openbts-discuss

[Openbts-discuss] Amplitude variation in TX?

[Ralph A. S. d. <ra...@sc...>]

Hi,

As I am back again with full TX power (thanks, Tom!), I noticed that the
signal produces the typical 1.7something KHz tone in the PC speakers here in
the office. As this is GMSK there should be only some frequency/phase
variation, at constant amplitude. Also the bandwidth is small enough that I
do not expect differing sensitivity of the PC speaker from the frequency
deviation. 283 KHz are nothing at 1800 MHz :) So why can I hear the frame
rate in the speakers? Did I miss something, or is there some inconsistency
in the modulation code, causing some amplitude variations? 

Ralph.

Re: [Openbts-discuss] Amplitude variation in TX?

[Tom T. <to...@ts...>]

On Tue, Mar 10, 2015 at 5:16 AM, Ralph A. Schmid, dk5ras
<ra...@sc...> wrote:
> As I am back again with full TX power (thanks, Tom!), I noticed that the
> signal produces the typical 1.7something KHz tone in the PC speakers here in
> the office. As this is GMSK there should be only some frequency/phase
> variation, at constant amplitude. Also the bandwidth is small enough that I
> do not expect differing sensitivity of the PC speaker from the frequency
> deviation. 283 KHz are nothing at 1800 MHz :) So why can I hear the frame
> rate in the speakers? Did I miss something, or is there some inconsistency
> in the modulation code, causing some amplitude variations?

There is a guard interval between bursts with full ramp-up and
ramp-down, so amplitude effects do exist.

As far as finding the exact cause of interference, that is difficult
to determine. When I used to have external PC speakers, before
receiving a call, I would 'hear' the phone through the speakers prior
to ringing.

Does the PC speaker behavior relate to a recent change? Or is this
just a new observation?

 -TT

Re: [Openbts-discuss] Amplitude variation in TX?

[Ralph A. S. d. <ra...@sc...>]

Hi,

>There is a guard interval between bursts with full ramp-up and ramp-down, so amplitude effects do exist.

Full ramp down? For more than 20 years my impression was that the carrier with the BCCH is CW.

>As far as finding the exact cause of interference, that is difficult to determine. When I used to have external PC speakers, before receiving a call, I would 'hear' the phone through the >speakers prior to ringing.

Sure, this is normal due to timeslotted uplink. But the downlink?!

>Does the PC speaker behavior relate to a recent change? Or is this just a new observation?

Well, this was at work, and I never had the B210 so close to the speaker like today. From the normal position there is no EMI.

By the way, for a full ramp down and up the EMI seemed not sharp enough, it was a softer effect. Starting or stopping the transmission gives a fat "pop", the 1.7 KHz noise is just a soft whine.

 -TT

Ralph.

Re: [Openbts-discuss] Amplitude variation in TX?

[Tom T. <to...@ts...>]

On Tue, Mar 10, 2015 at 10:40 AM, Ralph A. Schmid, dk5ras
<ra...@sc...> wrote:
>>There is a guard interval between bursts with full ramp-up and ramp-down, so amplitude effects do exist.
>
> Full ramp down? For more than 20 years my impression was that the carrier with the BCCH is CW.

Yes. Agilent has an application note on BTS power levels.

http://literature.cdn.keysight.com/litweb/pdf/5968-2320E.pdf

But, strictly speaking, I recall that ramping is not required in
certain cases or at all. In fact, my very old Agilent 8960 test set
does _not_ ramp between bursts. Interestingly, the Agilent E4406A
tester does not detect the Agilent generated test signal.

I took a quick look at the 3GPP specification, but couldn't find the
exact wording. In any case, practically all modern GSM downlinks do
ramp up and ramp down during the guard interval.

TS 05.05 Radio Transmission and Reception - 4.5 Output level dynamic operation

 -TT

Re: [Openbts-discuss] Amplitude variation in TX?

[Ralph A. S. d. <ra...@sc...>]

Hi,

> http://literature.cdn.keysight.com/litweb/pdf/5968-2320E.pdf

Thanks. I will read through when I find some time. At first glance I can't find any hint that the power of BCCH carrying signals is ramped up and down. 

> I took a quick look at the 3GPP specification, but couldn't find the exact
> wording. In any case, practically all modern GSM downlinks do ramp up and
> ramp down during the guard interval.

I do not see the sense in this. It would create more EMI, increase the bandwidth and the noise on the bands.
 
> TS 05.05 Radio Transmission and Reception - 4.5 Output level dynamic
> operation

Without having the time to read through this right now, the output level only should be dynamic on carriers without BCCH. For those only bursts are transmitted when there is something useful to transmit, of course guard intervals apply, and the power levels are adjusted dynamically as needed. 
 
>  -TT

Ralph.