Post subject: Re: GSM mobile phone
Posted: Fri Feb 26, 2010 12:18 pm
Joined: Sat Mar 04, 2006 10:12 am
The main two differences between any of the
telecom standards are the frequency bands of
operation and the modulation techniques. Some
use frequency modulation, others use phase modulation,
but all are some form of digital modulation.
Just as there are disparate groups of auto enthusiasts
that fervently believe their choice of car is
the best, groups exist in the mobile communications
cadres that think theirs is the best. Some systems
are better than other form the standpoint of
bandwidth efficiency, but that often comes with
a degradation in call quality - particularly
in weak signal conditions.
Take for example
16-QAM, where the receiver must correctly discern
16 unique phase angle/amplitude combinations
to properly decode a data package. With a strong
signal and a slow-moving platform (a walking
talker), a phone can usually get near 100% of
the data packages without the need for error
correction or maybe have to request that the
packet be re-sent from the tower. Same goes
in the other direction from talker to tower.
Wne a user is moving quickly in a low power
enviroment with lots of signal interferes around
(electronics and physical), the system often
breaks down. That is because of ambiguity in
phase, amplitude. Take a simple BPSK signal
in the same environment that only needs to discern
between two separate states and it has a much
better chance of surviving. Problem is, though,
is that BPSK takes 8x the BW of a 16-PSK signal.
You can look up the technical difference
between the various mobil phone systems, but
in a nutshell this is the difference.