OFDM Receiver and How it work for LTE?
The receiver is like in any other radio system the more complicated part. In radio systems and of course also OFDM there are two special points a receiver has to pay attention to: time/phase and frequency synchronization. Both are crucial for the performance of the receiver. A receiver gets its input from the antenna (or antennas) and the attached low noise amplifier.
A band pass suppresses signals out of the spectrum. The demodulator converts the signal back into the baseband and with this recovers the complex valued data signal. At this step we have the time domain representation of the signal. The time signal is now given to the “De-rotator” which applies to each time sample a phase offset to compensate frequency drifts and global phase offsets. A special unit in the receiver is responsible to determine and track the frequency and phase drifts and calculate the associated correction value for each sample.
This is a quite critical task, as errors made here, apply as additional (receiver intrinsic) noise to all data symbols. The frequency and time synchronization unit uses typically as input the autocorrelation of the input time sequence (especially cyclic prefix) and reference (or pilot) symbol interleaved with the data at predefined positions. The corrected signal is now fed into the Fast Fourier Transform (FFT) which implements a fast and efficient algorithm for the discrete Fourier transform to bring the signal back into the frequency domain representation.
In other words the FFT decodes the complex valued data symbols for each subcarrier. Of course before the FFT is applied, the cyclic prefix has to be removed. The recovered subcarrier data symbols are not useful yet, as there might be still distortion from phase offsets and from the channel propagation (multi-path propagation) on it. Thus the next step is to correct the data according to the known channel response.
The channel estimation uses the pilot and reference signals that are interleaved with the normal data at predefined positions to estimate and permanently correct the channel state information. A nice thing of the frequency domain representation is, that a distortion coming from channel propagation and time offset are in first order simple correction factors to each subcarrier, so that no complex filtering is required here.
After we have corrected our data symbols for each subcarrier, the symbol demapping can take place. Here we recover the original bit sequence either as hard decided bits or as soft decided bits. (Soft bits have some advantages in the further processing, namely in the channel decoding.)
OFDM Receiver and How It Works for LTE
The OFDM receiver in LTE is responsible for processing the transmitted signal, recovering the data, and ensuring reliable communication between the User Equipment (UE) and the network. The receiver works by performing several key steps to decode the data transmitted over the air using OFDM technology.
1. Signal Reception: The receiver first captures the transmitted OFDM signal over the air. The signal includes multiple subcarriers, each carrying a portion of the data. The signal is affected by noise, interference, and potential fading, which the receiver needs to handle efficiently.
2. Cyclic Prefix Removal: In the received signal, the cyclic prefix (CP) is the first part of each OFDM symbol, which was added at the transmitter to combat multi-path interference. The receiver removes this cyclic prefix to return to the original data sequence, ensuring that the symbols can be properly aligned for further processing.
3. Fast Fourier Transform (FFT): After the cyclic prefix is removed, the receiver applies the Fast Fourier Transform (FFT) to the signal. FFT transforms the time-domain signal back into the frequency domain, where each subcarrier can be analyzed separately. This step is crucial because the data is transmitted over several orthogonal subcarriers, and the FFT separates them for individual demodulation.
4. Subcarrier Demodulation: Each of the subcarriers is demodulated to recover the transmitted data. Depending on the modulation scheme used (e.g., QPSK, 16-QAM), the receiver decodes the signal into its corresponding bits. The demodulation process depends on accurately estimating the channel conditions to decode the symbols correctly.
5. Channel Estimation and Equalization: To combat distortion caused by the radio channel, the receiver uses channel estimation techniques to understand the channel’s characteristics. It uses reference signals or pilots to estimate the channel and then applies equalization to compensate for any interference or fading that occurred during transmission.
6. Error Correction: The receiver checks the integrity of the received data using error detection and correction techniques, such as Forward Error Correction (FEC) and Automatic Repeat Request (ARQ). These techniques ensure that any lost or corrupted data can be retransmitted and corrected for reliable communication.
7. Data Extraction: Finally, once the symbols are correctly demodulated and errors are corrected, the receiver extracts the original data and passes it on to higher layers of the LTE protocol stack for further processing, such as decoding and delivering the data to the application.
In summary, the OFDM receiver in LTE uses a combination of cyclic prefix removal, FFT, demodulation, and channel equalization to recover the transmitted data efficiently. These processes enable LTE to provide high-speed and reliable communication even in challenging radio environments.