CDMA vs. TDMA

[Flametamer]

I might not have the initials right but what is the difference between these two cellular formats? You don't have to give a very technical answer, just the basics.

Thanks
darrin

[ltec123]

I hope I am right!



CDMA = Code Division Multiplexing
TDMA = Time Division Multiplexing

CDMA = All subscribers communicate constantly via frequency or code modulation.

TDMA - Each subscriber has time when it communicates with the system (in ns)

[mts2000des2]

I might not have the initials right but what is the difference between these two cellular formats? You don't have to give a very technical answer, just the basics.

Thanks
darrin

CDMA stands for Code Division Multiple Access. CDMA assigns each call a specific code, and is transmitted spread spectrum over a single radio channel, simultaneously with other calls. CDMA is a superior method because it offers much more capacity than TDMA based systems, and also offers more security than TDMA since calls are encrypted and sent out over spread spectrum radio. CDMA networks also offer seamless "soft handoff" since your call can be rec'vd at multiple sites at the same time, and CDMA also offers less interference to nearby devices due to the lower power output of CDMA handsets. CDMA was developed by Qualcomm, an American company and has been adopted by many carriers such as Verizon, Sprint, Crickett, Metro PCS, Hargray Wireless and others in the US. It is also used by wireless companies in Canada. The next generation GSM world standard is gong to be based on a wideband variant of CDMA, known as wCDMA, to replace the exsisting TDMA based GSM systems in use today. Such "3g" systems will offer incredible data throughput, much more voice capacity, and multimedia capable handsets.

TDMA stands for Time Division Multiple Access, and is the original and first method of digital modulation for wireless networks. Basically, your call is broken down into packets (or frames) and sent at specified interavals on a single voice channel, with other users sharing time slots. Since TDMA is not wideband, only a certain number of users can share a channel, most carriers use either 3 to 1 or 6 to 1 compression. The higher the ratio of division means less time to send a given packet, so audio quality (and data throughput) decline. TDMA systems have been around longer than CDMA, and thus is more widely available, plus it is the current modulation scheme used by exsisting GSM worldwide wireless systems. U.S. carriers such as Cingular, AT&T, and other smaller operators use a TDMA system known as IS-136. Voicestream (now T-Mobile) and other GSM providers use a TDMA based GSM for their networks. Also, Nextel uses a TDMA based system called iDEN. It is important to note regarding security that most IS-136 carriers DO NOT enable voice channel encrytption, as it adds too much overhead. iDEN does also NOT support voice channel encryption whereas GSM does have built-in voice encrytption.

There are pro's and con's to both. CDMA systems generally offer more voice capacity than TDMA, though there aren't as many CDMA phones being made as their are TDMA/GSM phones. Most CDMA and TDMA phones also offer fallback capability for analog, so you will not be without service in rural areas and smaller towns. iDEN and GSM phones have NO analog fallback, so if you are out of range of the company or roaming partner's network, you are basically screwed.

I am a CDMA fan hands down. It is a much more efficient use of radio spectrum, about as "secure" as a consumer can get for commercial wireless, and the next generation CDMA systems are promising as far as data capability. Voice quality on a good CDMA phone and network also stand out above the rest, at least in my opinion.

[IHateEDACS]

Okay. So lemme get this straight. TDMA is where like 1 second worth of voice audio is compressed and therefore only takes 1/3 or 1/6 of a second to transmit ? Wouldn't that cause some sort of a delay in the phone that receives the audio ?

And CDMA is where the frequency you're talking on changes like 10 times a second or something ? That's crazy !! How can that even work ? Wouldn't that make the audio sound really bumpy ?

[mancow]

You're forgetting it's all digital. It's not analog so there are no "bumps" like a simple FM spread spectrum radio, but the audio on our nextel phones is crap. It's the trade off like he said due to the compression from what I understand.
I also get a-lot of cross talk in the background at times. As for delay, there is a 1/4 second or so delay from what I have noticed when hearing real time radio traffic in the background of people I'm talking to.
I wonder how the phone is instructed as to timing of data packets. When I use my nextel next to the AM radio it make a loud series of random clicks that sound like a snare drum. The guy at the next dest even starts air drumming at times

I guess the timing info is interweaved with the packets of data.

mancow

[Twisted_Pear]

And CDMA is where the frequency you're talking on changes like 10 times a second or something ? That's crazy !! How can that even work ? Wouldn't that make the audio sound really bumpy ?

A CDMA carrier seen on an analyzer looks like bart simpson's head. IIRC, it's a 1.25MHz wide carrier. You have paging, sync, and traffic channels that each are assigned a walsh code (two digit hex value). They're not frequency hopping in the literal sense, everything is just spread out.

It's similar to digital cable tv. You have a digital carrier that's somewhere around 5MHz wide, looks like a hay stack, and has something like a hundred or so channels within the stream.