On 2016-09-11 21:34, Horace Algier wrote:
> BTW, is there any *technical* advantage of CDMA over GSM?
you need to differentiate between CDMA and cdma.
CDMA is a proprietary implemenetation by Qualcomm (protocol IS-95) of a
network that uses a cdma air interface.
GSM 2G used a tdma air interface (time division multiple access).
GSM 3G and 4G use cdma air interface (code division multiple access)
tdma provides garanteed quality since each channel has its own time slot
and nobody else talks during that time slot.
cdma is basically technoplogy that lets you listen to a single person in
a crowd of people taking at same time. But when too many people talk at
same time, you may have problems picking up a single person's signals
and voice may break up. But it allows one to support much hyigher number
of people per frequency band than with tdma.
Qualcomm's CDMA essentially stopped at 2.5G speeds (though marketed as
3G). GSM's 3G (HSPA for data and UMTS for voice) offer far superior
speeds than the old CDMA.
Although both use a cdma air interface, the GSM implementation uses
wider channels, better voice codecs and compression than the older CDMA
whose development stopped.
CDMA topped out at about 3mbps while GSM with LTE now offers about
450mbps. (HSPA for 3G had about 45mbps).
The wider the channel, the more efficient the compression can be.
Consider this. If you have 1 hz channel of light, at each second, you
can transit either light or no light (1 bit per baud).
But if for each cycle, you can transmit no light, red light, green light
or blue light, then you have 4 possible values per cycle or 2 bits. you
have doubled your capacity.
And if you can transmit combinations of lights, you then have
black
red
green
blue
red-green
red-blue
blue-green
red-green-blue
So instead of transmitting 1 or 0 (1 bit) you are now able to transmit 8
possible values (3 bits) and have trippled your throughput.
This is why the ability to combine channels in LTE is such a big thing
because once channel of 10mhz can compressss far more bits per hertz
than 2 channels of 5mhz.
Initial "combinations" on LTE were for adjacent channels eg: A+B and
the more advanced LTE version can now combine channels that are not
adjacent. (eg: A+C).
the IS-95 CDMA is basically the 1200 baud dial-up of wireless while LTE
today liek like fibre to the home.
Where Verizon was able to stay in business is that the air interface is
one aspect, but the link between the antennas and the central office is
another one and this is where AT&T faltered big time and while the air
interface was 3G and capable of over 40mbps, its links on the ground
were grossly underprovisioned and on average provided less than 3mbps
throughput per user (so Verizon stayed faster for some time because of
AT&T incompetence on ground).