Multiplexing

Within a cell, mobile operators want to allow as many customers as possible to use the network. They achieve this by using multiple access technology, which allows the available spectrum to be shared between several users. Analog systems usually separate conversations by subdividing the spectrum into narrow frequency bands and by using directional transceivers at the base station. Digital systems also divide each frequency into time slots, or encode transmissions so that more than one can use the same airwaves at the same time.

Frequency

All radio systems use some kind of frequency division—they partition their available spectrum into sub-bands, each of which can be tuned into by one or more users. A system that gives each communication channel its own specific frequency is described as FDMA (Frequency Division Multiple Access). Examples include broadcast radio, television, and analog cellular networks. Digital cellular systems also use FDMA, but they combine it with other multiplexing schemes.

The only analog cellular network still in widespread use is AMPS (Advanced, or American, Mobile Phone System). It uses paired spectrum bands of 25 MHz each, sliced into 30 MHz channels. This gives a total capacity of 832 channels in each direction, though not all can be used for phone conversations. About 42 are needed for control information, and the entire spectrum is usually divided between two competing cellular operators. This would give each operator a maximum capacity of 395 calls.

The whole point of a cellular network is that channels can be reused by building more cells, so the 395-call limit applies to an isolated cell, not to an entire network. Unfortunately, interference means that adjacent cells cannot reuse the same frequencies, so no one cell can broadcast (or receive) on all channels. The most efficient pattern can use only one third of the channels in each cell, as shown in Figure 3.4. Because the pattern repeats itself in groups of three cells, it is said to have a cluster size of three. In AMPS, this gives each cell 131 channels.

Cluster sizes of three are unusual in real networks because cells are not exactly hexagonal and not all the same size. The interference between cells is still too great, and so larger clusters have to be used. The most common systems use cluster sizes of seven or twelve, but others are possible and may be necessary, depending on the particular structure of a network and whether it includes microcells. Figure 3.5 shows the number of channels per cell for various repeat patterns.

The biggest problem with pure FDMA is that nearby frequencies interfere with each other, something with which most radio listeners are familiar. Frequency needs to be separated by a gap, which is very wasteful of spectrum. Table 3.1 lists some analog cellular systems—for all of them, the spectrum actually used by a channel is less than that which needs to be allocated. Figure 3.6 shows how the frequency is used in some AMPS channels. Because of this spectrum waste, most of the systems in the table are now obsolete, in the process of being upgraded to more efficient technologies.

Table 3.1. Analog Cellular Systems

System	Frequencies(MHz)		Spectrum per channel	Channel size	Max. Data Capacity	Used in
	Downlink	Uplink
AMPS (Advanced Mobile Phone System)	869–894	824–849	24 kHz	30 kHz	9.6 kbps	Americas
C-Netz	461–466	451–456	8 kHz	20 kHz	5.3 kbps	Central Europe
J-TACS (Japanese TACS)	925–940	870–885	10 kHz	25 kHz	0.3 kbps	Japan only
NMT (Nordic Mobile Telephony)	463–467.5 935–960	453–457.5890–915	9.4 kHz	25 kHz	1.2 kbps	Scandinavia, Eastern Europe, Asia
TACS (Total Access Communications System)	935–950 917–933	890–905 872–888	19 kHz	25 kHz	8.0 kbps	Western Europe, Asia

Space

SDMA (Space Division Multiple Access) uses directional transmitters to cover only a part of an arc rather than an entire circle, as in a cell. It is most valuable in satellite systems, which often need a narrowly focused beam to prevent the signal spreading too widely and becoming too weak. A single satellite can reuse the same frequency to cover many different regions of the earth's surface.

In cellular networks, interference means that SDMA cannot actually reuse the same frequencies. It can, however, be used to reduce the number of base stations needed to cover a given number of cells. Instead of putting an omnidirectional transceiver at the center of each cell, three directional transceivers can be placed on a single site at the three-way cell boundary. Though the actual radio equipment is more complex, a single site makes it easier for the operator to obtain planning permission, build the fixed infrastructure, and perform maintenance.

Time

Most present-day digital systems, including the European GSM standard, rely on TDMA (Time Division Multiple Access). To American consumers, the name TDMA has come to mean the digital version of AMPS, but to engineers this is just one of many TDMA systems. It's also used by the Japanese PDC (Personal Digital Cellular), the world's second most popular cellphone standard, and some types of PMR.

TDMA works by dividing a band into several time slots, each of which corresponds to one communications channel. A cellphone usually transmits and receives in only one slot, remaining silent until its turn comes round again. The number of slots, cycle length, and bandwidth depend on the particular technology, shown in Table 3.2

GSM's wide frequencies give it advantages of scalability and reduce the wasted bandwidth shown in Figure 3.6. But the short time slots can cause problems with keeping phones synchronized with each other. Radio signals take just over 3.3 microseconds to travel a kilometer, which adds up to a round-trip delay of around 400 microseconds for a phone only 60 km from the base station. The time slot only lasts 577 microseconds, so this delay is enough to make the phone miss its slot entirely, even though it would be unnoticeable to a human listener. In practice, this means that GSM cells cannot have a radius of more than 35 km (22 miles), no matter how strong the signal.

TDMA systems provide an easy upgrade path to higher capacities: a mobile can simply transmit or receive on more than one time slot, without having to retune to different frequencies. This is the basis of HSCSD (High Speed Circuit Switched Data) and GPRS (General Packet Radio Service), described in Chapter 4, "PCS Standards."

Codes

CDMA (Code Division Multiple Access) sends every signal at once, but encodes each one differently so that they can be separated by receivers. It actually predates both computers and mobile phones, though was thought too complicated to be used in cellphone networks until the late 1990s. Many of the innovations in CDMA cell networks are due to a single company, Qualcomm, whose growth in the cellular technology market echoed that of Microsoft in PC software and Cisco in network hardware. But variants have also been developed by others, notably by ETSI (European Telecommunications Standards Institute) for its third-generation networks, and by manufacturers of wireless LAN equipment.

CDMA is a development of spread spectrum, a technology originally developed during World War II. Military chiefs were worried that the regular carrier waves used by most radio signals were very easy to detect, meaning that the enemy could either listen to or jam the signal with more powerful transmissions of their own. Instead, they tried to use carrier waves that resembled random noise, making it difficult for an eavesdropper even to know that a communication was taking place. The waves were not truly random, of course; they were agreed upon in advance so that the intended recipient would be able to decode the signal, the same principle as in modern encryption systems.

There are two types of spread spectrum systems.

• FHSS (Frequency Hopping Spread Spectrum) is the simplest kind. It uses narrow-band FM signals, but rapidly switches between each one in a seemingly random pattern known only to the sender and recipient. This doesn't stop the enemy knowing that a communication is in progress, but makes it very difficult to listen in or jam. FHSS is now used mainly for short-range radio signals, particularly in unlicensed bands; the ability to change frequency quickly can be useful in finding one that isn't already in use by someone else.

• DSSS (Direct Sequence Spread Spectrum) covers a very wide range of frequencies, transmitting on all at once. This means that the bandwidth required is very high, usually of the order of megahertz rather than kilohertz. The extra bandwidth is used to send extra copies of the transmitted signal and is known as the gain. The higher the gain, the more resistant the signal to interference.

All CDMA cellular technology uses DSSS. Qualcomm's original standard has a bandwidth of 1.25 MHz, while third-generation systems have even wider bands. Despite these apparently large requirements, they actually use spectrum more efficiently than other systems because it can be shared by many different transmissions. Unlike TDMA, every terminal broadcasts at the same time, but each using a different code.

If more than one phone can use the same frequency at the same time, so can more than one base station. This means that every cell can use all the spectrum available, a major saving as each cell in other systems can use at most only one third of it. Cells are designed to overlap. Instead of a cell boundary, CDMA has a handoff region, where the mobile unit is actually connected to two base stations simultaneously. This allows soft handoffs, and means that the exact location of a user can be pinpointed by triangulation, which is measuring the distance between the phone and each base station.

A CDMA channel is often compared to an airport transit lounge, where many people are speaking different languages. Each listener understands only one language, and so is able to concentrate on their own conversation and treat the rest as random noise. The analogy isn't exact, because a room full of people all talking at once soon becomes very loud. Everyone ends up trying to shout above the background noise, which just makes the problem worse.

To prevent such runaway growth in background noise, spread spectrum signals are not truly random; they are carefully calculated to cancel each other out as far as possible. This is the same phenomenon as the destructive interference that prevents adjacent cells from using the same frequencies in an FDMA network, but here is used to the system's advantage. It is also what prevented CDMA from being deployed for many decades—ensuring that the signals do actually cancel each other out is very difficult when in motion, as it requires that the transmission power vary depending on the distance between the phone and the base station. This is known as the near-far problem and is solved by sophisticated electronics, which can accurately measure the distance and vary the power.

Orthogonality

A few systems use OFDM (Orthogonal Frequency Division Multiplexing), an even newer and more complex technology. It is designed to solve what engineers call the multipath problem, destructive interference caused by waves that reflect off different surfaces. A wave's reflection tends to have the same frequency as the original, meaning they can cancel each other out or at least render the signal undecipherable.

Multipath interference is more of a problem for signals that have a high bit rate, because each modulation symbol is shorter and so more easily swamped by the reflections. OFDM reduces the bit rate by splitting a high-speed data stream into several lower-speed streams and sending each one separately. In this respect, OFDM is the opposite of CDMA and TDMA; they split a frequency between many users, while it splits a user between many frequencies.

This solves the multipath problem, but reintroduces an older one: the waste of bandwidth shown in Figure 3.6. Regular FDMA systems need a gap between each frequency to make them clearly distinguishable. OFDM eliminates this, moving the frequencies closer together until they overlap. Interference is prevented by using orthogonal carrier waves, which means that they are carefully calculated to cancel each other out as far as possible. Orthogonal is a mathematical term which means two quantities that result in zero when multiplied together. For example, the codes in CDMA are also orthogonal.

OFDM relies on very precisely tuned antennas and on sophisticated processing which requires a lot of computing power. This prevented it becoming available in cheap or portable devices during the twentieth century, though the technology has been adopted for several applications, including wireless LANs and digital broadcasting. It may also form the basis of future fourth-generation cellular systems.