Sprint PCS - Question on CDMA and data

Author

Question on CDMA and data

Bob

2007-10-20, 10:33 pm

I've been away from CDMA-based providers for awhile. I was wondering
if some of you knew the answer to a curiousity of mine.

I've heard that the more users on a CDMA system, the higher the noise
floor becomes, effectively shrinking the usable area of the cell.
When data is being used with EVDO, does this cause the same type of
effect?

Also, I've noticed many streaming audio and video features offered
with Sprint. What would happen if lots of people started using these
things all at once? How much data traffic can a cell realistically
handle before it can't provide the "400-800k" download speeds that are
mentioned in their advertising?

2007-10-21, 7:33 am

I think that the answer is different between the voice (1xRTT)and data
(EVDO) services.

For voice, things are 'classic' CDMA. Total carrier power does increase
as users are added. Each user uses a new spreading code (Walsh
code/channel) and his power looks like noise to all the others. The
protocol tries to make everyone deliver the same signal strength to the
receiver but C/N does suffer as users are added and there are some
issues which make things go to pot as users are added. Things get more
complicated as multiple cell sites are involved and RAKE effects need to
be considered.

However, for EVDO/data, things are quite different. On the downlink,
there is no code sharing at all. For each user, the entire RF carrier is
time-shared. At any instant, all the payload power is going to only one
user. A single user on a segment gets the whole shebang. Adding users
dilutes the capacity available and things slow down, just as for
multiple users on the same ethernet subnet, only traffic for one user
can be supported at any instant.

For data traffic, when a lot of people on the same segment/RF_carrier
start asking for a lot of capacity things break in a hurry. Where it
breaks is a pretty complex question but many EVDO systems only have a
single T1 backhaul so beyond 3 users they can't make 400kbps under the
best of situations. Even with two T1's and allowing the full rate of rev
A, after much more than a handful of users, nobody is getting the
'advertised rate'.

The real truth is that no cellular provider is able to provide true 3G,
never mind 4G, once the number of data users rivals the voice users of
existing 2G networks. To do that, every carrier needs a great deal more
infrastructures providing higher density sites, lower antennas and
probably more backhaul than they presently have. However, I don't
suspect most of them want to advertise this fact to Wall Street.

g

Bob wrote:
> I've been away from CDMA-based providers for awhile. I was wondering
> if some of you knew the answer to a curiousity of mine.
>
> I've heard that the more users on a CDMA system, the higher the noise
> floor becomes, effectively shrinking the usable area of the cell.
> When data is being used with EVDO, does this cause the same type of
> effect?
>
> Also, I've noticed many streaming audio and video features offered
> with Sprint. What would happen if lots of people started using these
> things all at once? How much data traffic can a cell realistically
> handle before it can't provide the "400-800k" download speeds that are
> mentioned in their advertising?
>

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