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From: | Marcus D. Leech |
Subject: | Re: phase difference between Tx/Rx1 Tx/Rx2 |
Date: | Tue, 13 Oct 2020 12:42:32 -0400 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 |
On 10/12/2020 11:32 PM, Xiang Ma wrote:
Let us for a moment think about how radios actually work, rather than how mathematical models of radios work. You construct some signal at baseband--for simplicity a sinusoid. That sinusoid is sampled at some rate, typically a rate that is a fraction of the clock rate of the hardware. The basebanded signal is passed through a special interpolating filter called a DUC, before being presented to the DAC at the master clock rate. Once it leaves the DAC it passes through an analog band-limiting filter, and then to a mixer. The mixer has an LO at some phase that is random with respect to the baseband signal. Once it leaves the mixer, it is amplified and then presented to the antenna port, where it heads down a cable or is propagated via an antenna--the new signal coming out of the antenna or down a cable will have a phase that is a combination of all of those stages in processing your baseband signal. Now, the signal arrives at the RX, and the logical inverse of all of that happens. Now, in the best of all possible worlds, the RX side is a complete and perfect "mirror" of the TX side. The LOs are perfectly synchronized (so you can eliminate them from the phase estimates), as are the ADC and DAC clocks, and all the digital transformations on the signal. Now, depending on the type of USRP hardware you have, you *MAY* be able to synchronize the TX and RX LOs. There are very few applications "out in the world" that assume/require that the TX and RX frequency-conversion machinery be perfectly in-phase and coherent. Radar is one of them. But 99% of telecom type applications out there *necessarily* MUST assume that the two "ends" of a conversation are not mutually coherent with respect to all of the frequency-translation machinery. Now, there are comms applications that require that the TX machinery be mutually-coherent with itself--because of MIMO, and this applies to the RX as well. But what it DOES NOT require is that the TX and RX be mutually coherent with each other, because the TX and RX will be separate pieces of hardware, potentially separated by a considerable distance, operated by different parties, and usually different equipment. So, in order to proceed further, it would be good to know, in a high-level way, what it is you're ACTUALLY trying to achieve rather than focussing on these small details. Because it's very easy to spend a long time on these details and with a "bigger picture" it may be easier and more expeditious to help you... |
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