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Cellular Telephone Basics: (Page Ten) continued . . .

 

Synchronization

To make this transmission method work it is not enough just to have a fancy coding scheme. To keep track of all this information flying back and forth we need to synchronize it with a master clock. As the CDG puts it, "In the final stages of the encoding of the radio link from the base station to the mobile, CDMA adds a special "pseudo-random code" to the signal that repeats itself after a finite amount of time. Base stations in the system distinguish themselves from each other by transmitting different portions of the code at a given time. In other words, the base stations transmit time offset versions of the same pseudo-random code."

Arrgh. Another phrase with the word 'code in it, one more term to keep track of! Don't despair. Even if "pseudo-random code" is fiercesomely titled, it's chore is simple to state: keep base station traffic to its own cell site by issuing a code. Synchronize that code with a master clock to correlate the code. Like putting a time stamp on each piece of information. CDMA uses The Global Positioning System or GPS, a network of navigation satellites that, along with supplying geographical coordinates, continuously transmits an incredibly accurate time signal.

What Every Radio System Must Consider

Radio systems, like life, demand tradeoffs or compromises. The CDG says, "CDMA cell coverage is dependent upon the way the system is designed. In fact, three primary system characteristics-Coverage, Quality, and Capacity-must be balanced off of each other to arrive at the desired level of system performance." Wider coverage, normally a good thing, means using higher powered mobiles which means more radio interference. Increasing capacity means putting more calls into the same amount of spectrum which means calls may be blocked and voice quality will decrease. That's because you must compress those calls to fit the spectrum allowed. So many things must be balanced. As the saying goes, radio systems aren't just sold, they are engineered.

G. CDMA Benefits

The CDG states that CDMA systems have seven advantages over other cellular radio transmission techniques. (GSM and IS-136 operators will contest this list.) CDG says benefits are:

1.Capacity increases of 8 to 10 times that of an AMPS analog system and 4 to 5 times that of a GSM system
2.Improved call quality, with better and more consistent sound as compared to AMPS systems
3.Simplified system planning through the use of the same frequency in every sector of every cell
4.Enhanced privacy
5.Improved coverage characteristics, allowing for the possibility of fewer cell sites
6.Increased talk time for portables
7.Bandwidth on demand

Good, readable information on CDMA is here:

http://www.cellular.co.za/celltech.htm

A Few More Details

IS-95, as I've mentioned before, is another cellular radio technique. It uses CDMA but is backward compatible with the analog based AMPS. IS-95 handles calls differently than TDMA schemes, although registration is the same. IS-95 queries the same network resources and databases to authenticate a caller. One thing that does differ IS-95, besides the different transmission scheme, are handoffs. It's tough transferring a call between cells in any cellular radio system. Keeping a conversation going while a cellular user travels at seventy miles per hour from one cell to the next finds many calls dropped. CDMA features soft handoffs, where two or more cell sites may be handling the call at the same time. A final handoff gets done only when the system makes sure it's safe to do so. Check out the file just below for a better summary:

Paul Bedell writes an excellent summary of CDMA, including information on soft handoffs, in this .pdf file. It's just six pages, about 273K.

I hope the above comments were helpful and that you visit the CDG site soon. Let's finish this article with some comments by Mark van der Hoek. He says that the most signifigant feature of CDMA is how it delivers its features without a great deal of extra overhead. He notes how CDMA cell sites can expand or contract, breathing if you will, depending on how many callers come into the cell. This flexibility comes built into a CDMA system. Here are some more comments from him:

"CDMA is already dominant, and 3G will be CDMA, and everyone knows it. The matter was really settled, though some still won't admit it, when Ericsson, the Big Kahoona of GSM, Great Champion of The Sacred Technology, capitulated to Qualcomm by buying Qualcomm's infrastructure division. The rest is working out the details of the surrender. TDMA just can't deliver the capacity. In fact, I understand that the GSM standard documents spell out TDMA as an interim technology until CDMA could be perfected for commercial use."

"A further note on CDMA bandwidth. IS-95 CDMA (Qualcomm) uses a bandwidth of 1.25 MHz. Anyone know why? I have fun with this one, because few people, even in the industry, know the answer. PhDs often don't know the answer! That's because it is not a technical issue. The key to the matter can be found in the autograph in one of my reference books, "Mobile Communications Design Fundamentals" by William C. Y. Lee. The inscription reads, 'I am very glad to work with you in this stage of designing CDMA system, with my best wishes. Bill Lee, AirTouch Comm Los Angeles, CA March 22, 1995'."

"Dr. Lee is a major figure in the cellular industry, but few know of the contribution he made to CDMA. Dr. Lee was one of the engineers at Bell Labs in the '60s who developed cellular. He later came to work for PacTel Cellular (later AirTouch) as Chief Science Officer. Qualcomm approached him in 1992 or 1993 about using CDMA technology for cellular. TDMA was getting off the ground at that time, and Qualcomm had to move fast to have any hope of prevailing in the marketplace. They proposed to Dr. Lee that PacTel fund them (I think the number was $100,000) to do a "Proof of Concept", which is basically a theoretical paper showing the practicality of an idea. Dr. Lee considered Qualcomm's proposal, and said, "No." Qualcomm was shocked. Then Dr. Lee told them we'll fund you 10 times that amount and you build us a working prototype."

"It is not too much to say that we have CDMA where it is today in part because of Dr. Lee. Qualcomm built their prototype system piggybacked on PacTel's San Diego network. During the development phase it was realized that deployment of CDMA meant turning off channels in the analog system. (What we call "spectrum clearing".) "How much can we turn off?" was the question. Dr. Lee considered it, and came back with the answer, "10%". Well, that worked out to 1.25 MHz, and that's where it landed. (All of this according to Dr. Lee, who is a brilliant and genuinely nice person.) By comparison, though, 3rd generation systems will have a wider bandwidth, than the 1.25 MHZ bandwidth used for CDMA in IS-95 . The biggest discussion about 3G is now what kind of CDMA will be used. Bandwidth is the sticking point. Will it be 3.75 MHz or 5 MHz? You can see discussions on it at the CDG site (external link)." Please see next page-->

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