Tx Beam forming

[cristian94]

Hello,

I would like to make a 'basic' Tx beam forming application using USRP 2954Rs.

I want to steer the direction of the beam towards the receiver, whose position I know, and compare data to the case in which there is no beam forming applied.

Method 1: Use one USRP as 'client' (whose position is known) and another one as transmitter (sort of AP) ;

                On this second USRP, I would use 2 Tx antennas (and transmit the same signal). I would have beam forming if I made a phase shift between these 2 transmitting channels, so that the receiver would be in the position of constructive combination. Is this possible? I read here [link removed], that there is a 'constant but arbitrary phase offset on each channel.' , which is undesired, but can I somehow control the phase shift between channels ? Is delaying the signal on one of the channels enough to achieve this , or is there any other method ?

Method 2: MIMO communication at transmitting side in order to correct the arbitrary phase shift between channels. In the same white paper, 'Beam forming applications however require phase alignment of the LO, so that phase can be controlled or captured through baseband I and Q'. Again, how?

I would really appreciate some advice.

Worth mentioning is that I have only 2 months experience with Labview & Labview Comms and USRPs.

Thanks!

[Chantastic!]

Hi Cristian,

With regards to your first question, the article states "During synthesis, as the reference is divided, the phase may lock on either rising or falling edges producing a constant but arbitrary phase offset on each channel." This means that each reference may lock one period between each channel. Because this is a physical hardware limitation, there is little you can do in terms of physically delaying the signal in one of the channel.

With regards to your second question, you can provide a 10MHz reference to the both USRP "Ref In" ports, and specifying that in your LabVIEW code as the reference source. See the functional block diagram to see how the REF IN port can be used as a LO reference.

Regards,

Stephen 

[cristian94]

Hi Stephen,

Thank you for your answer.

Any further ideas on how to modify the phase shift between USRPs ?

Regards,

Cristian

[Chantastic!]

Hi Cristian,

You could synchronize the USRPs, write phase shifted waveforms to each, and have them start at the same time. However, as we discussed, you will run into issues with that constant phase delay. 

Is there a reason why you are trying to accomplish this? Theoretically this sounds plausible, but I feel like there are more robust methods of achieving beamforming, as the constructive/deconstructive interference from reflections in the environment would limit the effectiveness of just a phase delay. 

Regards,

Stephen

[cristian94]

Hi Stephen,

Well, this is what I thought I could accomplish, also not really taking into account reflections, since I only want to see how this Tx beam forming could be done in its most basic forms. ( experiment in an environment with as small reflections as possible, LoS propagation, small distances etc.)

 Regarding other methods, I don't know any that is not using phase shifting..

Regards,

Cristian

[Chantastic!]

Hi Cristian,

In order to change the phase, you would have to implement that in the software to send a phase delayed waveform.

I would recommend looking at channel estimation and using that route. There's a few Youtube lectures with information on the math behind it. 

Regards,

Stephen

[vutran]

Hi Stephen,

I have the similar question too. I manage to control the phase and try the Tx beamforming. However, it is quite weird that the signal strength is not highest at LoS positions. So is it normal due to multipath or I didn't do the correct beamforming?

Thank you.

Regards,

Vu