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Thomas Farely

Tom has produced since 1995. He is now a freelance technology writer who contributes regularly to the site.

His knowledge of telecommunications has served, most notably, the American Heritage Invention and Technology Magazine and The History Channel.
His interview on Alexander Graham Bell will air on the History Channel the end of 2006.

Ken Schmidt

Ken is a licensed attorney who has worked in the tower industry for seven years. He has managed the development of broadcast towers nationwide and developed and built cell towers.

He has been quoted in newspapers and magazines on issues regarding cell towers and has spoke at industry and non-industry conferences on cell tower related issues.

He is recognized as an expert on cell tower leases and due diligence processes for tower acquisitions.

« Cellular, PCS, GSM, and Japanese Digital Cellular Frequencies | | Network Elements »

January 03, 2006

Posted by Tom Farley & Mark van der Hoek at 04:31 PM

Transmission and multiplexing

Transmission in telephony means sending information on electricity or light from one point to another. Voice or data makes up the transmission. We call the device or matter that the information travels on, be it wires, cable, or radio waves, the transmission media.

FDM, TDMA, and CDMA are different transmission technologies. Wireless folks call them transport mechanisms or access technologies. Whatever. They make up part of the overall operating system a cellular carrier uses. No transmission scheme stands by itself, that is, these techniques are not by themselves operating systems. They are part of one. When someone asks, "Is IS-136 TDMA?" they usually mean, or should mean, "Is IS-136 TDMA based?"

American PCS operating systems use TDMA or CDMA, two different transmission technologies. Usually it is either IS-136, a TDMA system, or IS-95, a CDMA based system. Analog cellular might use conventional frequency multiplexing division. GSM only works in TDMA.

Wireless systems use many ways to transmit information. Here are some:

1. Frequency division multiplex or FDM, used in analog cellular;
where calls are separated by frequency

2. Time division multiple access or TDMA, used in digital cellular and PCS;
where calls are separated by time

3. Code division multiple access or CDMA, used mostly for PCS;
where calls are separated by code

2. Frequency Division Multiplexing

Analog cellular use frequency division multiplexing or FDM. It's simpler than its name suggests. As we've seen, a carrier's assigned radio spectrum is divided into specific frequencies, each separated by space. Like AM radio, which is divided into 10 KHz chunks. Radio station 810, 820, 830, and so on. That's all FDM is. Think of FDM as a single train running on a single track, pulling just one freight car. But what if you've run out of frequencies to handle your customers? What if you need more capacity? You can either separate your existing frequencies by narrower amounts or you can separate your calls over time.

Motorola's Narrowband Advanced Mobile Phone system or NAMPS, used precise frequency control to divide the 30 Khz AMPS channel into three subchannels. Each call takes up just 10Khz. But NAMPS had the same fading problems as normal AMPS, lacked the error correction that digital systems provided and it wasn't sophisticated enough to handle encryption or advanced services. To increase capacity most cellular carriers moved instead to a digital solution, one separating conversations by time or by code.

[Look to my cellular basics article for more information on the now defunct NAMPS.]

3. Time Division Multiple Access

In TDMA first digitizes calls, then combines those conversations into a unified digital stream on a single radio channel. Time division multiple access or TDMA divides each cellular channel into three time slots. In conventional cellular or AMPS a single call takes up 10Khz. In TDMA based D-AMPS or digital AMPS, three calls get put on that single frequency, tripling a carrier's system's capacity. GSM, D-AMPS, and D-AMPS 1900 (IS-136), and Motorola's iDEN all use or can use TDMA. This scheme assigns a specific time slot, a regular space in a digital stream, for each call's use during a conversation.

Think of a not so drunken cocktail party, with each person speaking in turn. Everyone gets to speak over time. Or think of a train pulling three freight cars. In a TDMA analogy, each caller puts their supplies or payload, their part of the conversation, on every third boxcar in a long train. No need for three separate frequencies like in FDM. With TDMA a single radio channel is not monopolized, rather, it efficiently carries three calls at the same time.

An anonymous writer summed TDMA like this, "Effectively, the IS-54 and IS-136 implementations of TDMA immediately tripled the capacity of cellular frequencies by dividing a 30-kHz channel into three time slots, enabling three different users to occupy it at the same time. Currently, systems are in place that allow six times capacity. In the future, with the utilization of hierarchical cells, intelligent antennas, and adaptive channel allocation, the capacity should approach 40 times analog capacity." Webproforum 40 times analog capacity! That's quite a hope. Almost as hopeful at the old, unrealized promises that CDMA would increase capacity 20 times.

4. Code division multiple access

CDMA is another transmission technology. Rather than separating frequencies by space as in FDM, or by time as in TDMA, CDMA separates calls by code. Every bit of every conversation gets tagged with a specific code. The system receives a call, seeming at first like so much radio hash, and reassembles the conversation from the coded bits. Like at a cocktail party with most people speaking English but two people speaking French. The French speakers can easily understand each other above the din of the English. That's because they are speaking in a different language or code. To further punish you with the railroad analogy, think of shipping companies filling every boxcar with packages seemingly at random. Their order doesn't really matter since they each have a unique label on them, like a shipping number, and thus can be sorted out accordingly at the other end.

Each face represents a conversation or a part of a conversation. With FDMA we put different calls on different frequencies, like broadcast stations are separated or divided by frequency. You know, A.M. station 560, 570, 580, 590, 600, 610 and so on. With time division multiple access we divide each call on a single frequency by time, like talking in turn. With CDMA we assign an identifying code to each call and put bits and pieces of different calls on different frequencies as the conversation continues. AT&T's national wireless network, as well as GSM, use TDMA. Sprint's PCS network uses CDMA.

CDMA's greatest benefit is that it can use all cellular frequencies in every cell. We saw how TDMA and FDM carefully assigns channels to each cell in advance to prevent interference. But CDMA codes are so specific that interfering radio signals are treated like noise and disregarded. So you can increase capacity, theoretically, by making all frequencies available at all times. We'll see why that promised capacity doesn't quite work out in practice later. For now, let's look at the operating systems these transmission technologies are placed in.

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