when reading ACC_MEM, we are interested in a CIR, which is as detailed as possible, the first path or the distance calculated out of that information is not the goal. For that, it seems reasonable to use longer preambles, e.g. 1024 or even longer. In this context there are a few questions:
Is the assumption correct that a longer preamble will deliver more details in CIR? In my mind, the summation of more symbols should “lift of” the symbols or reflected parts of them out of the noise.
Is there a kind of limit, how much symbols might be summed up, before there is a clipping of the values due to the number of available bits in the register? It is clear that here the distance is of importance, but in a worst case, i.e. a short distance of maybe a few meters only, there might be such a limit.
If there is a clipping, at which value? In other words, will the register width of 18 bits (18 out of 24 according manual) will be used in total (0x3FFFF) or is there another max value?
Is there any influence of the PAC size in terms of CIR details? PAC size is in my mind the “portion of samples”, which is used to detect the presence of a preamble. There will be an influence on the preamble detection time, i.e. the starting point of symbol summation for CIR estimation, which might change the number of symbols to be summed up a little. I assume that this effect is not that important as long a detection is reliable, but is there any other effect?
A longer preamble can improve the SNR of the CIR estimate because more correlated preamble symbols are accumulated. It does not increase the CIR time resolution or number of useful delay samples; those are determined by the PHY bandwidth and accumulator sampling.
Each ACC_MEM real and imaginary component is stored as a signed 18-bit value in a 24-bit field. The nominal signed range is therefore -131072 to +131071, not 0x3FFFF as an unsigned value. Clipping behavior depends on the internal accumulator scaling, so it is good practice to verify signal levels on the target setup to ensure operation within the linear range.
PAC size primarily affects preamble detection. The PAC used to acquire the preamble does not contribute to the CIR accumulation, so a larger PAC can reduce the accumulated symbol count slightly. It does not otherwise improve CIR resolution or detail. PAC should be selected according to the recommended setting for the preamble length.
For preambles of 256 symbols or longer, also use the receiver’s Long operating parameter set rather than the default Short parameter set.
Hi Akash,
thanks for mentioning the long parameter set. We missed this point up to now and we will test it.
Just to be sure regarding the bits in ACC_MEM: According to the manual “Each value is actually 18-bit precision, with the upper 6-bits being all zero or ones depending on the sign of the value.”. This would mean we could have twice 262143 (or -262143 to +262142), since we have 18 bits for the value and the sign is coded in the upper 6 bits. Or does that mean we have an 18 bit signed number and the upper 6 bits are useless (but these bits are not constant and therefore might be the sign as written). We assume the value is given in two’s complement, right?