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CDMA Overview

September 12th, 2003 · No Comments

Russell Beattie spent time learning about CDMA and shares it with us:

First, here’s the deal about CDMA vs. GSM. The way that GSM works is really an extension of the old TDMA analog system. GSM adds digital technology to divide up the frequency allotments into 8 channels, which are then time shared across those channels. CDMA on the other hand uses the same piece of spectrum and separates the calls by encoding each one uniquely, allowing your phone to disregard other transmissions on the same frequency.

Just on the face of it, you can see that CDMA seems more scalable and easier to manage because the system uses just one frequency. However, enabling this requires a Rube Goldbergesque combination of adjusting power levels, GPS based time settings and other complex stuff that I don’t even have the faintest idea of. I can see why the GSM camp snuffed their nose at it in the beginning. It’s amazing it works at all.

Okay, so the basic CDMA that most people uses right now is now called “cdmaOne”. The next generation is the move to “CDMA2000 1x RTT.” This is what Verizon is spending all their money buying. The 1x stands for “single channel” and the RTT (which Qualcomm doesn’t like to use any more, though it was written in the article that way) stands for “radio transmission technology.” Even though the speeds of this new standard are really what has been considered 2.5G, the technology is the base for higher speeds and has been deemed 3G by marketing higher ups, so you’ll see it referred to that way.

The 1x is the important part: CDMA2000 uses from one to three 1.25 MHz carriers. This first rev of CDMA2000 uses just one of those three. cdmaOne already uses this frequency, which is why CDMA2000 is considered “backwards compatible” and I guess what the CDMA2000 standard adds is more efficient use of that spectrum. The next steps in CDMA are then CDMA2000 1xEv which uses a second channel (1xEv phase one uses the second channel only for data only: “1xEv-DO” and phase two uses both channels together “2x”), and 3x which uses all three channels as a single 3.75Mhz carrier. You can see how adding channels and infrastructure will naturally cause data bandwidth to go up, though, it’s important to note that unlike the GSM route, this allocation seems backwards compatible and isn’t just for data, but also for voice calls as well.

The GSM path goes to GPRS next, which can dedicate one or more of the channels in the GSM spectrum to packet data only. It works, but has lots of provisioning problems and bandwidth constraints. I’m not sure about this, but it seems to me that if you’re enabling GPRS, you’re cutting off at least one channel, and this must affect the GSM voice service. After GRPS is EDGE which works in a similar way, but uses a newer “modulation scheme” which allows higher data rates. I have no idea what a “modulation scheme” is, actually, but it’s easy to get the idea: same general functionality, but with faster moving bits.

After this, however, the GSM guys have to scratch all that equipment and move to WCDMA, which is a version of the CDMA technology and divides up calls by uniquely encoding them. Unlike the “multi channel” CDMA2000, “Wideband”-CDMA uses one big-ass 5Mhz frequency which allows for much greater capacity and data speeds (up to 2Mbps). How exactly this compares to CDMA2000 3x’s 3.75Mhz, I don’t know.

So that’s the general idea. The pairs are roughly cdmaOne/GSM, CDMA2000 1x/GPRS, CDMA 1xEv/EDGE, CDMA2000 3x/WCDMA . It’s important to note that where as the GSM guys are adding just data capabilities until WCDMA, the CDMA guys are also adding more voice capacity as well.

In India, Reliance and Tata are offering CDMA services (if I am not mistaken).

Tags: Telecom

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