Equalizers are used for broadband channel. Can any one tell the details of equalizers used in LTE ?
Dear Dr. Ready,
This is a good question. The equalizers used in LTE are frequency domain equalizers where a known input signal X(z) in the frequency domain at the transmitter is transmitted through the rf channel. Having the the transmitted signal X(z) and the received signal Y(z), H(z) is then calculated by the ratio of Y(z)/ X(z). Knowing H(z), and the received signal the transmitted signal can be computed. In the frequency domain z= exp jwTs where, TS is the sampling frequency.
The way to estimate the channel H(jw) is by means of using pilots tone distributed among the whole band width of the channel. The pilots have known amplitudes and frequency. The receiver detects these pilots and estimates their amplitudes and phases. Then H(jw) is determined at all other frequencies by interpolation.And so, H(jw) for the other frequencies i known and the received signal are divided by the channel response to equalize it and to compensate for the effect of the channel.
So, the equalizer is implemented by using pilots for channel estimation, and compensation is accomplished by dividing the received signal by the estimated channel response.
My best wishes to you
Dear Mr. Reddy,
This is a very interesting question, especially because standards usually don't specify the receiver. In what follows, I will only complement the answer by Dr. Zekry.
I have already worked with LTE, but with versions/releases earlier than LTE-Advanced. In those releases, LTE dedicated a whole OFDM symbol to transmit only pilot signals (also called reference signals). These signals are used for some purposes, such as synchronization and also to allow for channel estimation. This means that these signals are known by both transmitter and receiver and, with this information the receiver is able to estimate the channel frequency response (in the entire bandwidth of the OFDM symbol, i.e., in the entire transmission bandwidth). Then, in the other transmissions in which information is actually sent (i.e., not pilot), the receiver will be able to mitigate the effects of the channel through a process called equalization. Notice that the channel is estimated at a time instant 'n', but it is used to equalize the transmissions in other time instants, say 'n+1', 'n+2', etc. So, the equalization process will succeed as long as the channel does not vary (change its frequency response) too fast. And that's also one reason that LTE re-transmits pilot symbols periodically.
Regarding the equalizer, it is indeed very simple: it is a one-tap equalizer, which means that the info. arriving in each subcarrier is simply multiplied by a number (scalar) in order to estimate the original info. that was transmitted. The choice of this one-tap equalizer depends on how much knowledge of the environment you have. Typical choices for the equalizer are: (i) Zero-forcing (ZF) equalizer and (ii) Minimum Mean-Squared Error (MMSE) equalizer. While (i) uses only the information of the channel frequency response, (ii) also requires some knowledge about the environmental noise in each subcarrier. Thus, if you have quite accurate information of the channel and of the noise (and, in practice, you usually do!), then the MMSE equalizer will provide better results.
The equalizers mentioned in the previous paragraph are standard equalizers for OFDM systems. So, any book on OFDM should cover them.
Best Regards.
This is a good question.Adaptive Channel Equalization techniques with dynamic decision algorithms could be used, for effective data throughput in LTE systems.
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