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.
System GSM 800 P-GSM E-GSM GSM 1800 GSM 1900
824-849 880-915 1710-1785 1850-1910 Frequencies 890-915
MHz MHz MHz MHz • Uplink MHz
869-894 925-960 1805-1880 1930-1990 • Downlink 935-960
MHz MHz MHz MHz MHz
Wavelength 37.5 cm ~ 33 cm ~ 33 cm ~ 17 cm ~ 16 cm
Bandwidth 25 MHz 25 MHz 35 MHz 75 MHz 60 MHz
Duplex 45 MHz 45 MHz 45 MHz 95 MHz 80 MHz
Carrier 200 kHz 200 kHz 200 kHz 200 kHz 200 kHz
Radio 125 125 175 375 300
Transmissio 270 kbits/s 270 kbits/s 270 kbits/s 270 kbits/s 270 kbits/s
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 the wave oscillates per second. Frequency is measured in Hertz (Hz), where 1 Hz indicates one oscillation per second. We also describe radio waves in terms of amplitude and phase. In simple terms amplitude is the voltage or height of the wave and phase is the form, or shape, of one oscillation over time.
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 vary 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:
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 (3 x108 m/s or meters per second).The wavelength of a frequency can be determined by using thefollowing formula:
Wavelength = Speed/Frequency
Thus, for GSM 900 the wavelength is:
Wavelength = 3 x 108m/s/900 MHz
Wavelength = 300,000,000 m/s/900,000,000
Wavelength = 0.33 m (or 33 cm)
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 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 (30MHz each) and three other duplex blocks D, E, and F (10 MHzeach).
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 a city such as Los Angeles could be served for example by 6 operators: 2 MTA companies operating in California and 4 BTA companies operating in Los Angeles
Bandwidth is the term used to describe the amount of frequency BANDWIDTH
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.
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.
The use of full duplex requires that uplink and downlink transmissions are separated in frequency by a minimum distance, called duplex distance. Without it, uplink and downlink frequencies would interfere with each other.
In addition to the duplex distance, every mobile system includes a carrier separation 2. 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 to be transmitted within the channel. The greater the amount of information to transmit, the greater the amount of separation required. In GSM the carrier separation is fixed at 200 kHz
From the figure above, it can be seen that the information to be sent is carried on 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.
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 the interference, the lower the 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 the frequency re-use distance, the more capacity will be available in the network.
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
In GSM 900 a subscriber is allocated a timeslot, on a frequency,around 900 MHz. This is the frequency that will carry the voice or data, in digital format and so it is called a carrier frequency or,simply, a carrier. We shall examine shortly how voice is transformed from its original analog form into digital form, but, for now, let us look at how the carrier wave actually carries digital information.
At a basic level, for a carrier frequency to carry digital information we must be able to modify the carrier waveform in some way so that it represents digital one (1) and modify it again so that it represents digital zero (0). This modification process is called „modulation‟ and there are different methods available. We can modify the amplitude, frequency or phase of the carrier so that it represents the bit pattern or digitized version of the input signal.For example, we can modify the amplitude of a waveform so that a slightly higher amplitude represents digital 1 and the input, or unmodified, waveform represents digital 0. Depending on the modulation method used, each modulation of the waveform can represent one or several bits. Any modulation scheme increases the carrier load 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) and is a form of phase modulation, or „phase shift keying‟ as it is called. GMSK enables the transmission of 270kbit/s within a 200kHz channel. This gives a bitrate of 1.3 bit/s per Hz. This is a rather low bitrate but acceptable as GMSK gives high interference resistance level.
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, GMSK offers more tolerance of interference. This in turn enables tighter re-use of frequencies and leads to an overall gain in capacity, which out-performs other systems.
ACCESS METHOD: TIME DIVISION MULTIPLE ACCESS
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 a
corresponding frame in the uplink direction.
ANALOG AND DIGITAL TRANSMISSION
INTRODUCTION TO ANALOG AND DIGITAL
Analog information is continuous and does not stop at discrete values. An example of analog information is time. It is continuous and does not stop at specific points. An analog watch may have a second-hand, which does not jump from one second to the next, but continues around the watch face without stopping.
An analog signal is a continuous waveform which changes in accordance with the properties of the information being represented.
Digital information is a set of discrete values. Time can also be represented digitally. However, digital time would be represented by a watch which jumps from one minute to the next without stopping at the seconds. In effect, such a digital watch is taking a sample of time at predefined intervals.
For mobile systems, digital signals may be considered to be sets of discrete waveforms.
ADVANTAGES OF USING DIGITAL
Human speech is a form of analog information. It is continuous and changes in both frequency (higher and lower pitches) and amplitude (whispering and shouting).At first, analog signals may appear to be a better medium for carrying analog information such as speech. Analog information is continuous and if it were to be represented by discrete samples of the information (digital signal), then some information would be missing (like the seconds on the digital watch). An analog signal would not miss any values, as it too is continuous.All signals, analog and digital, become distorted over distances. In analog, the only solution to this is to amplify the signal. However,in doing so, the distortion is also amplified. In digital, the signal can be completely regenerated as new, without the distortion.
The problem with using digital signals to transfer analog information is that some information will be missing due to the technique of taking samples. However, the more often the samples are taken, the closer the resulting digital values will be to a true representation of the analog information. Overall, if samples are taken often enough, digital signals provide a better quality for transmission of analog information than analog signals.
Many problems may occur during the transmission of a radio signal. Some of the most common problems are described below.
Path loss occurs when the received signal becomes weaker and weaker due to increasing distance between MS and BTS, even if there are no obstacles between the transmitting (Tx) and receiving (Rx) antenna. The path loss problem seldom leads to a dropped call because before the problem becomes extreme, a new transmission path is established via another BTS.
Shadowing occurs when there are physical obstacles including hills and buildings between the BTS and the MS. The obstacles create a shadowing effect which can decrease the received signal strength.When the MS moves, the signal strength fluctuates depending on the obstacles between the MS and BTS.
A signal influenced by fading varies in signal strength. Drops in strength are called fading dips.
CARRIER TO INTERFERENCE C/I
Reusing an identical carrier frequency in different cells is limited by co-channel interference or C/I. Co-channel interference is the
relation between the desired signal C and the undesired re-used
signal I, both using the same carrier frequency.
CARRIER TO ADJACENT C/A
As the filters, limiting each carrier to its domain of 200kHz, are not ideal, the carriers will somewhat affect each other. This means that some the energy of the adjacent frequency will leak into the serving cell and cause interference. The relation between the desired signal C from the correct carrier and the undesired signal A from the carrier 200 kHz away is called adjacent channel interference or C/A.