RF Receiver: Calculating the ACS & blocking requirements

[MFayek]

Dear All,

I'm facing a little problem in the system design for Front-end wireless receiver. I'm searching for any way of calculating the Adjacent channel selectivity (ACS). the IEEE standards for any technology mention the value directly e.g LTE 33dB.I need to know how to calculate such value for different Channel BWs.???
As i know that value affects the attenuation of my filter for the adjacent channels .the selectivity as well need to be realized in the RF ,IF & baseband parts. But i'm confused in how too realize the selectivity in a block such a mixer:-?

if any one could give an advice in these issues ,plz don't hesitate;-)

 

[jiripolivka]

Like in any radio receiver, you need to know:
1. nominal channel bandwidth to pass a desired signal spectrum
2. the power level transmitted
3. frequency spacing between the two channels.

You have to specify the filters in each channel so that they pass the desired spectrum (pass-band to -3 dB), then you must specify filter reject band(s) so that the leaking power is X dB lower than the power level transmitted in the other channel. This way you find the adjacent-channel leaking power. If it is too high, you must redesign the filters to achieve the required lower leakage.

It is irrelevant if the filter is connected to an amplifier or a mixer. With mixers the problem often neglected is that mixers can be poorly matched to the nominal e.g. 50 Ohms, and also that their impedance depends on local-oscillator power level. A mismatch affects filter response- it can be improved by using an attenuator or a better mixer or amplifier.

 

[vfone]

The LTE spec have different ACS requirements vs BW. Should be better than 33dB @ 1.4MHz BW down to 27dB @ 20MHz BW.
All the wireless systems specify ACS value where sensitivity (or receive throughput in case of LTE) is met in the presence of a modulated adjacent signal, which characteristics (spacing from desired, modulation, BW, pattern, power, etc) is always specified in that spec. Knowing the sensitivity that you have to meet in the presence of an adjacent interferer, you can start designing the RF system, taking into consideration all the levels (in-channel and out of channel), channel filtering, stage linearity and compression point inside of the channel and outside of the channel, etc.
AGC design of the receiver can affect the ACS performance, especially for WCDMA and LTE where you have two cases for ACS (ACS at two different desired input levels).
I can say that ACS is the most elaborate part of the RF system design, because there are a lot o interactions between causes.

 

[MFayek]

All the wireless systems specify ACS value where sensitivity (or receive throughput in case of LTE) is met in the presence of a modulated adjacent signal, which characteristics (spacing from desired, modulation, BW, pattern, power, etc) is always specified in that spec.

The adjacent channel power in the standard of LTE is assumed to be 45dB above the reference sensitivity. the question is : Is that value of power is just for testing or for real case .Let me say it in another words,for a different power level of the adjacent channel do i need to recalculate the ACS again for the excess power .
What confuses me is that these values are tabulated as "Test Parameters" ,so should i consider this power of the interferer while designing or just for the test cases???. i think that power should be variable depending on the distance from the interferer's base station!!!

 

[vfone]

LTE spec has two cases. One use the desired signal at Reference_Sensitivity + 14dB, and second case where the desired has and absolute level of -56.5dBm (for 1.4MHz to 5MHz BW). The adjacent interferer signal for each case also has two different levels.
So, you have to meet ACS only at those particular levels.