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TDMA vs. CDMA

How the Feds Blew It, Once Again

Francis Vale

 

Overcrowding In The Cellular Tenement

It is no secret to frazzled users that cellular systems are rapidly becoming overcrowded. Fundamentally, the problem rests in the fact that cellular systems are fixed in channel capacity, with a limited number of available frequencies. This caller tenement overcrowding was foreseen by the industry. As far back as 1988, the industry had called for a new approach that offered at least a ten fold increase in cellular telephone call carrying capacity.

Even then, a digital solution was seen as the answer to the impending analog cellular logjam. From this call for industry salvation, two competing, incompatible, digital technology standards emerged. Each specifies how the bandwidth spectrum is allocated during your cellular call. One technique is called Time-Division Multiple Access (TDMA). The other is known as Code-Division Multiple Access (CDMA).

Both technologies share the common goal of allowing the maximum number of calls to simultaneously take place. And both techniques are equally applicable to the new PCS (Personal Communications Services) micro-cells, as well as to other types of wireless networks. TDMA was seen as a quick start technology because it already had an established market base in Europe; the TDMA-based GSM (Global System for Mobile Communications) digital cellular system. TDMA was thus selected in 1989 as a digital cellular standard.

TDMA multiplexes up to three conversations over the same 30-kilohertz transmission channel. However, TDMA's caller menage a trois was far less than the tenfold increase than was requested by the industry. Efforts are therefore underway to up this simultaneous user number by as much a six or eightfold. But TDMA offered access service vendors a quick way to get going, with some optimism for more calling capacity to come.

Had matters stopped there with TDMA, life would have been simpler, if not necessarily better. But as in all things digital, a technology dark house suddenly emerged from a west coast company called Qualcomm. This outfit had been in the business of supplying the military with spread spectrum technologies for things like spy satellites.

Spread spectrum, in a nutshell, allows multiple frequencies to be used simultaneously, rather than just one narrow slice of it. So, while TDMA attempts to shoehorn ever more calls into the same tight fitting frequency channel, CDMA provides a way out of the box. CDMA was the first to meet the ten fold call carrying capacity increase originally asked for by the industry. In fact, it can provide up to a twenty fold increase. CDMA pulls off this clever trick by assigning each call a unique code.

Every packet has an identifier, so the base station can recognize whether it's voice or data, and process it accordingly. CDMA further allows every customer (which can include a number of users), to utilize the entire 1.25-MHz frequency allocation of the cell. This CDMA code permits a call to be readily identifiable from all other calls swimming in the vast frequency sea -- presuming both ends of the same conversation know and share the unique code. The originating CDMA-encoded bit stream is thus locked into a receiver which continually scans only for the unique code. All other call codes are ignored.

This encoding scheme has a number of attractions. Call security, for example, is much easier to achieve, as the CDMA encoding scheme lends itself quite easily to crypto-techniques. Moreover, as your call is no longer living in an overcrowded, one room TDMA apartment, CDMA's spread spectrum caller high rise allows much more conversation carrying capacity. CDMA proponents also claim much better range, which means fewer calling cells to handle calls, and much lower power requirements.

With respect to PCS; like cellular systems, it now has two primary competing standards. One is CDMA-based, and the other is the TDMA-based, GSM European-derived standard. It used to be that the primary drawback to CDMA was that only one equipment supplier, Motorola, backed the technology. But all that has changed. AT&T, Goldstar, Hyundai, Northern Telecom, Samsung, Sony, OKI, and others, are now building PCS gear that supports CDMA.

It is interesting to note the large number of Asian manufacturers getting behind CDMA. The reason is quite simple. The Europeans managed to keep the big volume Far East manufactures out of the GSM/TDMA business, thus giving their home teams a virtual lock on the equipment industry. But CDMA-based PCS is a whole new ball game where everyone can play, and in the biggest single market to boot -- the U.S. Thus, Qualcomm not only offered superior technology, it also provided the off shore consumer manufacturing giants an entree into an explosive new market. All in all, a winning hand, and Qualcomm has played it very well.

This doesn't mean that TDMA is going away any time soon. Already, 'Composite' CDMA/TDMA technology is appearing. This hybrid technology won a Pioneer's Preference PCS license in the lucrative New York City market. This shotgun marriage notwithstanding, TDMA still has a real market share fight on its hands.

How the Feds Blew it

But much more importantly than this looming market battle, the spread spectrum technology used by CDMA calls into question the whole rationale of the Federal Communications Commission (FCC), and its highly publicized public auctions for thin slices of PCS bandwidth in the 1.9 GHz frequency range. This area of bandwidth was carved up by the FCC into six frequency blocks (tagged A through F) between 10 and 30 megahertz. In addition a separate 20MHz band, 1910-1930 MHz, was also allocated by the FCC for low power, unlicensed PCS; the so called U-PCS (tantamount to a PCS 'citizen band' radio).

At the FCC PCS license auctions, up for grabs were 51 MTAs, Major Trading Areas, and 492 BTA's, or Basic Trading Areas. (The MTA's and BTA's were based by the FCC on the1992 Rand McNally Commercial Atlas & Marketing Guide, 123rd Edition, pp. 38-39). The FCC intends to issue two PCS licenses for each MTA, and four for each BTA, for a total of 2,070 licenses. The FCC thus went out of its way to ensure that there would be several competing PCS service providers within each geographical market. Free market forces would therefore help the consumer get the best PCS deal, or so it was reasoned.

Unfortunately, the FCC may have sold off the wrong thing, at the wrong time. Instead of encouraging maximum and best free market use of PCS bandwidth, the FCC may have actually hindered the growth of the entire mobile wireless industry, to the consumer's detriment. If vast hunks of bandwidth can be used by one CDMA call (bandwidth on demand), then what's the point, or market value, of the FCC selling off narrow, 'beachfront' PCS bands? Shoehorning ever more calls into one constrictive frequency no longer makes economic or technical sense. CDMA's bandwidth on demand means that video and other high data volume systems can be equally happy roaming the 'open' ether, along with voice calls.

Most significant to contemplate, is what would happen if Steinbrecher Corp.'s radio (a system that can scan any range of frequency spectrum, and in real time, instantly detect which channels are free) were to be combined with CDMA. If this dynamic technology duo were to be deployed in parallel, they would completely destroy any pretense that narrow slices of frequency bandwidth were precious PCS commodities, to be auctioned off to the highest bidder.

With CDMA and the Steinbrecher radio, all available bandwidth, right up and down the frequency spectrum, could be dynamically allocated for your changing communications needs. PCS now seems to be a classic case of the government getting into a technology business it just did not understand, with the ultimate loser being the consumer -- the one whom the government was supposedly looking after in the first place.

So, score one for free market forces, even if the run didn't count.


Copyright 1996, Francis Vale, All Rights Reserved

 

21st, The VXM Network, http://www.vxm.com

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