# What is the Reference tap of an equalizer (FIR Filter)

Currently I'm using the built-in Equalizer (EQ) object from Matlab to equalize my signal. For the EQ there is a parameter called the Reference tap. With different value of the reference tap i got also different results and maybe also different delay of the EQ. My question is: what exactly is the reference tap of a FIR filter and how should i set it to achieve the best performance ? Thank you in advance LA

• There is not such thing as reference tap of a FIR filter. What command are you using in MATLAB? – Oliver Jun 13 '15 at 2:23

I haven't checked the code, but that parameter refers to the delay of the reference signal when computing the error used for updating the filter coefficients. If $y[n]$ is the output of the linear equalizer, and $r[n]$ is the reference signal (e.g., the training signal when in training mode), the error is computed as
$$e[n]=y[n]-r[n-d]$$
where $d$ is the delay defined by d=RefTap-1. This means that if the channel introduces a pure delay and the parameter RefTap is set to match that delay, the equalizer doesn't need to do anything. Otherwise, the equalizer would need to try to compensate for that delay, which is impossible for a causal filter (because it would try to implement an advance in time). So that parameter is indeed very important in order to achieve the best possible equalization for a given equalizer filter length. Since in practice the channel delay is often unknown, a reasonable choice is to set the parameter RefTap to half the equalizer length. Then, on average, the best use is made of the equalizer's weights. If the performance is not satisfactory, the number of weights needs to be increased (and the parameter RefTap must be changed accordingly).