Presentation

Data Transmission Project:

Global System for Mobile communications

GSM

Introduction

• The Global System for Mobile communications (GSM) is a digital cellular communications system. It was developed in order to create a common European mobile telephone standard but it has been rapidly accepted worldwide.

• But in the beginnings of cellular systems, each country developed its own system, which was an undesirable situation for the following reasons:  

• The equipment was limited to operate only within the boundaries of each country.

• The market for each mobile equipment was limited.

Introduction

• To overcome these problems, the Conference of European Posts and Telecommunications developed a standardized cellular system that meets certain criteria:

• Spectrum efficiency • International roaming • Low mobile and base stations costs • Good subjective voice quality • Compatibility with other systems such as ISDN

(Integrated Services Digital Network) • Ability to support new services

The cellular structure

• In a cellular system, the covering area of an operator is divided into cells.

A cell corresponds to the covering area of one transmitter or a small collection of transmitters  

The cellular structure

• The concept of cellular systems is to use low power transmitters in order to enable the efficient reuse of the frequencies.

• The cell size is determined by:

1. The transmitter's power.

2. The geographical layout of the coverage area.

3. Number of mobile stations in the coverage area.

Architecture of the GSM network

The GSM network can be divided into four main parts:

• The Mobile Station (MS).

• The Base Station Subsystem (BSS).

• The Network and Switching Subsystem (NSS).

• The Operation and Support Subsystem (OSS).

Architecture of the GSM network

• Mobile Station :

1. The mobile equipment or terminal. 2. The Subscriber Identity Module (SIM):

SIM card contains International Mobile Subscriber Identity (IMSI).which gives mobility for the user.

• The Base Station Subsystem (BSS):

The BSS can be divided into two parts:

1. The Base Transceiver Station (BTS) .2. The Base Station Controller (BSC).

Architecture of the GSM network

• The Network and Switching Subsystem (NSS):

1. The Mobile services Switching Center (MSC). 2. The Gateway Mobile services Switching Center (GMSC) .3. Home Location Register (HLR). 4. Visitor Location Register (VLR).5. The Authentication Center (AuC). 6. The Equipment Identity Register (EIR). 7. The GSM Inter-working Unit (GIWU).

• The Operation and Support Subsystem (OSS).

The GSM Radio Interface• Frequency allocation: Two frequency bands of 25 MHz :1. 890 - 915 MHz used for the uplink direction (from the mobile station to the base station).2. 935 - 960 MHz is used for the downlink direction (from the base station to the mobile

station). 3. The channel bandwidth is 200KHz.4. The spacing between the uplink and downlink for each channel is 45KHz5. The system is full Duplex.6. 125 carrier frequencies but the first carrier frequency is used as a guard band between

GSM and other services working on lower frequencies.

Physical Channel

• A timeslots on a carrier constitute a physical channel, which are used by different logical channels to transfer information - both user data and signaling.

• GSM uses a mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA), combined with frequency hopping.

•  

Burst Structure

• the burst is the unit in time of a TDMA system. • Four different types of bursts can be distinguished in GSM: 1. The frequency-correction burst is used on the FCCH. It has

the same length as the normal burst but a different structure.

2. The synchronization burst is used on the SCH. It has the same length as the normal burst but a different structure.

3. The random access burst is used on the RACH and is shorter than the normal burst

4. The normal burst is used to carry speech or data information. It lasts approximately 0.577 ms and has a length of 156.25 bits. Its structure is presented in below.

Burst Structure

• Different types of bursts are shown below:

Logical Channels • The traffic channels used to transport speech and

data information. (TCH).1. Full-rate traffic channels (TCH/F) are defined using a group of 26

TDMA frames called a 26-Multiframe.2. The 26-Multiframe lasts consequently 120 ms.3. The traffic channels for the downlink and uplink are separated by 3

bursts. • The frames that form the 26-Multiframe structure have different

functions: 1. 24 frames are reserved to traffic. 2. 1 frame is used for the Slow Associated Control Channel (SACCH). 3. The last frame is unused. This idle frame allows the mobile station

to perform other functions, such as measuring the signal strength of neighboring cells

Logical Channels

• The control channels used for network management messages and some channel maintenance tasks:

a. Broadcast channels:

The BCH channels are used by the base station, to provide the mobile station with the sufficient information it needs to synchronize with the network.

b. Common control channels:

The CCCH channels help to establish the calls from the mobile station or the network.

Logical Channels

c. Dedicated control channels:

The DCCH channels are used for message exchange between several mobiles or a mobile and the network.

d. Associated control channels:

The Fast Associated Control Channels (FACCH) replaces all or part of a traffic channel when urgent signaling information must be transmitted.

GSM Transmissions and Reception Process:

Speech Coding (source coding)

• Source coding removes redundancy from data resulting in compression of the data

• The speech signal is divided into blocks of 20 ms. these blocks are then passed to the speech coder, which has a rate of 13 kbps, in order to obtain blocks of 260 bits. i.e. it is a compression process. The 64 kbps resulting from PCM are compressed to 13 kbps.

Channel Coding

• Channel coding adds redundancy bits to the original information in order to detect and correct, if possible, errors occurred during the transmission.

• Channel coding reduces Pe

• The channel coding used in GSM are block coding and Convolution coding

• The GSM convolution code is characterized by R = 1/2 and a delay of K = 5 .

• it will add a redundant bit for each input bit.

• The convolution code uses 5 consecutive bits in order to compute the redundancy bit.

• Channel coding for data differs than that for speech and differs from that for control channels.

Channel Coding • An output block after channel coding is of 456 bits.

• The data rate after channel coding is 22.8 kbps.

• The channel coding for speech is shown in figure below.

Interleaving

• An interleaving rearranges a group of bits in a particular way. It is used in combination with FEC codes in order to improve the performance of the error correction mechanisms. The interleaving decreases the possibility of losing whole bursts during the transmission, by dispersing the errors. Being the errors less concentrated, it is then easier to correct them.

• Interleaving reduces the probability of bit error.

Modulation

• The modulation chosen for the GSM system is the Gaussian Minimum Shift Keying (GMSK).

• The GMSK modulation has been chosen as a compromise between spectrum efficiency, complexity and low spurious radiations (that reduce the possibilities of adjacent channel interference).

• The GMSK modulation is characterized by its(time bandwidth product) Bb T that is equal to 0.3.

• The modulator block diagram is shown below:

Modulation

• GMSK is minimum shift keying (MSK) (continuous phase frequency shift keying with its modulation index h=0.5) with a pre-modulation Gaussian filter. Where MSK is also considered as a special case of OQPSK.

• The demodulator block diagram is shown below:

Power Spectral Density for GMSK modulated data:

• the power spectral density of GMSK is given by:

Power Spectral Density for GMSK modulated data:

• The power spectral density functions for different filter for different WTb (time bandwidth product) is shown figure at right.

• The smaller WTb,the narrower the Gaussian filter the less the spectral re-growth.

Probability of bit error (BER) for GMSK modulated data:

• The Figure shows the BER performances of GMSK for different BbT

• we see that the less BbT

means less bandwidth of the Gaussian filter ,less spectral re-growth ,but the bit error performance is degraded

• GMSK has a better bandwidth from MSK ,but the BER of MSK is better.

Probability of bit error (BER) for GMSK modulated data:

)()( erfcPGMSK

)2(2)( QPGMSK

Probability of bit error (BER) for GMSK modulated data for fading channel and

Rayleigh channel

• BER for GMSK (BbT=0.25) with coherent detection

Multi-path fading: • Multi-path fading is fluctuation of the signal level due to

multi-path propagation. • The communication channel used in GSM is a Fading

Channel. • Multi-path fading results from a signal traveling from a

transmitter to a receiver by number of routes. This is caused by the signal being reflected from objects, or being influenced by atmospheric effects as it passes for example the layers of air of varying temperatures and humidity.

• Received signals will therefore arrive at different times and not be in phase with each other, they will have experienced time dispersion.

• On arrival at the receiver, the signals combine either constructively or destructively, the overall effect being to add together or to cancel each other out.

Multi-path fading:

• GSM offers five techniques which combat multi-path fading:

1.  Equalization.

2. Diversity.

3. Frequency hopping.

4. Interleaving.

5. Channel coding.

Equalization• Due to signal dispersion caused by multi-path signals the

receiver cannot be sure exactly when a burst will arrive and how to distorted it will be. To help the receiver to identify and synchronize to the burst, a training sequence is sent at the center of the burst. This is a set sequence of bits that is known by both the transmitter and receiver.

• An equalizer is in charge of extracting the 'right' signal from the received signal. It estimates the channel impulse response of the GSM system and then constructs an inverse filter. The receiver knows which training sequence it must wait for. The equalizer will then, comparing the received training sequence with the training sequence it was expecting, compute the coefficients of the channel impulse response. In order to extract the 'right' signal, the received signal is passed through the inverse filter.

Diversity:

• Signals arrive at receiver antenna from multiple paths. The antenna therefore receives the signals at different phases, some at peak and some at trough. This means that some signals will add together to form a strong signal, while others will subtract causing weak signal .

• When diversity is implemented, two antennas are situated at the receiver. These antennas are placed several wavelengths apart to ensure minimum correlation between the two receive paths. The two signals are then combined and the signal strength is improved

Frequency Hopping

• The propagation conditions and therefore the multi-path fading depend on the radio frequency. In order to avoid important differences in the quality of the channels, the slow frequency hopping is introduced.

• The slow frequency hopping changes the frequency with every TDMA frame(4.615 ms).

• The frequency hopping also reduces the effects of co-channel interference.

• There are different types of frequency hopping algorithms. The algorithm selected is sent through the Broadcast Control Channels.

Channel Coding and Interleaving

• Channel coding and interleaving reduce the error of received signal the BER curves will get better after using interleaving and channel coding