What is PN Code (PN Code technology for CDMA systems)

The principle of PN Code synchronization in 1.CDMA system

The transmitter and receiver adopt high accuracy and high stability clock frequency source to ensure the stability of frequency and phase. But in the practical application, there are many uncertain factors, such as the instability of sending and receiving clocks, uncertainty of launch time, transmission delay and interference, especially in mobile communication, these uncertainties are random, can not be compensated in advance and can only be eliminated by synchronous system. Therefore, in CDMA spread spectrum communication, synchronization system is necessary.

PN Code Sequence synchronization is a special part of spread spectrum system, and also a difficult point in spread spectrum technology. The CDMA system requires that the local pseudo-random code of the receiver be identical with the PN code in the structure, frequency and phase, otherwise it will not receive the information sent, and receive only a noise. If the transceiver synchronization is achieved but not synchronized, it is not possible to obtain the information sent by the data accurately and reliably. Therefore, the synchronization of PN code sequence is the key technology of CDMA spread spectrum communication.

The PN code synchronization process in CDMA system is divided into PN code capture (fine synchronization) and PN code tracking (fine synchronization) two parts. PN Code capture is the frequency and phase of the local PN code, so that the locally generated PN code and the received PN code between the timing error less than 1 yards interval TC, can be based on a sliding-related serial capture scheme or delay estimation problem of parallel capture scheme. PN Code Tracking automatically adjusts the local code phase, further reducing the timing error, so that it is less than a fraction of the interval of the code slice, to achieve the local code and receive PN code frequency and phase of accurate synchronization. The typical PN code tracking loop is based on the delay locking ring and the tau jitter Ring of the sooner or later gate timing error detectors. (Computer science)

The receiving signal is connected with the local PN code in the multiplier after the wideband filter. The capture device adjusts the voltage-controlled clock source to adjust the frequency and phase of the local PN code sequence generated by the PN code generator and captures the useful signal. Once a useful signal is captured, the tracking device is used to adjust the voltage-controlled Zhongyuan, so that the local PN code generator is synchronized with the foreign signal accurately. If, for some reason, the step is lost, a new round of capture and tracking is restarted.

The synchronization process involves capturing and tracking the automatic control and adjustment of the two-stage closed loop.

2.PN Code sequence Capture

PN Code sequence capture refers to the receiver in the beginning to receive the spread spectrum signal, select and adjust the receiver's local spread spectrum PN sequence phase, so that it and transmit the spread spectrum PN sequence phase, that is, the receiver to capture the spread of the PN sequence phase, also known as spread spectrum PN sequence initial synchronization. In the receiver of CDMA system, the general process of the expansion is in the carrier synchronization forward line, most of the implementation of the capture is incoherent detection. After receiving the spread spectrum signal, after the RF wideband filter amplification and the carrier demodulation, sent to the 2N spread Spectrum PN sequence Correlation branch to understand the diffuser (n is the spread spectrum PN sequence length). Which output is the largest in 2N output, corresponding to the output of the extension of the spread spectrum PN sequence Phase state, is sent spread spectrum signal spread spectrum PN sequence phase, so as to complete the spread spectrum PN sequence capture.

There are many methods of capturing, such as sliding coherence method, sequential estimation method and matched filter method, and the sliding correlation method is the most common method.

2.1 Sliding Correlation method

The receiver system in the search synchronization, its code sequence generator with the transmitter code sequence generator at different speed, resulting in these two code sequence in the phase of each other, only to reach the point of convergence, only to stop, so called the sliding correlation method.

The received signal is multiplied with the local PN code, the correlation value is obtained, and then a threshold value is compared with the threshold detector to determine whether the useful signal has been captured. It utilizes the correlation of PN code sequence, when two identical code sequence phase is consistent, its correlation value output is maximum. Once the capture is confirmed, the synchronous pulse of the signal is captured to control the search control clock, and the PN code repetition frequency and phase are adjusted to keep the signals in sync with the received signal.

Because the sliding correlator is related to the sequence of two PN codes, this method is also called sequential search method. Sliding Correlator is simple, the disadvantage is that when the time difference of two PN codes is too large, the relative sliding speed is guijie slow, which leads to the long searching times, especially the long PN code capturing time, must take measures to limit the capture range, speed up the capture time and improve the performance of the Guijie.

One of the effective methods to make the sliding correlator useful is to use special code sequence, which is short enough to search all code bits within a reasonable time. As to what extent, it is limited by satisfying the relevance requirement. This method of adding the front code is called the synchronous guidance method. The synchronization of Guide code is low, simple and easy to realize, and it is a synchronization method suitable for various applications.

The catch code is composed of several short code sequences, and the code sequence should maintain a certain correlation with the sequence of each constituent code. The most famous of these codes is the JPL code.

2.2 Sequential Valuation method

Sequential estimation method is another fast method to reduce the acquisition time of long code, it will receive the PN code sequence directly into the local code generator shift register, forced to change the starting state of the registers at all levels, so that the PN code produced by the same phase with the foreign code, the system can be immediately synchronized tracking State, Shortens the time required for local PN codes to be consistent with the foreign PN code phase.

The method first detects the PN code in the received code signal, and sends the shift register of the N-level PN code generator through the switch. When the whole code sequence is filled, the generated PN code is correlated with the received code signal in the Correlator, and the result is compared with the threshold in the comparator. If the threshold is not exceeded, the above process continues. If the threshold is exceeded, the search is stopped and the system goes to the tracking state. Ideally, the capture time is TS=NTC (TC is the PN code slice time width). This method has a short capture time, but there are some problems, it is necessary to detect the foreign PN code before it can be fed into the shift register, it is sometimes difficult to do this. In addition, the anti-interference ability of this method is very poor, because each time slice carries on the valuation and the judgment, does not use the PN code the anti-interference characteristic. But in the absence of interference, it still has good fast initial synchronization performance.

2.3 Matched Filter Method

The matched filter used for PN synchronization capture usually uses a delay line matching filter, which is designed to identify sequences, which can recognize special sequences in special structures and only recognize the sequence. Suppose an input signal is a 7bit code sequence 1110010 dual-phase modulated signal, whenever the code has 1-0 transitions, the counter believe number enters the delay line until the 1bit in T7, the first 2bit in T6. When all time delay elements are filled, and the signal modulation code is in accordance with the phase of the filter delay element, the phase of the T2 signal is the same as that of T5, T6 and T7, and the delay element T1, T3 and T4 also have the same signal phase. Put {T2, T5, T6, T7} and {T1, T3, T4} are added separately, the {T1, T3, T4} and the inverted output, and then add the two results, included in all 7 components of the signal energy together, the entire output is 7 times times the unhandled. The code sequence can be identified according to the energy relation.

To enhance the generated signal, it can be achieved by attaching more delay elements, and the processing gain in this structure is GP=10LGN (n is the number of Shiyan that participate in the summation).

It is advisable to use SAW-TDL-MF as Synchronizer in the CDMA spread spectrum communication which requires fast locking and long code use. For the undetermined signal, the matched filter has the time automatic ability, does not need the PN code clock synchronization and the RF carrier phase lock, not only avoids the data information bit synchronization, but also completes the spread spectrum signal correlation processing. After the guide code enters the programmed coding SAW-TDL-MF, its output is the bottom tip correlation peak on the noise base. In the spread spectrum communication, the noise power control receiver's AGC, thus the signal power (i.e. the correlation peak) varies greatly in the fluctuating noise environment. The function of the threshold calculator according to the envelope detection output, determines the dynamic gate power level, provides to the synchronous detector, guaranteed that has the permissible synchronization error in the low Snr. When the dynamic gate power is taken between the peak height and the maximum side peak, the bottom synchronization error caused by the noise is the smallest. When the SAW-TDL detector output envelope exceeds the dynamic gate limit, the synchronous detector provides a logic level synchronization signal for the receiver Wideband frequency synthesizer.

3.PN Code sequence Tracking

When the synchronization system completes the capture process, the synchronization system goes to the tracking state. The so-called tracking, is to make the phase of the local code with the received pseudo random code phase changes, and received the pseudo-random code to maintain a more accurate synchronization. The tracking loop constantly corrects the clock phase of the local sequence, which keeps the phase change of the local sequence consistent with the phase change of the receiving signal, realizes the phase locking of the receiving signal, makes the synchronization error as small as possible and receives the spread spectrum signal normally. Tracking is closed-loop operation, when the two-phase difference, the loop can automatically adjust according to the error size, reduce the error, so the synchronization system uses the lock phase technology.

The tracking loop can be divided into two kinds: coherence and incoherent. The former works under the condition of knowing the carrier frequency and phase of the originator signal, and the latter is working under unknown conditions. In fact, most applications belong to the latter. The commonly used tracking loop has two kinds of delay locking ring and tau jitter Ring, two independent correlator is used in the delayed locking loop, and the Tau jitter ring adopts a time-sharing single correlator.

3.1 Delay phase-locked loop

When the output PN code phase of the local PN code generator (n-2) and nth shift register is ahead of the received pseudo random code phase (i.e., the relative time difference of two yards) 0<τ

3.2τ jitter Tracking Ring

The dither ring is another form of the tracking loop, which is the same as the delay locking ring, and the receiving signal is related to the advanced hysteresis form of the local PN sequence, and the error signal is obtained by the correlation of the single correlator in alternating form. PN Code Sequence Generator by a signal-driven, clock signal phase two signal changes back and forth "swing", except to ensure that two channel transfer function the same requirements, so the jitter loop implementation is simple. Compared with the delay locking ring, the SNR performance deteriorates about 3dB.

The delay locking ring and the tau jitter ring can not only play a tracking role, but if the concept of sliding is adopted, the local VCO will have a certain frequency difference with the receiving signal at the beginning, and also can play the capturing function. In addition, an associated device can also play a decoding role.

The main tracking object of the two tracking loops is the single path signal, but in the mobile channel, due to the multipath fading and Doppler frequency shift and other complex factors, can not get satisfactory tracking performance, so CDMA spread spectrum communication system should be suitable for multipath fading channel tracking loop. The timing tracking ring based on the center of Energy window is one of them.

CDMA Digital Cellular mobile system adopts spread spectrum technology, and its spread spectrum bandwidth makes the system have strong multipath resolution ability. The receiver continuously searches for the distinguished multipath signal component, by selecting the most powerful J Multipath component as the Energy window, using the timing tracking algorithm based on the Energy window center of gravity, this paper observes the Multipath energy distribution in the adjacent two working windows, calculates the tracking error function, adjusts the local PN code clock according to the change of the energy barycenter, and controls the PN code sliding, To achieve tracking purposes. The aim of this tracking loop is to use the maximum of multipath energy in the working window which rake receives, and the receiver performance is better. The simulation results show that, compared with DLL tracking single diameter, it is better to use the time tracking method based on the Energy window center of gravity to track the effective multipath components.