[ale] OT cellular protocol versions

[Alex Carver]

On 2015-01-22 18:45, Michael H. Warfield wrote:
> On Thu, 2015-01-22 at 19:29 -0500, Phil Turmel wrote:
>> On 01/22/2015 06:32 PM, Alex Carver wrote:
>>> Sounds like a Sidekick.
>>>
>>> What network is he on?  That affects the communication channel.  Not
>>> everything is LTE just yet, lots of things are still using various
>>> modulations and channel types (2G GSM, 3G EDGE, CMDA, 4G LTE or WiMax).
>>>
>>> It might be his phone, it might not be. Tower sharing can cause
>>> interference and vegetation can cause other issues.  If the phone gets
>>> dropped a lot then the tiny SAW filters used in some phones can be
>>> damaged causing poor reception.
> 
>> OMG!  Industry is using SAW filters?  (I'm presuming that's "Surface
>> Acoustic Wave").  My college department head was an early researcher on
>> these, and I played (briefly) with them as a sophomore EE.  Talk about a
>> blast from the past.
> 
> You're kidding, right?  SAW filters have been used by industry for
> decades.
> 
> Back in the early 70's I helped install a Harris Corporation High
> Channel (channel 10) broadcast transmitter that used a little tiny SAW
> filter (about the size of the ceramic oscillators found on most computer
> mother boards) for it's vestigial sideband filtering at the modulator
> signal level as opposed to the massive reflection cavities and hybrid
> ring duplexers operating at 200KW with the old transmitter.
> 
> IIRC, most of our GPS receivers are using SAW filters to separate out
> the satellite signals in parallel.  That's where your "channels" derive
> from.  It's the number of parallel outputs from the filters that allow
> you to process multiple GPS signals simultaneously.

Actually for the GPS receivers working on the US GNSS, all the
"channels" are the same frequency (1575.42 MHz for L1).  The separation
is digital (spread spectrum) because each satellite uses an orthogonal
code to encode its data stream.  An autocorrelator in the receiver
demodulates the signals in parallel using each of the codes (sometimes
called a "chipping code").  When the autocorrelator gets the right code
for a particular satellite, the satellite signal pops up out of the
noise (SNR of the raw signal is actually below unity, looks like simple
white noise).  This chipping and autocorrelation is what makes it
difficult to jam GPS by using a simple transmitter with a plain carrier
on L1 (jamming being trying to scramble the signal, overloading the
front end so that it goes deaf is a different matter).  The number of
simultaneous channels receivable just depends on the power and quality
of the DSP in the receiver.  Most can receive 12, some can receive 20 or
more.

The receiver does use a single SAW filter on the front end to pull in L1
(and sometimes a second one for one of the other frequencies like L2
through L5 depending on the receiver) and keep the rest of the noise
out.  Bandwidth is about 100 MHz for L1 and 1 GHz for L2 (L2's chipping
code is 10 times larger than L1).