Tag: MIMO-OFDM system frame synchronization
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0 Preface
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier transmission scheme. It is characterized in that each sub-carrier is orthogonal to each other, and the spectrum after the spread-spectrum modulation can overlap each other, which not only reduces mutual interference between sub-carriers, but also greatly Improved spectrum utilization. OFDM systems are well resistant to frequency selective fading and narrowband interference. MIMO (multiple people out) is a revolutionary antenna technology. The MIMO system is characterized by the multipath propagation becoming a favorable factor. It effectively uses random fading and multipath delay spread. Without increasing the spectral resources and antenna transmit power, it can not only utilize the spatial multiplexing gain provided by the MIMO channel to improve the capacity of the channel, but also can be utilized. The spatial diversity gain provided by the MIMO channel improves the reliability of the channel and reduces the bit error rate. The combination of MIMO technology and OFDM technology can increase the data transmission rate without increasing the bandwidth and transmission power, making the realization of high-speed wireless communication system possible. Therefore, MIMO-OFDM technology is widely used in advanced mobile communication systems such as Beyond 3G.
Synchronization is one of the key technologies for current MIMO-OFDM system research. With the increase of the number of antennas and the number of users, the number of transmitting antennas increases, the transmitted signals are not only interfered by ISI and ICI, but also inter-antenna interference, and the uncertainty of wireless channels increases, resulting in the synchronization of MIMO-OFDM systems in comparison with SISO. - OFDM systems are much more difficult. At present, there are few literatures on MIMO-OFDM synchronization, so it is necessary to study the synchronization for high-speed multi-antenna systems. So far, a large number of literatures have studied the synchronization technology of OFDM systems, which are roughly divided into two types based on cyclic prefix and training sequence. Among them, the synchronization algorithm based on training sequence proposed by Schmidl and Tufvesson et al. It is widely used in various high-speed wireless communication systems. However, they did not give the key parameters of synchronization - the setting method of synchronization decision threshold is incomplete, so this paper improves on their basis and proposes an effective threshold setting method.
1 MIMO-OFDM system model
If the number of transmitting antennas of the system is Q and the number of receiving antennas is L, considering the influence of different transmission antennas to different receiving antennas, the MIMO-OFDM system model is shown in Figure 1.
Let the length of the IFFT be N, and the signal outputted by the N-point IFFT transform is subjected to the cyclic expansion operation. The cyclic extension length G should be larger than the maximum delay spread to avoid inter-symbol interference, and then the signal is converted into an analog signal by AD conversion, and then passed. The upconversion is converted into a radio frequency signal and sent to the air; after the channel is transmitted, the received sampling signal is converted into a digital baseband signal by down-conversion and DA conversion, and each transmitting and receiving antenna pair is synchronously synchronized in the digital domain, and then the synchronized signal is used. After de-circulating expansion, an N-point FFT transform is performed.
Let Si(k), i=1, . . . Q be the transmission signal of the ith antenna. Without considering the frequency offset, the signal received by the ith receiving antenna can be expressed as:
Where wj(k) represents additive white Gaussian noise (AWGN) with a mean of 0, which is irrelevant for different i, k, j. Hij(k) is the channel impulse corresponding to the ith transmit antenna to the kth subcarrier of the jth receive antenna. Dij is the transmission delay of the ith transmit antenna to the jth receive antenna.
2 Introduction to various synchronization algorithms
The synchronization technique includes frame synchronization and carrier frequency synchronization. In an OFDM system, the receiver needs to first determine the starting moment of receiving the OFDM symbol, and then estimate the carrier frequency offset between the receiver and the transmitter to perform carrier frequency offset compensation. Finally, FFT demodulation is performed. If the accurate frame synchronization is not achieved, the resulting symbol error will cause the FFT window to be misaligned, resulting in intersymbol interference, so that the receiving end cannot receive the data correctly.
The OFDM synchronization method can be divided into a data-assisted synchronization method and a blind synchronization method. The data-assisted synchronization method requires a training sequence, which reduces the efficiency of data transmission, but such methods have the advantage of high estimation accuracy, and generally have low computational complexity. In the data-assisted synchronization method, an earlier one was proposed by Classen. In this paper, the pilots scattered in the OFDM symbol are used for frequency coarse synchronization and fine synchronization. The coarse synchronization is blind search within a certain range, and the calculation amount is very large. Big. Later, Schmidl improved this method. Schmidl uses two OFDM symbols as training sequences for time and frequency synchronization. The first half and the second half of the first symbol are the same. It can be used for time synchronization and frequency fine synchronization. The inter-symbol relationship is frequency coarsely synchronized. In the time synchronization method proposed by Schmidl, the top function of the time synchronization is flat and the synchronization is not very accurate. Tufvesson proposed a time synchronization algorithm based on PN sequence, which obtains time synchronization information by using the locally pre-stored PN sequence and the received data to find the maximum value. The advantage of this method is that the accuracy is relatively high, and coarse synchronization and fine synchronization can be obtained at the same time. In this paper, based on the training sequence sent by the sender, the key parameters of the receiver are improved to obtain better performance.
The blind synchronization method mainly includes the time and frequency synchronization method proposed by Van de Beek et al. using the cyclic prefix, and most of the latter are also improved on this basis. Since the cyclic prefix is ​​used to resist multipath time domain expansion, it does not need to add new overhead for synchronization, which improves system bandwidth efficiency. The blind synchronization method does not require additional data for the training sequence, and it has the advantage of high bandwidth efficiency. However, the blind synchronization method generally has the disadvantage of high computational complexity, and in the multipath fading channel, the CP is highly susceptible to multipath interference and destroys OFDM. The periodic nature of the symbol. This paper discusses a frame synchronization algorithm based on synchronous training sequence, which uses local training sequence and received codeword sequence to obtain time synchronization information, and has more accurate synchronization estimation performance in multipath fading channel.
3 Frame synchronization based on training sequence
Frame synchronization is to find the starting position of the OFDM frame. The traditional frame synchronization algorithm based on the training sequence is shown in Figure 2. The transmitting end first sends a training sequence of a certain length, which is generally a PN sequence. In the MIMO-OFDM system, the training sequence can be selected as a G sequence with good cross-correlation. The receiving end intercepts a search window (searching within a certain range) when each frame of data arrives, and the size of the search window can be selected according to the processing capability of the chip and the sampling rate of the received data, and the search window size is F. First, the first sampling phase point in the search window is used as the starting point of the data, and the received data is correlated with the conjugate sequence of the pre-stored local synchronous training sequence IFFT transform result {C(k)}, that is, the summation is performed. Then, slide to the next phase point to find the correlation value, and so on, a total of F related results can be obtained in the search window. Let the receiving sequence be {R(n)}, then the dth correlation value can be passed through the formula
The calculation is generated. After the F correlation results are squared, the maximum value is found, and compared with the threshold, the symbol position corresponding to the correlation value larger than the threshold is the frame start position.
Finally, the lock frame processing is introduced. When a certain timing position occurs continuously for a certain number of times, the timing position is set as the locked frame start position, and the lock frame state at this time is locked; when the lock frame state is locked, If there is a timing value different from the locked timing position, change the lock frame state to unlocked, and record the number of times when different timing values ​​appear. When it reaches a certain number of times, clear the locked timing value and restart the search for the frame header.
4 frame header decision threshold scheme and simulation results
Since OFDM has cyclic prefix protection, the starting position of the sought frame is only required to be within the cyclic prefix range. Therefore, the MIMO-OFDM system does not require high frame synchronization accuracy, but it requires high stability and fastness. If stable time synchronization is not achieved, a series of data processing processes such as FFT will be affected. This greatly affects the performance of the entire system. Therefore, stable time synchronization is more important for the entire MIMO-OFDM system. Due to the time-varying characteristics of the channel and the influence of multipath fading, the stability of frame synchronization is very difficult. Therefore, a new mechanism needs to be added on the basis of predecessors to ensure the stability of synchronization. This frame header decision threshold is the main factor affecting stability.
The traditional synchronization algorithm often uses a fixed correlation threshold as the decision threshold of the frame header. However, in a multi-antenna system, the transmission signals of multiple transmitting antennas all arrive at the same receiving antenna, and the wireless channel performs uncorrelated fading on multiple antennas. This results in a more severe and faster fading of the received signal, so the use of traditional fixed thresholds will result in a multiplication of simultaneous false alarms and false alarm rates. Introducing an adaptive decision threshold can adaptively adjust the fading of the received signal, which will improve the synchronization performance. Tufvesson's synchronization method only mentions that the decision threshold should be changed, and no specific method is given. Here, a method for determining the decision threshold by using the correlation energy is given, that is, the maximum value of the Pthreshold times of all the correlation energy averages is determined as the frame start position in the search window, and the solution obtains the position of the main path. .
Where F is the search window size, Λ(d) is the correlation result between the received sequence and the local sequence, and Pthreshold is the threshold coefficient. The denominator can be multiplied by the threshold coefficient as a threshold to compare with the maximum correlation value. Since the mean value of the correlation value changes with the channel change, the average value of the received signal amplitude is large, and the mean value of the received signal amplitude is small, so the threshold is adaptively changed as the amplitude of the received signal changes. It can effectively combat inter-symbol interference and multipath of wireless channels. In the actual implementation, since the correlation result Λ(d) has been calculated, F can also be multiplied to the right side of the inequality, so only the sum of the F related energy values ​​in each search window needs to be calculated, so the scheme does not increase the implementation complexity. .
In the LTE channel, the vehicle speed is 3 km/h and 120 km/h, and the frame synchronization range Range is 5 (the synchronization is calculated within the range of ±5 points in the frame header). The performance when the threshold is 30, 40, and 50 is shown in Figure 3. It can be seen that, whether it is low speed or high speed, the threshold coefficient is 30 to 40 and 50, and the correct detection probability is high, and when the signal-to-noise ratio is above 2 dB, the threshold coefficient 30 can make the correct detection probability reach 1. 4 is the probability of false detection of different synchronization ranges when the threshold is 30. It can be seen that when the synchronization range is 5, the probability of false detection can be zero. This shows that this solution is more suitable for the high speed environment of high-speed mobile communication in the future. In practice, the threshold of 30 and the Range 5 scheme have been implemented in the B3G-TDD MIMO-OFDM hardware platform with a carrier frequency of 3.41 GHz and a maximum rate of 100 Mbps.
5 Conclusion
As an alternative to current high-speed communication, MIMO-OFDM systems have attracted more and more attention. As the number of antennas and the number of users increase, frame synchronization becomes more and more difficult in implementation. Based on the predecessor algorithm, this paper proposes an adaptive threshold frame header decision scheme, which makes the algorithm more perfect and achieves good performance without increasing the complexity of the system. It is suitable for future high-speed mobile communication such as Beyond 3G. system.
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