FREQUENCY CONCEPTS

The following table summarizes the frequency-related specifications of each of the GSM systems. The terms used in the table are explained in the remainder of this section.

Note: Every GSM network uses one channel as a guard channel. This reduces the number of channels available for traffic by one. This is used to separate GSM frequencies from the frequencies of neighboring applications, e.g. 889 MHz. In this way extra protection (and quality) for GSM calls is ensured.

FREQUENCY                                                          

An MS communicates with a BTS by transmitting or receiving radio waves, which consist of electromagnetic energy. The frequency of a radio wave is the number of times that the wave oscillates per second. Frequency is measured in Hertz (Hz), where 1 Hz indicates one oscillation per second. Radio frequencies are used for many applications in the world today.

Some common uses include:

• Television: 300 MHz approx.

• FM Radio: 100 MHz approx.

• Police radios: Country dependent

• Mobile networks: 300 - 2000 MHz approx.

The frequencies used by mobile networks varies according to the standard being used. An operator applies for the available frequencies or, as in the United States, the operator bids for frequency bands at an auction. The following diagram displays the frequencies used by the major mobile standards:

As these frequencies are used to carry information, they are often referred to as carrier frequencies.

Wavelength

There are many different types of electromagnetic waves. These electromagnetic waves can be described by a sinusoidal function, which is characterized by wavelength. Wavelength (λ) is the length of one complete oscillation and is measured in meters (m). Frequency and wavelength are related via the speed of propagation, which for radio waves is the speed of light (3x).

From this formula it can be determined that the higher the frequency, the shorter the wavelength. Lower frequencies, with longer wavelengths, are better suited to transmission over large distances, because they bounce on the surface of the earth and in the atmosphere. Television and FM radio are examples of applications, which use lower frequencies. Higher frequencies, with shorter wavelengths, are better suited to transmission over small distances, because they are sensitive to such problems as obstacles in the line of the transmission path. Higher frequencies are suited to small areas of coverage, where the receiver is relatively close to the transmitter. The frequencies used by mobile systems compromise between the large-coverage advantages offered by lower frequencies and the closeness-to-the-receiver advantages offered by use of higher frequencies.

Example of Frequency Allocation - United States

In 1994, the Federal Communications Commission (FCC) in the United States (US) auctioned licenses to prospective mobile network operators. Each network operator owns the rights to the license for ten years. Further auctions will be held following that period for the same licenses. The FCC has specified six blocks within the frequency band: three duplex blocks A, B, and C (30 MHz each) and three other duplex blocks D, E, and F (10 MHz each).

For telecommunications purposes, the US is divided into 51 regions or Major Trading Areas (MTA) and 493 Basic Trading Areas (BTA). One MTA can be as large in geographical area as a state, while a BTA can be about the size of a large city. The FCC issued two PCS 1900 licenses for each MTA and four licenses for each BTA. Thus, if a city such as Los Angeles will be served by 6 operators: 2 MTA companies operating in California and 4 BTA companies operating in Los Angeles.

BANDWIDTH

Bandwidth is the term used to describe the amount of frequency range allocated to one application. The bandwidth given to an application depends on the amount of available frequency spectrum. The amount of bandwidth available is an important factor in determining the capacity of a mobile system, i.e. the number of calls, which can be handled.

CHANNELS

Another important factor in determining the capacity of a mobile system is the channel. A channel is a frequency or set of frequencies which can be allocated for the transmission, and possibly the receipt, of information. Communication channels of any form can be one of the following types:

A simplex channel, such as a FM radio music station, uses a single frequency in a single direction only. A duplex channel, such as that used during a mobile call, uses two frequencies: one to the MS and one from the MS. The direction from the MS to the network is referred to as uplink. The direction from the

network to the MS is referred to as downlink.

Duplex Distance

The use of full duplex requires that the uplink and downlink transmissions must be separated in frequency by a minimum distance, which is called duplex distance. Without it, uplink and downlink frequencies would interfere with each other.

Carrier Separation

In addition to the duplex distance, every mobile system includes a carrier separation4. This is the distance on the frequency band between channels being transmitted in the same direction. This is required in order to avoid the overlapping of information in one channel into an adjacent channel. The length of separation between two channels is dependent on the amount of information which is to be transmitted within the channel. The greater the amount of information to transmit, the greater the amount of separation required.

From the figure above, it can be seen that the information to be sent is modulated around the carrier frequency of 895.4 MHz. The same is true of the information to be sent on 895.6 MHz. To avoid interference between the two sets of information, a separation distance of 200 kHz is required. If less separation were used, they would interfere and a caller on 895.4 MHz may

experience crosstalk or noise from the caller on 895.6 MHz. Carrier separation is sometimes referred to as carrier bandwidth.

Capacity and Frequency Re-use

It is the number of frequencies in a cell that determines the cell’s capacity. Each company with a license to operate a mobile network is allocated a limited number of frequencies. These are distributed throughout the cells in their network. Depending on the traffic load and the availability of frequencies, a cell may have one or more frequencies allocated to it. It is important when allocating frequencies that interference is avoided. Interference can be caused by a variety of factors. A common factor is the use of similar frequencies close to each other. The higher interference, the lower call quality. To cover an entire country, for example, frequencies must be reused many times at different geographical locations in order to provide a network with sufficient capacity. The same frequencies can not be re-used in neighboring cells as they would interfere with each other so special patterns of frequency usage are determined during the planning of the network.

These frequency re-use patterns ensure that any frequencies being re-used are located at a sufficient distance apart to ensure that there is little interference between them. The term “frequency re-use distance” is used to describe the distance between two identical frequencies in a re-use pattern. The lower

frequency re-use distance, the more capacity will be available in the network.

TRANSMISSION RATE

The amount of information transmitted over a radio channel over a period of time is known as the transmission rate. Transmission rate is expressed in bits per second or bit/s. In GSM the net bit rate over the air interface is 270kbit/s

MODULATION METHOD

In GSM 900, the frequency that is used to transfer the information over the air interface is around 900 MHz. Since this is not the frequency at which the information is generated, modulation techniques are used to translate the information into the usable frequency band. Frequency translation is implemented by modulating the amplitude, frequency or phase of the so called

carrier wave in accordance with the waveform of the input signal (e.g. speech). Any modulation scheme increases the carrier bandwidth and hence is a limit on the capacity of the frequency band available. In GSM, the carrier bandwidth is

200 kHz. The modulation technique used in GSM is Gaussian Minimum Shift Keying (GMSK). GMSK enables the transmission of 270kbit/s within a 200kHz channel. This gives a bitrate of 1.3 bit/s per Hz. This is rather low bitrate but acceptable as the channel used has high interference level in the air. The channel capacity in GSM does not compare favorably with other digital mobile standards, which can fit more bits/s onto a channel. In this way the capacity of other mobile standards is higher. However, GSM’s GMSK offers more tolerance of interference. This in turn enables tighter re-use of frequencies, which leads to an overall gain in capacity, which out-performs that of other systems.

ACCESS METHOD: TIME DIVISION MULTIPLE ACCESS (TDMA)

Most digital cellular systems use the technique of Time Division Multiple Access (TDMA) to transmit and receive speech signals. With TDMA, one carrier is used to carry a number of calls, each call using that carrier at designated periods in time. These periods of time are referred to as time slots. Each MS on a call is assigned one time slot on the uplink frequency and one on the downlink frequency. Information sent during one time slot is called a burst. In GSM, a TDMA frame consists of 8 time slots. This means that a GSM radio carrier can carry 8 calls.

Note: Only the downlink direction is shown. There is also acorresponding frame in the uplink direction.