The ping-pong secrete of GPRS and GSM.
One basic thing of GSM and also of GPRs is
that the handset doesn't send while recieving, greatly
boosting battery efficiency, cheaper to achieve
higher dynamics (difference between max and min)
of transmit and recieve signal, size of cell.
For example, a GSM handset never transmits
at the same time it receives, no expensive
"duplex filters" (except for those USA models
needing to be AMPS compatible) the network
is responsible for selecting different time slots
for the two functions, even maybe considering neighbouring,
competing opertor cells and optimally leaving one "silent"
slot inbetween.
...speak........listen.........speak........listen
instead of doing both at the same time
For example, having a competing operator in a
next-next-cell having it's slots "sliding by"
will disturb "my cell", cooperation helps!!
(compare to CDMA situation were 3-4 basestations
interfere or cooperate with the same channel code,
synchronization, nonideal correlation, one basestation
indoors or underground, not GPS synchronizable,etc)
Anyways, it is obvious that a GPRS handset cannot
receive all 8 timeslots while transmitting on one
or all 8 timeslots, except if theoretically very close
to a basestation.
send.send.listen.listen.listen-listen.send.send.etc
The 4-4 division is one, except that the more practical one
is to transmitting one or two while receiving 2-4 or 5-7
max.
The packet-thing is obviously to only use the slots
needed, shut up if nothing to say (save battery
and cell capacity)
Another basic thing is that GSM "wastes" at least half of
the "capacity" to increase the efficiency of the
power amplifier to gain battery time, GSM is by design
working in C or D-class, transmitting only 01010101
sequences, not a "HiFi" linear signal (A or B class,
to stick to HiFi terms)
This is the "secrete" of EDGE, by using a linear
power amplifier, seemingly wasting power and
battery efficiency, cost,etc, one can easily
double the raw data rate.
However, if the grid of basestations is dense enough
or one is close enough to a basestation this doesn't
add any cost!!
Additionally one can use "higher orders" of modulation,
as 8-16QAM to further increase the bitrate when
the threshold of adding linear PA-stages is done.
Note that:
CDMA demands linear PA-stages not to waste most of
the neighbouring bands as "guardbands" (GSM cellplanning
includes this "loss in capacity")
With GSM-like PA amplifiers in the "pure" CDMA handsets
would have "severe problems" coexisting with other systems.
Basestations always have to have expensive, linear power
amplifiers!!
That is, the important thing is the cost of the number
of paying customers and handsets one basestation can
support compared to the cost of those handsets!!
(sometime setting up the mast and the connecting links might
be the major cost)
This including the problems of neighbouring basestations,
maybe competing operators, and number of handsets needing
one or three basestations to function (soft handoff,
sectoring,etc)
Anyway, back to GPRS:
8 time slots, max 7 or more practical 4 used to recieve,
one or two to transmit, battery and power saving
ping-pong, half duplex,TDAM system (like the ping pong
WCDMA of China and HDR).
If one is in a dense basestation grid one can additionally,
with EDGE, double, tripple or more, the basic datarate,
switch on more battery wasting EDGE power stages in
linear mode, use "higher order" modulations.
If one additionally moves slowly or not at all, these
"higher order" modulations can, just like with CDMA,
be used more easily, reaching a higher efficiency,
transmission rate per Hz!!
Additionally one use the "raw data rate", no extra
coding, bit interleaving needed to cope with
"moving channels", neighbouring cells,etc.
And while continuosly sending,recieving, non-TDMA-CDMA
always need to fight non-optimal, non-theoretical
channelcorrelations,etc, power ratios,etc, EDGE-GSM-TDMA
has everything under control!!
Thats why the one "in the business" takes many CDMA and
WCDMA claims with a kind of cynicism, reminding to
consider the whole system as well as the business idea!!
The mythical single cell Q-CDMA, isolated from reality
in the middle of the desert beating any GSM cell in
crowded Manhattan, but using either linear power stages,
tripple size batteries or quadraple band guardbands
for 100 miles is a regular joke!!
If one adds smart, adaptive antennas, like flash lights
in the night, at least tripling the cost of basestations
and making a handset user wearing a funny looking antenna
hat, one can hype the business even more..
Luckil most operators figure out these things by
actually trying out systems..
Ilmarinen.
Need to reboot another set of WinBoot installations... |
