US is primed to overtake Europe and Japan as the technological leader in cell phone technology

USS Clueless ^ | 5 Oct 2002 | Steven Den Beste

[_Jim]

I think FH is a CD

Nope, not quite.

The key is this: Look at the IF filter bandwidth.

A DSSS (direct sequence spread spectrum) system (which CDMA is) literally occupies a bandwwidth as wide as the 'chipping' code - as wide as 1.2 MHz or so for CDMA as used in cellular systems ...

Whereas a FH system has *only* has an IF BW as wide as the data or modulation rate expected ... and the LO is 'frequency hopped' to keep track of the transmitted 'hopped' data ...

There are other differences too, such as a FH system *still* requires a S/N ratio of some 10 to 15 dB or more whereas a CDMA system can have Eb/No (Energy per bit to noise ratio) of just over one ... equivalently a unity S/N ratio ...

All non-coherent 'codes' during recovery (mixing with the original chipping code again, a demodulation process if you will) still results in noise whereas the *desired* signal, when the recovered bit stream is low-pass filtered, will result in the original bit stream ...

[KayEyeDoubleDee]

As I understand it, CD is at the physical layer, and MA is basically the concept of multiplex access protocol, so it can be considered to exist at the network layer, although CDMA apparently could be conceptually lumped together and considered as the physical layer for other link/network layers, such as TCP/IP, but the latency calls for clever work-arounds

I will have to pick nits and note that CDMA is a very specific physical layer solution to the problem of multiple access of the air interface.

Not to get too technical, but compared to TDMA and FDMA, it is a more efficient use of the observation that multiplication in the time domain equals convolution in the frequency domain. This latter fact is the single most important fact behind the old school analog stuff (FM,AM) and most of the modern digital stuff (which is helped out by them fast little chips that crunch the little bits)

[#3Fan]

Thanks for the explanation. I thought it could've been something else. :^)

[apochromat]

Sorry for going a bit off topic, here.

I thought the MA in CDMA was a quick way of describing the idea that one shouldn't necessarily be able to immediately take any valid channel in a well-designed network of limited resources.

[_Jim]

MA in a cellular network practically requires a reservation protocol, as I see it.

How about a strict assignment to either a chipping code or a time slot?

Chipping codes (one of 64 orthogonal Walsh codes if I recall correectly) for a DSSS - CDMA system or a particular time slot for a time division system ...

Of course, each one also requires a frequency during a channel assignment and the TDMA system also sends a time alignment value to correct for distance from the cell site (otherwise your slot starts to overlap into adjacent slots) ...

[apochromat]

True, but I think you are somwhat confusingly using the term "physical layer" to lump together that which is actually a radio physical layer and a cellular network layer.

[#3Fan]

Of the other four nationwide mobile telephone operators, Sprint PCS and Verizon Wireless use CDMA as their digital technology, VoiceStream uses GSM...

Is Voicestream's GSM service available nationwide?

[KayEyeDoubleDee]

Sorry, the linked demo didn't do what I thought it would and I can't immediately find a good demo showing it.

[KayEyeDoubleDee]

lump together that which is actually a radio physical layer and a cellular network layer.

I don't know what the latter means. Do you mean the network of base station controllers?

[apochromat]

Right. Direct Sequence spread spectrum typically uses correlation filtering sets for each coded channel. Correlation is a variation of convolution.

[_Jim]

From the FCC's 7th report to congress:

Two-way SMS was introduced in the United States in May 2000 when VoiceStream began to offer the service.458

As of year-end 2001, the six nationwide mobile telephone carriers, as well as handheld providers and some smaller mobile telephone carriers, were offering SMS.459

At the end of 2001, AT&T Wireless became the first carrier to offer cross-carrier network SMS capabilities.460

A short time later, most of the nationwide carriers announced similar plans.461 Mobile telephone carriers that offer crosscarrier SMS have reported increased traffic since implementing this capability on their networks.

AT&T Wireless stated that it has experienced 29 percent SMS growth from November 2001 through the end of March 2002, while Verizon has reported SMS traffic up to 4 million messages per day.

This isn't *quite* correct -

- I know for a fact that it has been possible to send messages between various phones, PDAs and 2-way wireless pagers for sometime now - IF they had internet e-mail addressability through their various carriers ... we were doing 2-way e-mail messaging via Skytel's 2-way pagers in '98!

[_Jim]

Is Voicestream's GSM service available nationwide?

It seems they have entered into agreements with at least Cingular to provide some coverage in three states mentioned in the FCC report - they also have a name change coming up, to "T-Mobile" ... since they are the newest PCS provider on the block they are still building-out some parts of their network in areas they won in the spectum auction.

http://www.voicestream.com

[apochromat]

I'm saying that the MA grant mechanism can be distinct from the CD-based physical channel format. From an engineering standpoint, a empty channel CD multiplex link format can be built without concern for MA protocol, and MA capability can then be applied in a wide variety of ways.

[_Jim]

I'm saying that the MA grant mechanism can be distinct from the CD-based physical channel format

I think you're putting waaaay too much emphasis on these terms - practiaclly speaking it simply indicates the methodology used to attain 'separate' voice or data streams:

1. FDMA - Frequency Division Multiple Access (analog FM, uses different freq for each 'stream')
2. TDMA - Time Divsiion MA (uses same 'channel' (time shared spectrum) but different time slot for each 'stream')
3. CDMA - Code Divsion MA (uses same spectrum but different Walsh 'spreading' code for each stream)

And with that - I gotta get outa here!

[KayEyeDoubleDee]

I'm saying that the MA grant mechanism can be distinct from the CD-based physical channel format. From an engineering standpoint, a empty channel CD multiplex link format can be built without concern for MA protocol, and MA capability can then be applied in a wide variety of ways.

I think I see your point and we are definitely talking about the same things, but I always assumed that the MA (ie. "multiple access") does stand for the "CD multiplex link format" to which you refer.

Not a very good forum for technical discussions...

[apochromat]

To put it simply, the coded channels are one thing that defines part of the physical protocol, and getting an open coded channel is completely another. Link protocol (data formatting) and "MA" (network control protocol) are essentially separately designed and can be separately reprogrammed without changing the channel codes, which are the "CD" part.

[Ernest_at_the_Beach]

Thanks for posting this.

I skimmed thru it last night and found it to be very interesting!

OFFICIAL BUMP(TOPIC)LIST

[KayEyeDoubleDee]

Gotcha. See my 75.

[Erasmus]

Tks for the update on Cingular's plans.

[SauronOfMordor]

More precisely, Headly Lamarr and George Antheil invented a frequency-hopping scheme, not a direct sequence scheme such as is used in CDMA. And they did get a patent, but the technology to implement it widely was not available until after it expired.

And one interesting part of the story is that Hedy Lamarr, besides being the engineer who helped develop spread-spectrum technology, was also a gorgeous movie actress

See The Birth of Spread Spectrum