Analysis of principles for sending and receiving OFDM Signals

The principle of OFDM signal transmitter is that user signals are input in serial mode, and the rate is r codeword/second. These codes are first sent to a serial-parallel converter, so that the serial input signal is output to M lines in parallel. The data transmission rate on any one of the M lines is R/M codeword/second.

The OFDM code is subsequently sent to a fast Fourier inverse transformation module for Fast Fourier inverse transformation. 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. After the fast Fourier inversion sample value is calculated, a cyclic prefix is added before the sample value to form a circular extended OFDM information code word.

The circular prefix technology utilizes a concept in the principle of Discrete Linear Systems. We know that in the continuous time domain, the convolution of two time domain signals equals the product of these two signals in the frequency domain form. However, this is generally not true in discrete time domains unless we use an infinitely large sample Value Point N or at least one convolution signal is cyclical (in this case, signals can be convolution by circumference ). Because we can only use a limited sample value of N, we can only use the cyclic prefix to make the OFDM Information Code appear periodically in the time zone we are interested in.

The sample value of the circular expansion information code is re-transmitted through a parallel-serial converter module. Then pass through the channel in serial mode (through proper filtering and Modulation ). During transmission, the impact response of the channel causes interference to the time domain signal. Due to the cyclic prefix, the transmitted OFDM signal shows periodicity, and this convolution becomes a circular convolution. Based on the discrete-time linear system principle, this circular convolution is equivalent to the product of the frequency response and channel frequency response of the OFDM signal.

The receiver completes operations opposite to the sender. The receiver receives a time-domain signal. Due to some changes in the impact of wireless channels, the received signal goes through a serial-parallel converter and the cyclic prefix is cleared.

Clearing the loop prefix does not delete any information. Information in the loop prefix is redundant. The circular prefix is used to ensure the establishment of the convolution feature mentioned above.

Another benefit of the cyclic prefix is that it can eliminate inter-code interference. We require that the cyclic prefix value be greater than the channel memory. The multi-path signal causes the lag of the first information code word to affect the current information code word, thus generating inter-code interference. However, in fact, inter-code interference only interferes with the cyclic prefix of the current information code. Therefore, the use of appropriate cyclic prefixes can eliminate inter-code interference in OFDM technology.

After the cyclic prefix is cleared, the signal will go through a fast Fourier transform module to change the signal from the time domain to the frequency domain. After a parallel-serial conversion module and serial conversion, the signal receives the original OFDM signal.

In order to improve the information transmission capability of OFDM, We have extensively studied the OFDM loading algorithm. Each sub-channel of the OFDM system has two parameters to be determined, namely, the transmission power and data transmission rate. The efficient distribution of power and data between sub-channels can improve the system efficiency. Such algorithms that effectively distribute power and data are called load algorithms.

Load algorithms can be classified by optimized resources and specified restrictions. In the rate adaptation algorithm, everyone is interested in how to maximize the total data transmission rate under the limits of the total power, of course, to meet certain bit error rate requirements.