OFDM technology and Performance Analysis

Abstract: As more and more wireless devices enter people's lives, a large number of multimedia services also emerge. As multimedia transmission services require a certain amount of bandwidth, the bandwidth of traditional wireless networks also shows some shortcomings in expanding multimedia services. The emergence of OFDM modulation technology solves the bandwidth requirements of users to a certain extent. In addition, the OFDM modulation technology is designed for wireless networks. It can make full use of the existing bandwidth and effectively defend against frequency selective fading or narrowband interference. The 4G standard will be used as the underlying modulation technology to provide higher transmission quality.

1. Introduction to OFDM

In traditional multi-carrier communication systems, the entire system band is divided into several separate sub-channels, that is, the so-called carrier. In order to avoid interference between channels, there is usually a certain width of protection interval between channels. The receiving end uses a filter to separate each sub-channel and then receives the required information. In this way, although mutual interference between different channels can be avoided, the frequency utilization rate is sacrificed. In addition, when the number of subchannels is large, it is almost impossible to set a filter that separates the signal of each subchannel.

In the middle of the 20th century, people proposed a multi-carrier communication scheme with overlapping bandwidths, and used orthogonal carrier frequencies as subcarriers, this is what we call orthogonal frequency division multiplexing (OFDM) technology. This "orthogonal" represents the exact mathematical relationship between carrier frequencies. According to this assumption, OFDM can not only make full use of channel bandwidth, but also avoid using high-speed balancing and anti-Burst Noise errors.

Currently, OFDM has been adopted by many foreign standards, such as the hiperl-An/2 standard of IEEE 802.11a and ETSI (European Institute of Communication Standards, the application of the wired transmission system also uses the OFDM-based modulation multiplexing technology, such as the application of the XDSL discrete multi-Audio System and the wired modulation.

OFDM is a special multi-carrier modulation technology. User information must first be converted into multiple low-rate data streams through serial to parallel conversion. After encoding, modulation is a RF signal. The traditional modulation technology can transmit data at only one frequency at the same time, while OFDM can send multiple signals at the same time, it can be said that it is a parallel transmission of multiple signals, so that OFDM can make full use of the channel bandwidth. OFDM does not use a band-pass filter to separate subcarriers, but uses the Fast Fourier Transform (FFT) to select waveforms that can maintain orthogonal even if they are overlapped.

Although OFDM is still a type of Frequency Division Multiplexing (OFDM), it is completely different from the previous OFDM. In fact, the OFDM receiver is a set of receivers implemented by FFT. It moves different carriers to zero frequency, and then points in a code element cycle. Because other carrier signals are orthogonal to the signals of the points, the information extraction will not be affected. The data transmission rate of OFDM is also related to the number of subcarriers.

Subcarriers In the OFDM system can select modulation modes based on the channel conditions and switch between modulation modes. The principle of selection and switching is the balance between spectrum utilization and bit error rate. In general communication systems, in order to maintain a certain degree of reliability, we choose to adopt the power control and adaptive modulation coordination technology. When the channel is good, the transmit power remains unchanged. The modulation method (such as 64 QAM) can be enhanced, or the transmit power can be reduced when the channel is low (such as PSK. Power Control and adaptive modulation should be balanced, that is, for a remote transmitter, it has a good channel. If the transmission power remains unchanged, a high modulation scheme such as 64 QAM can be used; if the power can be reduced, the modulation scheme will also be reduced accordingly, and you can use the PSK.

2. Principles of OFDM signal sending and receiving

The basic principle of the OFDM system is to divide the specified channel into many orthogonal subchannels and conduct narrowband modulation and transmission on each subchannel. The signal bandwidth is less than the relevant bandwidth of the channel.

The following steps are required to send OFDM signals:

(1) encoding: In Systems Based on OFDM modulation technology, the encoding uses Reed-Solomon Code, convolution Error Correction Code, vitercode or Turbo code.

(2) intertwined: the intertwined machine is used to reduce unexpected errors in the data channel. The intertwined data is mapped to a corresponding constellation chart through a series of parallel converters. Here I represents the same phase signal, and Q represents the orthogonal signal.

(3) digital modulation: In the OFDM method, the constellation chart is used to map the symbol to the corresponding constellation point. This process generates IQ values that are filtered and sent to IFFT for transformation.

(4) Insert guide: four guide information is inserted into each 48 sub-carriers to ensure stable reception.

(5) serial parallel conversion: enables the serial input signal to be output to M lines in parallel. The data transmission rate on any one of the M lines is R/M codeword/second.

(6) Fast Fourier inverse transformation: The Fast Fourier inverse transformation can convert discrete data in the frequency domain into discrete data in the time domain. As a result, the user's original input data is processed by OFDM according to the frequency domain data.

(7) Parallel String Conversion: used to convert parallel data into serial data.

(8) insert the cyclic prefix and add a window: the cyclic prefix creates a protection band for a single OFDM symbol, which is discarded in the signal-to-noise ratio (SNR) Edge Loss, greatly reducing inter-symbol interference.

The receiver completes operations opposite to the sender. The receiver receives a time-domain signal. Due to some changes in the influence of wireless channels, we must first estimate the training sequence timing and frequency offset, and transmit the timing information of the symbol to the decirculating prefix function module, here, the training sequence and pilot information are mainly used for channel error correction. Then, the signal goes through a serial converter and the cycle prefix is cleared. Clearing the cyclic prefix does not delete any information. The information in the cyclic prefix is redundant. The cyclic prefix is used to ensure the establishment of the convolution feature mentioned above. In general, the entire receiving process requires the following steps:

① Timing and frequency synchronization, ② decyclic prefix, ③ string parallel conversion, ④ fast Fourier transformation, ⑤ parallel string conversion, ⑥ channel correction, 7digital demodulation, cosine deinterlace, and Cosine demodulated.

3. Key Technologies of OFDM

3.1 Synchronization Technology

In the OFDM system, parallel transmission of N symbols will extend the duration of the symbol, so it is not sensitive to Time deviation. For wireless communication, there is a time-varying channel. The frequency offset in the transmission will cause the orthogonal behavior between subcarriers of the OFDM system to be damaged, and the phase noise will also cause great damage to the system.

Because the sampling clock deviation between the sending and receiving ends, each signal sample deviates from its correct sampling time to a certain extent. This deviation increases linearly as the number of samples increases, although the time deviation damages the orthogonal behavior between subcarriers, it is usually negligible. When sampling errors can be corrected, the interpolation filter can be used to control the correct time for sampling.

Phase Noise has two fundamental influences. One is to introduce a random phase variable to all subcarriers. The tracking technology and difference detection can be used to reduce the influence of the Common Phase Error, A certain amount of inter-channel interference (ICI) will also be introduced, because of the phase error, the sub-carrier interval is no longer accurate 1/T.

The offset of the carrier frequency can cause interference between sub-channels. The output signal of the OFDM system is the superposition of multiple subchannels that overlap each other, And the orthogonal between them has strict requirements. A specific manifestation of time-varying wireless channels is the Doppler frequency shift, which is proportional to the carrier frequency and the moving speed of the Mobile Station. Doppler broadening may cause frequency dispersion and signal distortion. From the frequency domain, the signal distortion will increase with the increase of Doppler expansion of the sending channel. Therefore, for OFDM systems that require strict sub-carrier synchronization, the impact of carrier frequency offset will be more serious. If we do not take measures to overcome this inter-channel interference (ICI, the system performance is hard to be improved.

Synchronization in OFDM usually includes three aspects:

① Frame detection, ② carrier frequency deviation and correction, ③ sampling deviation and correction.

Because synchronization is a difficult point in OFDM technology, many people have also proposed many OFDM Synchronization algorithms, mainly for loop expansion and special training sequences and pilot signals, among them, esprit synchronization algorithm and ML estimation algorithm using Singular Value Decomposition are commonly used. Although the estimation accuracy of ESPRIT algorithm is high, the calculation is complicated and the calculation is large, the ml algorithm uses the cyclic prefix of the OFDM signal to effectively perform joint estimation of the frequency offset and Time offset of the OFDM signal. In addition, compared with the ESPRIT algorithm, the algorithm requires much less computation. There are many researches on the synchronization algorithms of OFDM technology. It is feasible to use various algorithms for joint estimation based on the specific system design and research. The timing frequency offset of the OFDM system is less than 4% of the OFDM symbol interval, and the frequency offset is less than 1% of the subcarrier interval ~ 2%, the system generates 0.01% ~ 0.1%.

Solution of 3.2 Power Peak to mean ratio (PARP)

The impermanence of the OFDM envelope can be expressed by PAPR. PAPR (peak to average power ratio) is the ratio of peak power to average power. The larger the PAPR, the greater the impermanence of the system envelope. Therefore, to improve system performance, we must try to reduce PAPR.

Because the OFDM signal is superimposed on multiple sine waves, when the number of subcarriers reaches a certain level, the OFDM symbol waveform will be a Gaussian random process from the central limit theorem, its envelope is not constant. This phenomenon is not expected in nonlinear band-limited channels. After a nonlinear amplifier, the ups and downs in the envelope can be weakened or eliminated, but at the same time, the signal spectrum is expanded, its Side-lobe will interfere with signals from neighboring channels. This will cause interference between adjacent channels in the OFDM system and disrupt its orthogonal. In general, the high frequency amplifier hPa in the transmitter has strong nonlinear characteristics. In order not to extend the spectrum too much, HPA must work in a large back off state, which will waste a lot of power. Therefore, without any measure to improve the nonlinear sensitivity of OFDM, OFDM technology will not be used in battery transmission systems, such as mobile phones. It is generally solved through the following technologies.

(1) Limiting (clipping) technology: it is a simple and effective method to reduce PAPR, but it can cause distortion of in-band signals and out-of-band spectrum dispersion, this degrades the bit error rate. High-speed encoding is a simple encoding of the message code, which can reduce the probability of a large PAPR from the statistical characteristics.

(2) encoding technology: The grouping encoding method can definitely reduce PAPR and have a certain error correction capability. The complex envelope of OFDM signals depends on the non-periodic auto-correlation function of the sent data signal sequence. If the side lobe is small, the fluctuation of the signal is small, that is, the PAPR is small, and the quasi-constant (quasi-constant) amplitude signal can be obtained. Therefore, it is necessary to find a sequence of sending signals with a small number of auto-correlation functions. The Golay binary sequence (complementary) is a sequence with a small number of sides. Even if it is extended to a multi-phase sequence, it still meets the characteristics of a small side lobe. It can be proved that the PAPR of the Golay sequence should not exceed 3 dB. The principle of block code based on the mutual cool sequence is to avoid the use of PAPR high code sub. By adopting the block code based on the mutual cool-down sequence, the system can get a large coding gain when PAPR is controlled in 3-6 dB, and the error-floor performance is improved.

(3) Scrambling Technology: The scrambling technology is used to make the mutual correlation of the generated OFDM as far as possible 0, thus reducing the PAPR of the OFDM. Here, the Scrambling Technology can reset the phase of the generated OFDM signal, typical of which are PTS and SLM.

3.3 training sequence and guide and Channel Estimation Technology

The receiver does not require channel estimation when using differential detection, but still requires some pilot signals to provide initial phase reference. Differential detection can reduce the complexity of the system and the number of pilot frequencies, however, the signal-to-noise ratio is lost. Especially in OFDM systems, the system is sensitive to frequency offset, so coherent detection is generally used.

Channel estimation is required when the system uses coherent detection. In this case, the training sequence and the guide can be used as auxiliary information. The training sequence is usually used in non-time-varying channels and the guide signal is generally used in time-varying channels. In the OFDM system, the pilot signal is a two-dimensional time-frequency signal. In order to improve the estimation accuracy, We can insert the continuous and scattered guide. The number of guides is a compromise between the estimation accuracy and the complexity of the system. The interval between the pilot signals depends on the coherent time and coherent bandwidth of the channel. In the time domain, the frequency must be less than the coherent time. In the frequency domain, the frequency guide interval should be less than the coherent bandwidth. In practical application, the design of the pilot mode depends on the actual situation.

4. Advantages of OFDM technology

(1) a large amount of data can be sent in narrowband bandwidth. The OFDM technology can separate at least 1000 digital signals at the same time, moreover, the ability to run securely around interference signals directly threatens the development and growth of the popular CDMA technology in the market, it is precisely because of this special signal "penetration ability" that the OFDM technology is favored and welcomed by European telecom carriers and mobile phone manufacturers.

(2) The OFDM technology can continuously monitor the sudden changes in the communication characteristics of transmission media. Because the ability to transmit data through communication paths changes over time, therefore, OFDM can dynamically adapt to it, and connect and disconnect the corresponding carrier to ensure continuous and successful communication. This technology can automatically detect which particular carrier has a high signal attenuation or interference pulse under the transmission media, then appropriate modulation measures are taken to make the carrier at the specified frequency successfully communicate.

(3) The biggest advantage of OFDM technology is the protection against frequency selective fading or narrowband interference. In a single carrier system, a single fading or interference can cause the entire communication link to fail. However, in a multi-carrier system, only a small portion of the carriers will be affected. Correction Codes can also be used for correction of these subchannels. The OFDM technology is particularly suitable for use in areas with high-rise buildings, densely populated and geographically prominent buildings, and where signals are transmitted. High-speed data transmission and Digital Speech broadcast both hope to reduce the influence of the multi-path effect on the signal.

(4) OFDM technology can effectively combat interference between signal waveforms and is suitable for high-speed data transmission in multi-path environments and fading channels. When the channel suffers frequency selective fading due to multi-path transmission, only the subcarriers that fall into the band sag and their information are affected. Other subcarriers are not damaged, therefore, the overall error rate of the system is much better.

(5) The OFDM technology has strong anti-fading ability through the joint coding of each sub-carrier. The OFDM technology itself has used the frequency diversity of channels. If the fading is not serious, there is no need to add a Time Domain balancer. The system performance can be improved by associating various channels for encoding.

(6) OFDM technology can use hardware modules to integrate IFFT/FFT-based algorithms. The operating speed of the OFDM system in this way depends mainly on the operating speed of the hardware circuit, it also simplifies the complexity of system implementation.

(7) The channel utilization rate of OFDM technology is very high, which is particularly important in wireless environments with limited spectrum resources. When the number of subcarriers is large, the spectrum utilization rate of the system tends to be 2 baud/Hz.

5. disadvantages of OFDM technology

(1) sensitive to frequency offset and phase noise. The method used by OFDM to distinguish sub-channels is to use strict orthogonal conditions between sub-carriers. The frequency offset and phase noise will make the orthogonal characteristics of each sub-carrier worse. only 1% of the frequency offset will reduce the signal-to-noise ratio by 30 dB. Therefore, the OFDM system is sensitive to frequency offset and phase noise.

(2) high ratio of peak power to average power (PAPR), resulting in low power efficiency of RF amplifiers. Compared with a single-carrier system, because the OFDM signal is composed of multiple independently modulated sub-carrier signals, such synthetic signals may generate relatively large peak power, this will bring about a large peak-to-average ratio of power, referred to as the peak-to-average ratio. For an OFDM system that contains n sub-channels, when n sub-channels all sum with the same phase, the obtained peak power is n times the average power. Of course, this is a very extreme situation. Generally, the peak mean in the OFDM system will not reach this level. The peak-to-average ratio increases the requirement on the RF amplifier, resulting in reduced power efficiency of the RF signal amplifier.

(3) Load algorithms and Adaptive Modulation Technology increase system complexity. The use of load algorithms and adaptive modulation technology will increase the complexity of the transmitter and receiver, and when the terminal moves at a speed higher than 30km every hour, the adaptive modulation technology is not very suitable.

6. Conclusion

The OFDM system is suitable for communication systems with multiple businesses and high flexibility. It features high spectrum utilization and good system stability. Currently, OFDM has been widely used in digital broadband audio systems and digital broadband video systems in Europe and Australia. It is based on OFDM communication technology, this enables low-latency and high-speed data transmission during transmission. The bandwidth of 54 Mbit/s can basically meet the wireless network requirements of most users. With the continuous improvement of OFDM technology, its application scope will be expanded to various fields.

There are still many problems to be solved in OFDM standards related to the fourth generation mobile communication. The main reasons for selecting OFDM as the core technology of the Fourth Generation Mobile Communication include high spectrum utilization and strong anti-noise ability, suitable for high-speed data transmission and other factors.