The GSM Radio Link

The air interface Um is the interface between the BTS (Base Transceiver Station)
and the moving MS (Mobile Station). The air interface utilizes a radio wave which
is subject to attenuation, reflections, Doppler shift, and interference from other transmitters. These effects will cause loss of signal strength and distortion, which will impact the quality of the individual radio link and the voice channel.
To cope with the harsh conditions of the VHF/UHF (Very High Frequency/Ultra
High Frequency) cellular land-mobile radio environment, and given the required high spectral efficiency, GSM makes use of an efficient and protective signal processing. In addition, proper cellular RF design must ensure that sufficient radio coverage is provided in the area.

Aspects of Radio Propagation

Types of signal strength variations:

· Macroscopic variations:
Macroscopic variations are due to local mean, long term, or log-normal fading. Its variation is due to the terrain contour between the BTS and the MS. The fading effect is caused by shadowing and diffraction (bending) of the radio waves.

· Microscopic variations:
Microscopic variations are due to multipath, short-term, or Rayleigh fading. It is caused by the fact that as the MS moves, radio waves from many different reflection paths will be received.

Digital TDMA Implementation

Advantages of digital transmission:
The analog cellular system is known as the first-generation system. Second generation cellular systems are digital. GSM systems are second-generation systems.

The digital transmission over the air interface Um has a number of advantages over analog transmission:
· Better speech quality
· Speech privacy and security (improved through encryption)
· High spectral efficiency (traffic density per MHz bandwidth, due to extensive frequency reuse)
· Better resistance to interference (also by frequency hopping)
· Data services and ISDN compatibility
· Efficient use of battery power by RF power control.

Access methods

Cellular radio as a network does not specify how the individual subscribers have access to the network. The two main access methods are: analog and digital.

Analog access

Analog systems use the familiar single channel per user concept, known as Frequency Division Multiple Access (FDMA). World-wide there are up to six incompatible analog cellular standards, such as NMT. The available spectrum is divided into channels A,B,C,D, and so on. During the call, a single user will occupy completely one channel of e.g. 25 kHz bandwidth irrespective whether the modulation is analog or digital. The signaling over the network is digital, the speech is modulated analog narrow-band FM

Digital access

The aim digital networks is to have:
· Better compatibility with the network supporting the cellular radio system
· Alternative access method to achieve a better spectral efficiency.

Digital systems let each user have access to the frequency band for a short time
(traffic burst), during which time the user transmits data at a high rate.

Time division multiple access:

Time Division Multiple Access (TDMA) is used in GSM-900 and GSM-1800 digital cellular radio. In TDMA, the user's frequency allocation is shared with other users (seven in case of GSM) who have time slots allocated at other times. Hence, there are eight physical channels per frequency carrier.

In fact, the GSM system uses a mix of:
· TDMA (time slots on one carrier) and
· FDMA (a number of carriers within the band), although frequency hopping makes the FDMA somewhat more complex.

From Source Data to Digital Radio Transmission:

Transmitting/ receiving processes:

There are two major processes involved in transmitting and receiving information over a digital radio link: coding and modulation.

Coding
Coding is the information processing that involves preparing the basic data signals so that they are protected and put in a form that the radio link can handle.

The coding includes:
· Speech coding or transcoding
· Channel coding
· Bit interleaving
· Encryption
· Multiplexing

The first step is performed in the STF (and MS), while the remaining steps are performed in the BTS (and MS). The decoding process follows these steps in the reverse order.

Modulation:

Modulation is the processing that involves the physical preparation of the signal so that the information can be transported on a RF carrier.

The demodulation process retrieves the information from the RF carrier.

Time Slot Data Bursts:

GSM radio transmission is accomplished by sending data in bursts. The burst is the physical content of a time slot. Each burst consists of 148 usable bits of 3.69 msec each. Between the bursts there is a guard period of 30.5 msec (= 8.25 bit) to distinguish the consecutive bursts. Hence, each time slot interval has a fixed length of 156.25 bits or 15/26 ms. The actual burst varies in length, depending on the type of burst. The different parts in a burst have special functions. Note that the number of bits used for a particular function may vary with the type of burst.

Examples of burst parts are: training sequence, encrypted bits, tail bits, guard period and stealing flag bits.

Training sequence

A fixed bit pattern, called the TSC (training sequence code) is known by both the MS and the BTS. It is used to train the MS in predicting and correcting the signal distortions (due to Doppler and multipath effects) in the demodulation process. The TSC has either a 26, 41 or 64 bit pattern.

Encrypted bits

The encrypted bits represent the useful bits serving for speech, data transmission, or signaling.

Tail bits

The tail bits (TB) at the beginning define ("flag") the start of a burst. The tail bits at the end define the end of a burst.

Guard period

The guard period (GP) between to consecutive bursts is necessary for:

· Switching the transmitter on and off. The transmitted amplitude is ramped up from zero to a constant value over the useful period of a burst and then ramped down to zero again. This is always required for the MS, and the BTS may do so if the adjacent burst is not emitted. Switching off will reduce interference to other RF channels.
· Timing advance

Stealing flag bits

The network has the option to use the information bits in the normal burst to send signaling data as needed. By setting a flag, using the stealing flag bits, the receiver can distinguish between traffic (user data) and signaling information.

The stealing flag bits indicate whether the adjacent 57 bits in the associated data field contain speech/data information or are "stolen" from the traffic channel for carrying pre-emptive FACCH (fast associated control channel) signaling information. The FACCH is used for sending signaling data if the capacity of the SACCH (slow associated control channel) is not sufficient.

Burst types

The different types of bursts are defined in GSM:
· Normal burst
· Dummy burst
· Access burst
· Synchronization burst
· Frequency correction burst.

Time Alignment

Problem
Because of the time division multiplexing scheme used on the radio path, the BTS receives signals from different mobile stations very close to each other. However, when a mobile station is far from the BTS, the BTS must deal with the propagation delay. It is essential that the burst received at the BTS fits correctly into the time slot. Otherwise, the bursts from the mobile stations using adjacent time slots could overlap, resulting in a poor transmission or even in a loss of communication.

Solution: timing advance
In order to solve the problem of the propagation delay, a compensation mechanism is necessary in the mobile station; the mobile station is able to advance its transmission time by a time known as the timing advance.

Time alignment definition
Time alignment is the process of transmitting early the bursts to the BTS (the timing advance) to compensate for the propagation delay.
Once a connection has been established, the BTS continuously measures the time offset between its own burst schedule and the reception schedule of the mobile station burst. Based on these measurements, the BTS is able to provide the mobile station with the required timing advance via the SACCH. Note that the timing advance is derived from the distance measurement which is also used in the handover process. The BTS sends to each mobile station a timing advance parameter according to the perceived timing advance. Each mobile station advances its timing by this amount, with the result that signals from different mobile stations arriving at the BTS are compensated for propagation delay.

Time alignment process

The RF communication experiences a propagation delay over the distance between the BTS and the MS. This is described in GSM.
In order to synchronize the MS to the BTS, a timing advance is used to align the time slots arriving at the BTS receiver:

1. The BTS measures the reception time of the incoming MS burst.
2. The BTS requests the MS to advance its transmission to compensate for the delay over the distance. A 6-bit number indicates how many bits the MS must advance its transmission. This time advance is Ta
3. The time advance value can have a value between 0 and 63 bit lengths, which corresponds to a delay of between 0 and 233 ms.
4. This leads to a maximum mobile range of 35 km, which is rather determined by the Ta than by the signal strength.

Modulation

Methods of modulation:
· ASK (Amplitude Shift Keying)
· FSK (Frequency Shift Keying)
· PSK (Phase Shift Keying)

Amplitude Shift Keying
Amplitude Shift Keying (ASK) is the oldest form of radio modulation, having its beginnings in the early days of radio in the form of sending dots and dashes in what was called Morse Code signaling. This was known as wireless telegraphy and is still used in radio today. This form of modulation can also be used to send binary (via on-off keying) or M-ary (via multilevel ASK) digits in digital radio communications. However, it is not used in mobile radio because of its susceptibility to interference and noise.

Frequency Shift Keying
Frequency Shift Keying (FSK) is a form of modulation that varies the radio carrier's transmitted phase. FSK is used in many "analog" cellular systems as the means of sending control data across the radio link, either on separate control radio channels, or on traffic radio channels using pre-emptive signaling (known as blank-and-burst) or out-of-band signaling on a subcarrier. FSK can be multilevel (i.e., M-ary).

Phase Shift Keying
Phase Shift Keying (PSK) and Frequency Shift Keying (FSK) are related forms of
modulation that vary the radio carrier's transmitted phase. PSK can also be multilevel (i.e., M-ary).

Modulation processing in GSM
GSM uses a form of phase modulation known as Gaussian filtered Minimum Shift Keying (GMSK). This method shows a good spectrum efficiency and requires a reasonable demodulation complexity.

1. In this method, the input bit stream is differentially encoded.
2. The encoded signal is passed to a filter which has a Gaussian impulse response function. The filtered bit waveform shows smooth transitions resulting in a narrow occupied frequency spectrum. (Steep phase jumps would require more bandwidth).
3. The Gaussian filtered waveform is then applied to a phase modulator which produces a phase +or-90 C shift for each differential bit value.

This form of modulation was chosen to allow a constant envelope (amplitude) modulator to be used. Thus, non-linear RF power amplifiers can be used in both base and mobile radio equipment.