Overlaying GSM on TDMA

When Cingular says they are overlaying GSM on TDMA frequencies, there
are certain things which don't make sense to me. I would appreciate it if
someone could help me understand how this works technically.

1) Do GSM and TDMA occupy the same timeslots at the same time? Since
that seems impossible, I'm going to assume the answer is "no".

2) If GSM and TDMA cannot occupy the same timeslots at the same time,
then how does the switch allocate bandwidth? Is it done dynamically such
that the GSM handsets request the bandwidth and it is given to them from
all available bandwidth including bandwidth which could potentially have
been used by TDMA handsets? Or are some frequencies given for exclusive
GSM use and some for exclusive TDMA use?

3) Can wideband-CDMA be used in the same frequencies at the same time as
GSM (using a TDMA air interface)? I know, in theory, it can be done, but
the question I'm asking is whether it will be done when the carriers start
upgrading to W-CDMA. Has anyone read any information about hybrid
CDMA/TDMA systems using the same bandwidth at the same time?

RDT
--
"The inherent vice of capitalism is the unequal sharing of blessings; the
inherent virtue of socialism is the equal sharing of miseries."
--- Sir Winston Churchill

---

› See More: Overlaying GSM on TDMA

[email protected] ("RDT") wrote in message news:<[email protected]>...
> When Cingular says they are overlaying GSM on TDMA frequencies, there
> are certain things which don't make sense to me. I would appreciate it if
> someone could help me understand how this works technically.
>
> 1) Do GSM and TDMA occupy the same timeslots at the same time? Since
> that seems impossible, I'm going to assume the answer is "no".
>
> 2) If GSM and TDMA cannot occupy the same timeslots at the same time,
> then how does the switch allocate bandwidth? Is it done dynamically such
> that the GSM handsets request the bandwidth and it is given to them from
> all available bandwidth including bandwidth which could potentially have
> been used by TDMA handsets? Or are some frequencies given for exclusive
> GSM use and some for exclusive TDMA use?
>
> 3) Can wideband-CDMA be used in the same frequencies at the same time as
> GSM (using a TDMA air interface)? I know, in theory, it can be done, but
> the question I'm asking is whether it will be done when the carriers start
> upgrading to W-CDMA. Has anyone read any information about hybrid
> CDMA/TDMA systems using the same bandwidth at the same time?
>
> RDT

I'm not technical enough to answer all your questions, but from what i
have read, in the case of a carrier that is overlaying GSM in a TDMA
area using the TDMA bandwith, they basically take away a given
percentage from TDMA and set it up for GSM. Then, as usage grows on
GSM and shrinks on TDMA they will go back and re-adjust their
allocation for each. I'm sure getting that equation just right is not
an easy task.

RDT wrote:
> When Cingular says they are overlaying GSM on TDMA frequencies, there
> are certain things which don't make sense to me. I would appreciate it if
> someone could help me understand how this works technically.
>
> 1) Do GSM and TDMA occupy the same timeslots at the same time? Since
> that seems impossible, I'm going to assume the answer is "no".

You're right, it's no.

>
> 2) If GSM and TDMA cannot occupy the same timeslots at the same time,
> then how does the switch allocate bandwidth? Is it done dynamically such
> that the GSM handsets request the bandwidth and it is given to them from
> all available bandwidth including bandwidth which could potentially have
> been used by TDMA handsets? Or are some frequencies given for exclusive
> GSM use and some for exclusive TDMA use?

GSM and TDMA cannot share a radio channel. Each is allocated a
percentage of what the RF engioneers believe is needed in each cell
sector, and can be adjusted almost daily.

>
> 3) Can wideband-CDMA be used in the same frequencies at the same time as
> GSM (using a TDMA air interface)? I know, in theory, it can be done, but
> the question I'm asking is whether it will be done when the carriers start
> upgrading to W-CDMA. Has anyone read any information about hybrid
> CDMA/TDMA systems using the same bandwidth at the same time?

RDT, where do you come up with this? )) Honest, I don't have a
clue about this one - maybe somebody else?


--
jer email reply - I am not a 'ten' ICQ = 35253273
"All that we do is touched with ocean, yet we remain on the shore of
what we know." -- Richard Wilbur

On 28 Oct 2003 13:57:28 -0500, [email protected] ("RDT") wrote:

> When Cingular says they are overlaying GSM on TDMA frequencies, there
>are certain things which don't make sense to me. I would appreciate it if
>someone could help me understand how this works technically.
>
>1) Do GSM and TDMA occupy the same timeslots at the same time? Since
>that seems impossible, I'm going to assume the answer is "no".
The timing for D-AMPS (TDMA) and GSM have very little in common.
There are 3 time 'slots' in TDMA, and compared to GSM, the slots are
VERY long. This greatly reduced the timing advance issues. The
modulation schemes and data structures are quite different. About the
only thing the two share is the fact that they are both Time
Divisision Multiple Access. The timings, modulations scheme, codec,
signaling, and data formats have essentially nothing in common. A GSM
phone would probably see a TDMA phone as noise, and vice vesa.
>
>2) If GSM and TDMA cannot occupy the same timeslots at the same time,
>then how does the switch allocate bandwidth?
They do not share bandwidth at all.
> Is it done dynamically such
>that the GSM handsets request the bandwidth and it is given to them from
>all available bandwidth including bandwidth which could potentially have
>been used by TDMA handsets? Or are some frequencies given for exclusive
>GSM use and some for exclusive TDMA use?
>
>3) Can wideband-CDMA be used in the same frequencies at the same time as
>GSM (using a TDMA air interface)?
Yes as long as the processing gain is fairly high. CDMA is a digital
analog of synchronous detection, so all signals except the one you are
interested in tend toward zero power when demodulated. The GSM signal
is narrow compared to the CDMA, and simply raises the noise floor for
CDMA. The link margins for GSM are such that the GSM phone probably
doesn't even notice the CDMA noise. GSM active calls simply reduce the
capacity of the CDMA channel, much as other CDMA calls in the same
cell and block reduce the capacity, although a GSM call probably
whacks out more capacity then a CDMA call. How well this will work
for high bandwith CDMA applicactions, where the processing gain is
orders of magnitude lower is a more interesting question.

> I know, in theory, it can be done, but
>the question I'm asking is whether it will be done when the carriers start
>upgrading to W-CDMA. Has anyone read any information about hybrid
>CDMA/TDMA systems using the same bandwidth at the same time?
>
>RDT

[email protected] ("RDT") wrote in message news:<[email protected]>...
> When Cingular says they are overlaying GSM on TDMA frequencies, there
> are certain things which don't make sense to me. I would appreciate it if
> someone could help me understand how this works technically.
>
> 1) Do GSM and TDMA occupy the same timeslots at the same time? Since
> that seems impossible, I'm going to assume the answer is "no".

Correct- it's "no".

> 2) If GSM and TDMA cannot occupy the same timeslots at the same time,
> then how does the switch allocate bandwidth? Is it done dynamically such
> that the GSM handsets request the bandwidth and it is given to them from
> all available bandwidth including bandwidth which could potentially have
> been used by TDMA handsets? Or are some frequencies given for exclusive
> GSM use and some for exclusive TDMA use?

AFAIK, it's not dynamic. They use x# of the available channs for TDMA
and x# for GSM, and simply reallocate as the "balance of power"
changes.

> 3) Can wideband-CDMA be used in the same frequencies at the same time as
> GSM (using a TDMA air interface)? I know, in theory, it can be done, but
> the question I'm asking is whether it will be done when the carriers start
> upgrading to W-CDMA. Has anyone read any information about hybrid
> CDMA/TDMA systems using the same bandwidth at the same time?

No, but I assume any carrier cramming in TDMA, GSM, and CDMA as well
as analog (like a rural "roaming" carrier trying to be
jack-of-all-trades) would simply allocate x# channels for each type of
technology based on the anticipated traffic "mix".

""RDT"" <[email protected]> wrote in message
news:[email protected]...
> When Cingular says they are overlaying GSM on TDMA frequencies, there
> are certain things which don't make sense to me. I would appreciate it if
> someone could help me understand how this works technically.
>
> 1) Do GSM and TDMA occupy the same timeslots at the same time? Since
> that seems impossible, I'm going to assume the answer is "no".
>
> 2) If GSM and TDMA cannot occupy the same timeslots at the same time,
> then how does the switch allocate bandwidth? Is it done dynamically such
> that the GSM handsets request the bandwidth and it is given to them from
> all available bandwidth including bandwidth which could potentially have
> been used by TDMA handsets? Or are some frequencies given for exclusive
> GSM use and some for exclusive TDMA use?
>
> 3) Can wideband-CDMA be used in the same frequencies at the same time as
> GSM (using a TDMA air interface)? I know, in theory, it can be done, but
> the question I'm asking is whether it will be done when the carriers start
> upgrading to W-CDMA. Has anyone read any information about hybrid
> CDMA/TDMA systems using the same bandwidth at the same time?
>
> RDT
> --
> "The inherent vice of capitalism is the unequal sharing of blessings; the
> inherent virtue of socialism is the equal sharing of miseries."
> --- Sir Winston Churchill
>
1) No
2) Frequencies are assigned for exclusive use by each technology
3) WCDMA is a 5 MHz carrier. Remains to be seen how it will be deployed in
the US. Watch ATT Wireless since they are deploying in limited locations per
the DoCoMo partnership.

Jer, thanks for your answers! Let me address your point.

In article <[email protected]>, Jer <[email protected]> wrote:
>RDT wrote:
>> 3) Can wideband-CDMA be used in the same frequencies at the same time as
>> GSM (using a TDMA air interface)? I know, in theory, it can be done, but
>> the question I'm asking is whether it will be done when the carriers start
>> upgrading to W-CDMA. Has anyone read any information about hybrid
>> CDMA/TDMA systems using the same bandwidth at the same time?
>RDT, where do you come up with this? )) Honest, I don't have a
>clue about this one - maybe somebody else?

CDMA and TDMA can co-exist because one appears nearly invisible to
the other. Notice I say nearly invisible. Apparently, using CDMA and
TDMA in the same channel increases the noise level for both. Several
authors have said it is possible for both CDMA and TDMA to co-exist on the
same channels at the same time, however, at this time, it would require
more sophisticated equipment to reduce interference. Frankly, it's a
great idea if they could get it to work because Cingular might effectively
double their capacity by using both modulation techniques in the same
bandwidth. W-CDMA would be used for data, GSM (TDMA) for voice, with
W-CDMA used as bleed over if the GSM (TDMA) sub-channels filled up.

RDT
--
"The inherent vice of capitalism is the unequal sharing of blessings; the
inherent virtue of socialism is the equal sharing of miseries."
--- Sir Winston Churchill