the bigger system of Doppler frequency-shift, then adopt the communication mode of time diversity to communicate.Thereby overcome existing technology or can only overcome the delay expansion that multipath causes, powerless to higher Doppler frequency-shift, or can only overcome higher Doppler frequency-shift, and helpless shortcoming is expanded in the delay that multipath causes.Utilize code division multi-address radio communication system of the present invention, can improve the adaptive capacity of system significantly, guarantee the speech quality under various mal-conditions.

Description of drawings

For making the purpose, technical solutions and advantages of the present invention clearer, by the following examples, and with reference to accompanying drawing, the present invention is described in more detail.

Fig. 1 shows the switching flow figure of the code division multi-address radio communication system that utilizes adaptive approach of the present invention.

Embodiment

The communication system that the present invention relates to is a code division multiple address communication system, and promptly each user's signal is to distinguish by different sign indicating numbers.The transmission of the signal of a user's a symbol is that the chip pulse sequence of n obtains by sending length, wherein n claims spreading factor, this length is that the chip pulse sequence of n becomes a spreading code, and also promptly a spreading code is made up of several chip pulse (claiming chip again).Chip pulse can only be got+1 ,-1 two possible values with mathematical symbolism.The time interval between two chips (being chip pulse) of spreading code can be the same or different.

The residing environment of communication system real-time detecting system, delay extended by tau that multipath signal causes in the environment of measuring system and Doppler frequency-shift D.Wherein postpone expansion and be the continuation that the signal of receiving that the multipath effect of signal causes produces in time, reflected the characteristic of multipath transmisstion.Doppler frequency-shift is then because the frequency displacement in the frequency displacement that causes apart from the variation along with the time between travelling carriage and the base station reflects the characteristic that channel changed along with the time.The big more then Doppler frequency-shift of movement velocity is big more.According to judgment criterion (1) τ＞X1, (2) D＞X2, wherein X1 and X2 are two thresholdings, does not set up simultaneously criterion (1) and (2) in general, if set up simultaneously then channel is in the very serious stage that time and frequency spectrum all spread, be difficult to accurate receiving demodulation signal.

If (1) set up, then the environment at system place is in the environment of frequency selective fading.Be that the time delay expansion is enough big.At this moment, can select suitable sign indicating number and spreading rate, the correlation bandwidth that makes multipath cause is less than signal far away and propagates shared bandwidth, from the time consideration, gets final product so that the time interval between the chip expands much smaller than the time delay that multipath causes.Therefore can adopt precision the recipient is the tapped delay line structure of chip-spaced, obtains the different multipath of signal.Here when set up (1), its main feature of the frequency diversity communication mode of employing was to adopt different sign indicating numbers to distinguish different users, and the spaced far between chip is expanded less than time delay.

If (2) set up, then the residing environment of system is the environment of time selective fading.Be that Doppler frequency-shift is bigger.Doppler frequency-shift has determined the coherence time of channel, and both are roughly reciprocal relation.Like this, from the time, the speed that channel changed along with the time is very fast, and promptly coherence time is less.At this moment, adopt another code division multiple access mode, i.e. the code division multiple access mode of time diversity.Equally, different users distinguishes with different code words.But because coherence time is less, at this moment the code word that different user adopts in the system can have following characteristic: promptly the auto-correlation of code word and cross-correlation correlation in a window is zero, perhaps all less than a thresholding v.With the cross-correlation is example, i.e. the translation of a code word and another code word is carried out relevant, when translation chip number between [m, m] time, correlation is zero, perhaps less than thresholding v.This window [m, m] is zero correlation window, perhaps minimum associated window.

Because coherence time is less, we can select the length of associated window greater than coherence time.Like this, in correlation window, different code word correlations is very little, and the fading signal that we just can strict cause Doppler frequency-shift makes a distinction.The key of this communication mode is that the length of zero correlation window or this minimum associated window surpasses or near size coherence time, like this, just looks like to be to repeat to have sent several times at recipient's signal.

For the consideration on designing, can adopt for example address code of LA sign indicating number.The time interval that is chip under the time diversity mode can be irregular, and promptly the interval between two non-zero chips is not necessarily identical.

Fig. 1 shows the switching flow figure of the code division multi-address radio communication system that utilizes adaptive approach of the present invention.Please refer to Fig. 1, according to measurement, select to adopt frequency diversity mode or time diversity mode, and proceed real-time measurement between destination and the originator, and continue the diversity mode that selection adapts system environments.Environment around the basis of system self-adaption switches between time diversity mode and frequency diversity mode, reaches the optimum of performance.

Preferably, described originator and destination can be respectively base station or travelling carriage.

Preferably, described alternative address code can be distinguished corresponding different spreading rates, when the transceiver both sides that communicate by letter have dynamically selected to communicate by letter employed address code, and the also selected simultaneously and corresponding spreading rate of selected address sign indicating number.

Preferably, described alternative address code can correspondingly respectively be applied on the different frequency bands, in the described employed address code of dynamically selecting to communicate by letter, and the also selected and corresponding frequency band of selected address sign indicating number.Also promptly utilize different frequencies that two kinds of address codes are separately used, travelling carriage switches between two kinds of frequencies and address code according to the measurement result to environment, thereby reaches best communication efficiency.

Preferably, described alternative address code also can be applied in respectively on the different time bursts, in the described employed address code of dynamically selecting to communicate by letter, and the also selected and corresponding time bursts of selected address sign indicating number.Also promptly utilize different time bursts that two kinds of address codes are separately used, travelling carriage, also switches on the corresponding time bursts when switching address code according to the measurement result to environment, thereby reaches best communication efficiency.

Preferably, postponing under the situation of expansion greater than predefined thresholding X1, the address code that is adopted between originator and the destination, the time interval between its chip can be much smaller than expanding with the corresponding time delay of thresholding X1, to reach better effect.

Preferably, under the situation of Doppler frequency-shift greater than predefined thresholding X2, the address code that is adopted between originator and the destination, the width of its zero correlation window or minimum associated window can greater than and near with thresholding X2 corresponding coherence time, to reach better effect.

Preferably, under the situation of Doppler frequency-shift greater than predefined thresholding X2, between originator and the destination the address code that adopts can be the LA sign indicating number.

Above embodiment is only unrestricted in order to explanation the present invention, although the present invention is had been described in detail with reference to preferred embodiment, those of ordinary skill in the art is to be understood that, can make amendment or be equal to replacement the present invention, and not breaking away from the spirit and scope of the present invention, it all should be encompassed in the middle of the claim scope of the present invention.

Claims (6)

1. An adaptive method for a code division multiple access wireless communication system, characterized in that at least two alternative address codes are set, between the sending party and the receiving party of the system, based on multipath signals resulting in Real-time measurement of the delay spread in time and Doppler frequency shift, dynamically select the address code used for communication; and when the delay spread is greater than the preset threshold X1, the communication between the sender and the receiver The address code used between the chips, the time interval between the chips is less than the delay spread corresponding to the threshold X1, when the Doppler frequency shift is greater than the preset threshold X2, the distance between the sender and the receiver The address code used in the interval has a zero correlation window or a minimum correlation window, and the width of the zero correlation window or minimum correlation window is greater than the coherence time corresponding to the threshold X2. 2. The self-adaptive method of the code division multiple access wireless communication system according to claim 1, characterized in that, the sender and the receiver are respectively a base station or a mobile station. 3. The adaptive method of the code division multiple access wireless communication system as claimed in claim 1, characterized in that, said alternative address codes correspond to different chip rates respectively, and said dynamically selecting the used code for communication At the same time as the address code, the chip rate corresponding to the selected address code is also selected. 4. The adaptive method of the code division multiple access wireless communication system as claimed in claim 1, characterized in that, said alternative address codes are correspondingly applied to different frequency bands respectively, and are used in said dynamically selected communication At the same time as the address code, the frequency band corresponding to the selected address code is also selected. 5. The self-adaptive method of the code division multiple access wireless communication system as claimed in claim 1, characterized in that, said alternative address codes are respectively applied to different time bursts, and said dynamically selected communication place At the same time as the address code used, the time burst corresponding to the selected address code is also selected. 6. The adaptive method of the code division multiple access wireless communication system as claimed in claim 1, 2, 3, 4, 5, characterized in that, when the Doppler frequency shift is greater than the preset threshold X2, The address code used between the sender and the receiver is the LA code.