Hello,
If we consider a 8Ghz Microwave Link. Which is currently running on 4QAM modulation and 28Mhz channel spacing. What affect will be on the overall link performance specially the Receiver Sensitivity and Capacity, if we change the modulation scheme to 16Qam or 256Qam?

Detailed answer will be helpful as my concepts are not very good.

Thanks!

 

(Verify this with Web research of your own)

Receiver sensitivity will be lowered. S/N ratio will be lowered.

Increasing the number of sub carriers, means overall each sub channel has lower peak power, so noise levels can increase data error rate. Meaning capacity could be lowered drastically when data transmission rates allow background noise to affect the receiver. It begins to generate large numbers of error bits and may drop content or otherwise malfunction from buffer overflow.

 

What Kermit2 says is true except that some of that loss in sensitivity can be offset by increased error correction.

 

You mention that increasing the number of sub-carriers from changing the modulation scheme from 4Qam to 256Qam will drastically decrease capacity. But in most microwave radio's the increase in modulation scheme (higher) results in more capacity. For example here is a page from a brochure which mentions traffic capacity as per modulation scheme and channel spacing. http://min.us/jbeAqO.png

Can you please clarify this.