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Design >> RF Design >> Channel Estimation
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Message started by aaron_do on Nov 17^th, 2008, 12:30am

Title: Channel Estimation
Post by aaron_do on Nov 17^th, 2008, 12:30am

Hi all,

the way i understand it, RXs usually have some form of channel estimation which allows them to decide what kind of data rate/TX output power is possible/required for proper transmission.

So i was wondering what the channel estimator estimates...is it EVM or BER or maybe just interferer power? Also is it done on the digitized signal or at some part of the analog chain?

Any references would be extremely helpful...

thanks,
Aaron

Title: Re: Channel Estimation
Post by didac on Nov 18^th, 2008, 1:40am

Hi,

aaron_do wrote on Nov 17^th, 2008, 12:30am:

Hi all,
the way i understand it, RXs usually have some form of channel estimation which allows them to decide what kind of data rate/TX output power is possible/required for proper transmission.

The answer to this question it's specific to each system, in general the information that the terminal it's receiving it's useless for it's own transmission since the channel could have changed before he is able to access to the medium(except maybe for ACK-acknowledgment- packets, and some standards define the data rate and power of the ACK packet for reliable transmission).
Channel estimators typically use one of the following(depends on the System Level Architecture-by System I mean protocol,standard):

-Training sequences: if there is a preamble in the received packet this is known so it can be calculated the EVM or use of a MMSE algorithm for channel estimation.

-Some standards(GSM and UMTS if I remember well) sent continuous or periodic carrier pilots of a known power, so the terminal can estimate the power needed for transmission.Here you can use an analog RSSI to estimate the power needed for transmission(if the pilot it's continuous you have real time information on the channel).

-In a OFDM system exists the possibility to configurate the subcarriers for channel estimation:some subcarriers are not data modulated and some other carriers are nullified so it can be done a SNR estimation.

This information typically it's feedback to the terminal that it's transmitting so it can adjust the output power level and/or the modulation, so for maximum efficiency it's necessary a full-duplex system and the method should be chosed carefully taking into account the coherence time of the channel since information could be old when it's sent.

-One simple way to adjust dynamically the data rate it's to count the ACK's, if after N packets all the ACK's are received increase the data rate if some are lossed decrease the data rate(I think that method was one of the first used in WLAN).

As you can see most of this techniques are into the DSP domain(except RSSI), you can look to your particular system and see what they do.

It's difficult to give references since its a topic that falls between radio engineering,digital signal processing and network engineering...

Hope it helps,

Title: Re: Channel Estimation
Post by rf-design on Dec 15^th, 2008, 2:27pm

There are two possible interpretations:

1. Channel Estimation

A channel have typical linear distortions. If the method to number this distortion is frequency domain, as most systems today, each frequency of the channnel have a phase and amplitude. This information is used to equalize the channel and receive a much less distort signal. To measure the channel special signals which are known to the receiver are used.

The other type is blind channel estimation where the assumpion is that the mean signal is of noise type and any linear distortion could be read from long time statistics for each frequency.

2. Channel Quality Estimation

For each frequency a symbol must be decided. The symbol is choosen which has the least distance to the signal. The difference could collected over time and is a measure for the quality, e.g. noise, of the channel or previous equalisation.