What is UMTS?

Last Updated: 05-Nov-2007

If UMTS is part of GSM, then what exactly is the difference between it and what we presently know as GSM? There is a tendency to refer to the old service as strictly GSM, but I need to clarify that calling it that is a bit of a misnomer. GSM is actually the name given to the entire system upon which a cell phone and its service are part. The term GSM encompasses everything from the air interface to the way in which the switches interact with one another, as well as to the landlines to which they connect. However, in this context we are using GSM to refer to the old 2.5G service that most North Americanís know as GSM. UMTS is the new iteration of the GSM air interface.

Okay, so now that we have the nitpicks out of way, letís delve into the differences between the old and the new air interfaces. The original GSM service was based on a TDMA (Time Division Multiple Access) scheme. This approach broke up a single channel into various ďslotsĒ, which phones took turns transmitting on in order to share the channel. The new UMTS service uses a CDMA (Code Division Multiple Access) scheme.

That immediate begs comparison to the existing networks that most North Americans refer to generically as CDMA (which, like GSM, is a bit of a misnomer). Currently the big CDMA providers include Bell Mobility, Telus, Sprint PCS, and Verizon. The comparison is certainly a valid one, but rest assured that both of these systems are quite different, though they are based on a similar air interface technology.

But getting back to GSM, a big difference between a TDMA-based system and a CDMA-based system is how the phone deals with handing off from one cell site to another. In TDMA-based systems the phone must wholeheartedly switch from one channel to another in order to switch towers. There are no halfway measures here, itís all or nothing. As a result, all TDMA-based GSM phones suffer from slight (but rather annoying) interruptions in the audio stream whenever a handoff occurs, and if one doesnít occur in a timely fashion, the user can experience rather devastating degradation of the call quality.

In a CDMA-based system all callers and all cell sites operate on the SAME FREQUENCY. The calls are separated by the magic of encoding differences that can be sorted out by the receiver. For this reason handoffs can be achieved without using an all-or-nothing approach. A phone can gently shift from one tower to the next by selectively receiving the data stream from multiple sites simultaneous. Subsequently handoffs are not audible.

However, there is a price to be paid for this approach. In order to CDMA to function correctly, each and every phone on the network must be operating just a hair above the noise floor. To prevent them from occasionally falling through the noise floor, rapid corrections in transmit power must be made. The concept works exceptionally well in practice, but occasional bouts of frame errors do occur, and so while CDMA doesnít have the interruptions from handoffs, it does offer its own unique type of audio problems. However, the overall effect on a conversation is markedly less annoying in a CDMA system than in the current TDMA system employed by GSM (though for some reason, handoffs on the old iDEN system are surprisingly non-invasive).

So, does that mean that UMTS sounds the same as existing CDMA networks? The answer to that is a resounding NO. I long ago realized that the decision by Qualcomm to employ noise-cancellation technology into their EVRC CODEC was a huge mistake. It has resulted in virtually all CDMA phones sounding ďslushyĒ and distorted whenever there is even a small amount of background noise present. It is the one reason why I personally have avoided CDMA systems and I cringe whenever I have to take a call from someone using one.

I was therefore very concerned that UMTS would suffer the same fate. Now that UMTS is here, Iíve had plenty of opportunity to test it out and Iím delighted to find that the GSM engineers were wise enough to avoid making the same mistake. While Iím not 100% sure, it seems that the exact same AMR CODEC presently used in 2.5G GSM networks is used in UMTS. If any attempt is made to reduce background noise by the CODEC, I certainly canít detect it, and so as a result there is no slushiness or distortion caused by background noise.

Many years ago I wrote an editorial entitled Future CODECS and what we might have to put up with. Near the end of the article I wrote:

ďI've got my fingers crossed that the engineers working on the CODECs for W-CDMA (UMTS) are not quite so single-minded. GSM users have a much higher expectation of sound quality and I rather suspect that their development teams understand this.Ē

Fortunately I was right, and it seems that GSM users have no reason to fear that UMTS will sound anything like the present CDMA networks.

Throughout my testing of the Sony-Ericsson K850i I was extremely impressed with the overall lack of interruptions to the sound as I travelled far and wide. I was also impressed with how the sound quality on UMTS was indistinguishable from that of regular GSM, but without all of the annoying handoffs and prolonged bouts of frame errors when a handoff did not occur in a timely manner.

Way back over 10 years ago I was mesmerized by the hype Qualcomm published about the capabilities of CDMA, but ever since the day the first CDMA network went online around here Iíve been terribly disappointed. UMTS however has proven that the hype was actually true, but it took a bunch of engineers who cared more about sound quality (than they did about stuffing as many users into the available spectrum as possible) to finally achieve it. In short, UMTS is CDMA done right.