References: Principles and Applications of OFDM mobile communication I. Concepts of inter-symbol interference (ISI) and inter-channel interference (ICI)
Ii. reduced protection interval (ISI)
3. reduce ICI by the cyclic prefix
Iv. Block Diagram of the OFDM system after protection interval and cyclic prefix are added
5. Select the protection interval method ==================================== =====
The concept of intersymbol interference (ISI) and interchannel interference (ICI)Multi-path latency expansion refers to the extended width of the received signal pulse caused by multiple paths. The extended time Delta is the difference between the maximum transmission latency and the minimum transmission latency. Latency scaling varies with the environment, terrain, and terrain, and is generally independent of frequency. Due to the influence of the multi-path effect, there may be a collision when the symbol arrives at the receiving side through the multi-path transmission, that is, the delay of the pulse signal is extended, and inter-symbol interference (ISI, inter-symbol interference, which is also known as inter-code crosstalk, because a chip in CDMA is a symbol, But OFDM does not have the concept of chip .). It seriously affects the transmission quality of digital signals. OFDM symbol transmission requires a high degree of orthogonal. If the sub-carrier's orthogonal character is damaged, it will affect the receiving side's demodulation, that is, inter-channel interference (ICI, inter-channel interference, also known as inter-Carrier Interference and frequency interference) There are two ways to eliminate multi-path interference in OFDM: 1. OFDM performs serial and conversion of high-speed data streams, this increases the length of data symbols on each sub-carrier, effectively reducing inter-symbol interference (ISI) caused by time dispersion of the wireless channel, thus reducing the complexity of the receiver's internal equalizer, sometimes, you can even eliminate the negative effects of the ISI by inserting the loop prefix instead of using the balancer. 2. OFDM solves multi-path interference through protection interval. The OFDM system is equipped with guard interval (GI) at the transmitting end to eliminate the ISI and ICI caused by multi-path. The method is to fill in the cyclic prefix (cyclic prefix, CP) within the protection interval of the OFDM symbol to ensure that the number of waveform cycles contained in the delayed copy of the OFDM symbol within the FFT period is also an integer. In this way, signals with lower latency than the protection interval will not generate ISI during the demodulation process. Explanation: The interference within the OFDM symbol (produced by the frequency selectivity of the channel in the OFDM symbol) is eliminated through channel balanced \ channel estimation. Inter-OFDM symbol interference (between two consecutive OFDM symbol blocks of time K and k + 1) is eliminated by protection interval. The receiver discards the first G sampling points of the received signal (the cyclic prefix within the protection interval)
Reduced protection interval (ISI)OFDM reduces the Signal Rate and increases the symbol cycle by concatenating input high-speed data streams into N parallel subchannels, so that each data symbol cycle used for modulation sub-carrier can be expanded to n times the original data symbol cycle, so the ratio of latency extension to symbol cycle is also reduced by N times (explanation: because the period used as the denominator is increased by N times, and the latency is characteristic of the Time Domain Waveform and channel, the latency will not change ). This reduces the time selectivity. However, the multi-path still causes inter-symbol interference (ISI), for example. If there is no protection interval, the receiving side receives a multi-path signal. Because the transmission paths vary, the time it takes to reach the receiver is also different. The signal of each diameter produces intersymbol interference at the cross signal. It may cause difficulties in demodulation. Explanation: The signals after the superposition of two multi-path signals are shifted to the right on the time axis of the time domain (the initial phase also changes ). After the receiving end is scheduled, the tail of a symbol sweeps to the header of the next symbol, that is, interference between symbols. The protection Interval Length is greater than the maximum multi-path latency of the channel, so that the multi-path components of an OFDM do not interfere with the next OFDM symbol. That is, because the next symbol starts with the protection interval and is followed by the actual data, it will not be affected by the previous symbol, because part of the previous symbol will fall within the protection interval. Therefore, the integral interval starts from the real data to the end (falls within the protection interval of the next symbol) to eliminate the interference between symbols to the maximum extent, protection intervals can be inserted between each OFDM symbol (not a sub-carrier symbol, but an OFDM symbol superimposed on each sub-carrier, that is, the waveform in the time domain, in addition, the duration of the protection interval Tg is generally greater than the maximum latency extension of the wireless channel, so that the multi-path component of a symbol will not interfere with the next symbol. See. During this protection interval, no signal can be inserted (that is, no sampling), that is, a idle transmission period.
Reduce ICI by Cyclic PrefixHowever, after the protection interval is added, due to the influence of multi-path propagation, inter-channel interference (ICI) occurs, that is, the orthogonal behavior between sub-carriers is damaged, interference occurs between different subcarriers, as shown in. Due to the existence of the multi-path Delay, the idle protection interval enters the integral time of the FFT. As a result, the integral time cannot contain an integer waveform, undermining the orthogonal between sub-carriers, causes inter-Carrier Interference with ici. The actual effect is that the frequency offset of the sub-carrier occurs. Explanation: The multi-path and delay results in Doppler effect, and the received signal neutron carrier is no longer orthogonal. Because each OFDM symbol contains all non-zero sub-carrier signals, and the delay signal of the sub-carrier is also displayed, the delay signal of the first sub-carrier and the second sub-carrier is given, it can be seen that the difference between the number of cycles between the first sub-carrier and the second sub-carrier with a delay is no longer an integer within the FFT operation time (explanation: Due to the frequency offset, the sub-carrier interval changes, and the sub-carrier is no longer orthogonal), so when the receiver tries to demodulated the first sub-carrier, the second sub-carrier will interfere with this (that is, the sub-carrier credits in the OFDM symbol cycle are not 0, because they are no longer orthogonal ). Similarly, when the receiver demodulation the second sub-carrier, sometimes there will be interference from the first sub-carrier. To avoid the loss of sub-carriers due to multi-path propagation during idle protection interval, the OFDM symbol must be filled with the cyclic prefix signal (I .e: copy the samples in the last time of each OFDM symbol to the front of the OFDM symbol. In this way, the number of waveforms in the delayed copy of the OFDM symbol within the FFT period is also an integer. In this way, the delay signal with a latency less than the protection interval TG will no longer produce ICI during the demodulation process. As long as the delay of each path does not exceed TG, the integral waveform of each sub-carrier can be included in the FFT integral interval.
Of course, adding the protection interval also pays the bandwidth price and brings about the energy loss: the longer the CP, the greater the energy loss. It is generally considered that the CP must be less than 1/4 of the length of the OFDM symbol. For example, if an OFDM symbol has 256 characters, the cyclic character length is 64 bits, and the total symbol length is 256 + 64 bits. explanation: CP is mainly used to satisfy the cycle difference of different carriers in the same sampling interval as an integer to overcome inter-Carrier Interference and resist the multi-path latency (so the length of CP depends mainly on two factors, the first is the coherent length of the channel, and the second is the duration of the OFDM symbol.
CP:
1. CP occupies a period of time in the time domain. This period must be greater than the maximum latency extension. Therefore, it can suppress ISI (intersymbol interference, from this point, CP can be understood as a GP (Protection interval );
2. CP content: For GP, we know it is blank, that is, the transmitter is silent during this period of time, but CP is not. This is another feature of CP: the CP content enables the implementation of cyclic convolution, which can effectively suppress ICI (Inter-Carrier Interference). That is to say, the CP content effectively guarantees the orthogonal loss caused by frequency offset to some extent.
Block Diagram of the OFDM system after protection interval and cyclic prefix are addedGenerally, when the protection interval accounts for 20%, the power loss is less than 1 dB. However, the loss of information rate reaches 20%, while the loss of information rate (bandwidth) exists in traditional single-carrier systems. However, the insertion protection interval can eliminate the impact of ICI caused by the ISI and multi-path, so this cost is worthwhile. It has been proved that the ISI and ICI can be completely eliminated by selecting the number of sub-carriers and the protection interval properly. Summary: adding a protection interval to the symbol interval ensures crosstalk between codes, and filling in the cyclic prefix CP during the protection interval ensures that sub-carriers are orthogonal to each other.
Protection Interval MethodA good system design must avoid ISI and ICI, or at least suppress them to an acceptable level. That is to say, select a sufficient CP to prevent the ISI and ICI caused by frequency selective fading, and select the appropriate OFDM symbol length to enable channel Impulse Response (CIR) it remains unchanged during at least one OFDM symbol.
Because the OFDM system is sensitive to frequency offset and phase noise, the width of the sub-carrier must be carefully selected, neither too large nor too small. Because the OFDM symbol period is inversely proportional to the sub-carrier bandwidth, the smaller the sub-carrier width, the larger the symbol period, the higher the spectrum efficiency (because a CP is inserted before each OFDM symbol, CP is the system overhead and does not transmit valid data ). However, if the sub-carrier width is too small, it is too sensitive to frequency offset and is difficult to support high-speed mobile terminals.
The choice of CP length is closely related to the delay extension of the wireless channel and the radius of the cell. The longer the latency extension and the larger the cell radius, the longer the CP is required. In addition, in the macrodiversity broadcast system, because the terminal receives the signal sent by each base station at the same time, in order to avoid interference caused by poor transmission latency, the CP needs to be extended.
Optimization design is very important for the OFDM system. The actual system needs to process various environments (the channel parameters are very different ). One solution to the problem is to optimize the parameters according to the worst case (high-speed mobile users in macro cells, another optional method is to optimize parameters based on different environments (indoor, outdoor, Macro, micro, etc.), but this requires a highly flexible transceiver. ==================================
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