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# How to remove interferers placed one channel away

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#### Debdut

##### Full Member level 3
I'm in a situation as follows:
Suppose there are two transmitters and two receivers. The transmitters are transmitting BPSK signal at 402.3 MHz and 402.6 MHz (one channel apart).
One receiver must decode the 402.3 MHz signal, the other the 402.6 MHz signal.
How will it be possible?

It is difficult (or impossible) to make bandpass filters with the sharp rolloff you need. You might try combining two filters: a bandpass and a notch filter. One is tuned to the desired frequency, the other is tuned to reject the other frequency.

The reliable method is to make a transmitter wait until both channels are clear, then transmit. Encode the signal and make each receiver examine the coding so that it accepts only the signal which is intended for it.

Carriers only 300 kHz apart should be selected at the IF level. Blocking of the adjacent channel is a matter of RF front end and mixer linearity and IF bandwidth. You didn't mention the bit rate so that we could get an idea of an approriate IF bandwidth.

The reliable method is to make a transmitter wait until both channels are clear, then transmit. Encode the signal and make each receiver examine the coding so that it accepts only the signal which is intended for it.

Thanks Brad, I'll definitely consider that.

The bandwidth of each channel as you can guess is 300 kHz. So data rate can be = bandwidth * log2 (1 + SNR) (Shannon's Theorem).
I thought of 'mixing' the 402.3 MHz and 402.6 MHz with 402 MHz. Then, at each receiver we would receive 300 kHz and 600 kHz. The receiver that decodes 300 kHz will filter out the 600 kHz and viceversa. But then comes the problem of decoding.
We have to synchronize the incoming signal, either 300 kHz or 600 kHz, with the local LO in order to convert the signal into data (we are converting the input BPSK signal to bits).
There is the problem, the LO is generating 402 MHz, how to synchronize with 300 kHz or 600 kHz...

The bandwidth of each channel as you can guess is 300 kHz. So data rate can be = bandwidth * log2 (1 + SNR) (Shannon's Theorem).

Sounds like you are talking about a theoretical problem rather than a real receiver.

No its a real receiver I'm trying to design.

Consider broadcasting on one frequency. Receiver A demodulates it, then sends it to two bandpass filters. One decodes a '0' as 101 kHz. The other decodes a '1' as 117 kHz.

Receiver B contains bandpass filters which decode a '0' as 155 kHz, and a '1' as 173 kHz.

This is only a crude description of the concept. I made up odd frequency values to avoid harmonics which might result in misinterpreted data. (I'm pretty sure this is already used in certain equipment, although I don't know in detail.)

You can transmit channel A and B simultaneously or at independent times. Each will be detected only by the receiver which is intended for it.

You can add channel C, etc.

Debdut

### Debdut

Points: 2
OK I'll try this. Thanks for the replies.

It sounds rather theoretical that the signal occupies the full channel without the smallest guard band.

Technically, I would start the design from the BSPK demodulator side. You have usually a vco and a quadrature demodulator in a costa's loop.

Yeah, thanks for the input.
I was seeing the Costas Loop architecture, however in addition to that I have to either do frequency planning or transmit at different times.
Any thoughts on these...

Hi,

I'm not experienced with RF.

But just looking at the frequency values makes me think it is not possible, because the carriers are 300kHz from each other and the bandwidth is also 300kHz, so there is a certain frequency that meet both criteria.

Klaus

Communication is not impossible, as you say, but it can be difficult. In a way you're right.
The channels I am using fall into the FCC MICS band 402 - 405 MHz with 300 kHz channel spacing.
I'm trying hard to find out the other specifications for communication in this band from the FCC web, but not getting much information.
Parameters like guard band, maximum distance between transmitters and receivers, modulation to be used, data rate, etc are still unknown to me.
All the information I can get about this band is from papers. The authors have cited the FCC website for MICS band, but there's not much information there.

FCC 47 part 95 specifies channels, radiated power respectively field strength, frequency tolerance, spectrum masks, but not modulation type or data rate. These parameters are beyond regulations requirements.

Debdut

Points: 2