CN1201500C - Downlink Synchronization Acquisition Method in Code Division Multiple Access System - Google Patents

Abstract

The invention discloses a synchronous capturer applied to a down link of a CDMA communication system and a broadcast channel receiver for realizing the method. The method mainly comprises the following steps: adding a broadcast channel in a downlink, and placing a unique code in a fixed time slot in a broadcast channel frame; the mobile station broadcast channel receiver firstly finds out the position of the energy concentration of the main peak of each symbol, then determines the time for judging each symbol according to the position and judges the symbol; then the unique code is detected by a unique code matching filter, and if the unique code is detected, the synchronous information is given. The invention is realized by using a broadcast channel, a matched filter structure and a sliding correlation technique, quickly realizes the synchronous receiving of the downlink, and has high synchronous precision, simple method and easy realization.

Description

Downlink Synchronization Acquisition Method in Code Division Multiple Access System

technical field

The invention relates to any technical field of code division multiple access (CDMA) wireless communication, in particular to a method for capturing synchronization in a receiver device of a CDMA communication system.

Background of the invention

In recent years, due to the unique advantages of large capacity, soft capacity, soft handover, high voice quality and low transmission power, anti-interference and security, the CDMA system has developed rapidly, and has become the preferred technology for the third-generation cellular communication system .

CDMA is a modulation and multiple access connection technology based on spread spectrum communication. In the CDMA communication system, the signals used by different users to transmit information are not distinguished by different frequencies or different time slots, but by different code sequences. The receiver correlator can select a signal using a predetermined code pattern among a plurality of CDMA signals. Other signals using different code patterns cannot be demodulated because they are different from the code patterns generated locally by the receiver.

Usually in a CDMA system, both the base station transmitter and the mobile station transmitter use the spread spectrum code to spread the spectrum of the digital information transmitted, and the receiving device uses the local spread spectrum code to despread the spread spectrum signal to extract useful information . Synchronization is to realize that the local spread spectrum code is completely consistent with the received spread spectrum code in structure, frequency and phase. In addition to the carrier synchronization, bit synchronization, and frame synchronization of the general digital communication system, the CDMA system is unique to the synchronization of the spreading code. Therefore, the synchronization problem of the spread spectrum system is more complicated than that of the general digital communication system.

Generally speaking, the use of high-precision and high-stability clock frequency sources in transmitters and receivers can remove many frequency and phase instabilities. However, objectively there are many uncertain factors that cannot be estimated in advance, especially in mobile communications, these uncertain factors are random, cannot be compensated in advance, and can only be eliminated by a synchronization system. Therefore, the synchronization system is indispensable and very important in the CDMA system.

To sum up, the main causes of uncertainty are time uncertainty and frequency uncertainty, which are summarized as follows:

1. The distance between the transceivers causes the phase difference caused by the propagation delay.

2. The frequency difference caused by the relative instability of the clock frequency of the transceiver.

3. The Doppler frequency shift caused by the relative motion of the transceiver.

4. Frequency and phase changes caused by multipath effects.

Then, the synchronization of the spread spectrum code is divided into two steps: one is to capture, that is, to roughly adjust the frequency and phase of the local spread spectrum code, so that the phase difference between the local spread spectrum code and the received spread spectrum code is less than one symbol width Tc. The second is tracking, that is, to automatically adjust the phase of the local spreading code so that it is precisely synchronized with the frequency and phase of the receiving spreading code.

There are many traditional CDMA system acquisition methods, such as sliding correlation method, sequential estimation method, matched filter method, etc., among which the most basic and most commonly used is sliding correlation method.

Refer to Figure 1 for the block diagram of sliding correlation capture principle. In practice, the frequently used spreading codes are m-sequences or Gold sequences. The spread spectrum code has good autocorrelation characteristics. When the period N is very large, the autocorrelation function is very large and the secondary peak is small. The correlator is used to calculate the correlation function value of the received signal spread spectrum code and the local spread spectrum code, and according to This judges the phase difference. When the phase difference is greater than Tc, the signal output from the integral to the decision device is lower than the decision threshold, and a stepping pulse signal is sent at this time. When the relative time difference between the two codes is small, the output signal amplitude of the envelope detector is very large, and the signal level after integration is higher than the judgment threshold, and the judgment circuit sends out a tracking pulse signal after making a judgment, indicating that the time difference between the two codes is within the trackable range. range, start the tracking circuit, and the synchronization system is in the tracking state.

Sliding correlation acquisition is a sequential correlation calculation method for two spreading codes, which is relatively simple, but the disadvantage is that when the time or phase difference between the two spreading codes is too large, especially for long codes, the acquisition time is too long, so it must be Measures are taken to limit capture range and speed up capture time.

The sliding correlation method above calculates the autocorrelation function value R(τ) within the period of the spreading code, and thus determines and realizes the acquisition of the spreading code. However, when the spreading code is a long code for sliding correlation acquisition, the received code and the local code must be correlated and integrated within a complete cycle of the long code, and the correlation function needs to be gradually sliding to obtain the peak value of the correlation function. The disadvantage of this is that the capture time is too long and the implementation is complicated.

In the IS-95 CDMA system, the pilot spread code is a long code with a period of 215, and the chip rate is 1.2288Mcps. If there is no time reference, it is unrealistic to use the sliding correlation method to solve the capture of the spread code of. Therefore, the downlink channel of the IS-95 CDMA system is set with a pilot channel, and the mobile station adjusts and synchronizes with the spread code of the pilot channel, and transmits with the same phase of the spread code.

Contents of the invention

In order to solve the above-mentioned problems and defects in the prior art, the purpose of the present invention is to provide a new downlink synchronization acquisition method used in a code division multiple access communication system.

The present invention combines the broadcast channel, the matched filter structure and the sliding window to quickly realize the synchronous reception of the downlink, and has high synchronization precision, simple method and easy realization.

Objects and advantages of the present invention will become more apparent from the following description of a downlink synchronization acquisition method for a mobile communication CDMA system.

The downlink synchronous acquisition method of the present invention is mainly realized by using broadcast channel, matched filter and sliding correlation technology. Implementation of the method includes the following steps:

1) Add a broadcast channel in the downlink, and put a unique code in a fixed time slot in the broadcast channel frame; when the broadcast channel receiver of the mobile station detects the unique code, the frame positioning information can be determined.

2) In order to overcome the influence of the multipath fading channel, the broadcast channel synchronization loop first finds the position where the energy concentration of the main peak of each symbol appears, and then determines the decision time of each symbol of the broadcast channel receiver according to the position. If the judgment is correct, When the unique code matching filter detects the unique code, a relatively accurate synchronization information can be given.

Brief description of the drawings

Figure 1 is a schematic diagram of sliding correlation capture.

Fig. 2 shows a schematic structural diagram of a mobile station broadcast channel receiver designed according to the method of the present invention.

Fig. 3 shows a schematic diagram of a synchronization loop designed according to the method of the present invention.

Modes of Carrying Out the Invention

The present invention will be described in detail below through the embodiments and accompanying drawings.

Referring to Fig. 2, the broadcast channel receiver passes the received signal 21 through the demodulator 22 and the low-pass filter 23, and then sends the output I road and the Q road value of the matched filter 24, 24, after the square sum operation 25, the output The signal is fed into the synchronous loop.

The detailed structure of the synchronous loop is shown in Figure 3. The specific process is as follows:

A threshold value V0 is preset, and its value is determined according to actual experience, for example, it may be half of the main peak signal value. After the operation, the signal output by 25 is compared with the fixed threshold V0 in the comparator 301 . When a value exceeds the threshold V0, record this moment as T0, and skip 2I For the sample value of the width, take this moment as the starting time T=T0+ITc, the selection of I should consider the actual channel environment, and it is advisable to skip all the main peak positions of the symbol.

If the data entering the delay accumulator 302 at time T is recorded as D _T ,

through the delay accumulator 302 to put

$\underset{i=0}{\overset{K}{\mathrm{\Σ}}}{\mathrm{D.}}_{T+i\×\mathrm{NTc}}$ The value of is stored in RAM, denoted as R[1];

$\underset{i=0}{\overset{K}{\mathrm{\&Sigma;}}}{\mathrm{D.}}_{T+i\&times;\mathrm{NTc}+\frac{\mathrm{<figure-callout\; id="2"\; label="Tc"\; filenames="C0081877300101.png"\; state="\{\{state\}\}">Tc</figure-callout>}}{2}}$ The value of is stored in RAM, denoted as R[2];

$\underset{i=0}{\overset{K}{\mathrm{\&Sigma;}}}{\mathrm{D.}}_{T+i\&times;\mathrm{NTc}+\frac{\mathrm{<figure-callout\; id="2"\; label="Tc"\; filenames="C0081877300101.png"\; state="\{\{state\}\}">Tc</figure-callout>}}{2}\&times;2}$ The value of is stored in RAM, recorded as R[3];

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$\underset{i=0}{\overset{K}{\mathrm{\&Sigma;}}}{\mathrm{D.}}_{T+i\&times;\mathrm{NTc}+\frac{\mathrm{<figure-callout\; id="2"\; label="Tc"\; filenames="C0081877300101.png"\; state="\{\{state\}\}">Tc</figure-callout>}}{2}\&times;(2N-1)}$ The value of is stored in RAM, recorded as R[2N];

Among them, N is the length of the broadcast channel spreading code, and 2N is recorded because the receiver uses twice the chip rate. The purpose of the memory 303 is to write the first value accumulated by the accumulator into the RAM at T+KNTc moment. $T+\mathrm{KNTc}+\frac{\mathrm{<figure-callout\; id="2"\; label="Tc"\; filenames="C0081877300101.png"\; state="\{\{state\}\}">Tc</figure-callout>}}{2}$ Write the second value into RAM at all times until the $T+\mathrm{KNTc}+\frac{\mathrm{<figure-callout\; id="2"\; label="Tc"\; filenames="C0081877300101.png"\; state="\{\{state\}\}">Tc</figure-callout>}}{2}\&times;(2N-1)$ Write the 2Nth value into RAM at any time. The above operation is to circularly accumulate the corresponding position values of the K+1 symbols. The selection of K should ensure that most of the symbols in the broadcast frame are cyclically accumulated, so that the unique code can be detected in the next frame. The unique code is placed at the end of the broadcast frame.

With L as the window width, the sliding window 304 with a sliding step size of 1 slides from front to back in the 2N values in the RAM, and calculates the energy sum every time it slides to find out the window of the energy sum maximum (Emax); The location number of a window, Location (each number in RAM has a location number, there are 2N numbers in total, and the number of locations corresponds to 1, 2, 3...2N, and the number of locations written first is small).

The output of the sliding window 304 is sent to the comparator 305, if Emax＜E0, that is, most of the symbols of the broadcast channel frame are not captured, then return to enter the re-search state; if Emax＞E0, that is, most of the symbols of the broadcast channel frame are captured , then in $T+\mathrm{FT}+\mathrm{location}\&times;\frac{\mathrm{<figure-callout\; id="2"\; label="Tc"\; filenames="C0081877300101.png"\; state="\{\{state\}\}">Tc</figure-callout>}}{2}+i\&times;\mathrm{NTc}$ At the moment, the differential demodulator 306 starts differential demodulation, at $T+\mathrm{FT}+(\mathrm{location}+L)\&times;\frac{\mathrm{<figure-callout\; id="2"\; label="Tc"\; filenames="C0081877300101.png"\; state="\{\{state\}\}">Tc</figure-callout>}}{2}+i\&times;\mathrm{NTc}$ The moment differential demodulation is completed, and at $T+\mathrm{FT}+(\mathrm{location}+L)\&times;\frac{\mathrm{<figure-callout\; id="2"\; label="Tc"\; filenames="C0081877300101.png"\; state="\{\{state\}\}">Tc</figure-callout>}}{2}+i\&times;\mathrm{NTc}$ Make a decision for each symbol at any time, where i=0, 1, ... K, and send each decision value into the unique code matched filter 307 .

After unique code matching filter 307 detects unique code, enter counter 308, if through FT ms (frame length) unique code detection circuit detects unique code again, then through FT, unique code detection circuit detects unique code again, counter The value 308 is 3, indicating that the unique code is detected at the same position of the broadcast frame for three consecutive frames, then the synchronization capture is completed, and a synchronization signal 27 is sent.

After the synchronization capture is completed, delay Q×NTc+GapTc time, and the traffic channel receiver starts to work, where Q is related to the time slot where the unique code is placed in the broadcast channel frame, and Gap is the protection between the broadcast channel frame and the traffic channel frame interval. Afterwards, every time FT ms passes through the synchronization capture time, if the broadcast channel receiver synchronization loop detects a unique code, it will re-send synchronization information to the service receiver.

If there is no unique code detected in the time synchronization loop less than 4 frames, then the service receiver maintains the previous synchronization state; if the synchronization loop does not detect the unique code for 4 consecutive frames, the service receiver is turned off, The broadcast receiver performs sync acquisition again.

Claims (3)

1. synchronization capture method that is applied to the down link of cdma communication system is characterized in that this method may further comprise the steps at least:

In down link, add broadcast channel, and in broadcast channel frame fixedly time slot place the unique word be exclusively used in synchronous seizure; The broadcast channel receiver behind demodulation, low-pass filtering, matched filtering and related operation, is sent into synchronization loop with the signal that receives; Synchronization loop is at first found out the position of the concentration of energy that the main peak of each symbol occurs, and determines moment that each symbol is adjudicated according to this position then, and carries out symbol judgement; Secondly detect this unique word through the unique word matched filter, then provide synchronizing information if detect.

2. method according to claim 1 provides synchronizing information after it is characterized in that described unique word matched filter is continuous and detecting unique word three times.

3. mobile radio station broadcast channel receiver of implementing the described method of claim 1 is characterized in that this receiver comprises at least:

Demodulator, low pass filter, matched filter, related operation device and synchronization loop, wherein received signal after demodulation, low-pass filtering and matched filtering, is carried out related operation successively, and the output signal after the computing is sent into synchronization loop;

Synchronization loop comprises first comparator, delay accumulation device, memory module, sliding window module and second comparator, differential ference spiral device, unique word matched filter, counter; Form the connection of series connection form between described first comparator, delay accumulation device, memory module, sliding window module and second comparator, differential ference spiral device, unique word matched filter, the counter in order, another output of second comparator is connected with the input of synchronization loop; Synchronization loop moment of finding out the position of the concentration of energy that the main peak of each symbol occurs with first comparator, delay accumulation device, memory module, sliding window module and second comparator and determining according to this position each symbol is adjudicated wherein, separate the mediation symbol judgement with the differential ference spiral device, detect this unique word with unique word matched filter and counter, and provide synchronizing information.

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