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#1 2020-Dec-22 21:46:44

wlJ
Member
Registered: 2019-Sep-02
Posts: 8

Beamforming does not increase the power of signal?

Hi , I am currently doing experiments about beamforming with 4 Tx antennas and 1 Rx antenna.
And we observed that the power of 4-elements BF signal is almost equal to that of the received signal with 1 Tx antenna.
I guess if the total output power is devided to each antenna equally by WARP v3? 
The antenna used in the experiment is common omnidirection antenna.

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#2 2020-Dec-23 19:36:04

murphpo
Administrator
From: Mango Communications
Registered: 2006-Jul-03
Posts: 5154

Re: Beamforming does not increase the power of signal?

I guess if the total output power is devided to each antenna equally by WARP v3?

No - the Tx power of each RF path is set by the Tx gain in that RF path's MAX2829 transceiver. The Tx power from one antenna does not automatically decrease when other antennas are transmitting.

For open-loop beamforming it's important to understand the phase relationships between the RF carrier signals on multiple Tx antennas. On WARP v3 hardware each radio interface uses a separate PLL (internal to the MAX2829). There is a random phase offset between any pair of MAX2829 Tx circuits; this phase offset changes any time the PLL is re-tuned (radio powerdown, reset, or channel change). For open-loop beamforming you must somehow estimate this phase offset among all Tx circuits and account for these phase offsets when setting your per-antenna beamforming weights.

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#3 2020-Dec-23 21:00:52

wlJ
Member
Registered: 2019-Sep-02
Posts: 8

Re: Beamforming does not increase the power of signal?

Thanks a lot for your reply.
When I implement beamforming, I have measured the phase offset among the four antennas and compensate these offset to each antenna.
What I mean is that assuming that the output power of WARP is 10dBm, does this mean "the power of each antenna is 10dBm" or "the total power of the 4 antennas is 10dBm"? In other words, will the total output power increase as the number of antennas increases?

I did an experiment to verify the transmit power of each antenna.
Mode 1: 1Tx-1Rx, we only enable the RFA port, connect one antenna to send a signal, and the receiver uses RF A to measure the LTS amplitude.
Mode 2: 4Tx-1Rx, we enable all the RF ports (A B C D ports), only connect one antenna to RF A or RF B or RF C or RF D to send a signal, and the receiver uses RF A to measure the LTS amplitude.

The test results show that:
(1) the signal strength measured by Mode 2 is less than the signal strength of Mode 1;
(2) The total amplitude obtained by adding the signal amplitudes measured by the 4 antennas of Mode 2 is approximately equal to the signal amplitude measured by Mode 1.

Thanks a lot.

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#4 2020-Dec-24 11:07:42

murphpo
Administrator
From: Mango Communications
Registered: 2006-Jul-03
Posts: 5154

Re: Beamforming does not increase the power of signal?

What I mean is that assuming that the output power of WARP is 10dBm, does this mean "the power of each antenna is 10dBm" or "the total power of the 4 antennas is 10dBm"? In other words, will the total output power increase as the number of antennas increases?

Yes, the total output power increases for each Tx RF interface that is enabled. The hardware does not automatically adjust any Tx powers - you adjust Tx powers from your code via the Tx gain settings of each RF interface.

A phase offset between antennas is the most likely explanation for non-constructive addition of waveforms from multiple Tx antennas.

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