Data Transmission Project: Global System for Mobile communications GSM

1
Data Transmission ProjectGlobal System for
Mobile communicationsGSM
2
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.
  

3
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

4
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
   

5
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
• The transmitter's power.
• The geographical layout of the coverage area.
• Number of mobile stations in the coverage area.

6
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).

7
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
• The Base Transceiver Station (BTS) .
• The Base Station Controller (BSC).

8
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).

9
The GSM Radio Interface

• Frequency allocation
• Two frequency bands of 25 MHz
• 890 - 915 MHz used for the uplink direction (from
  the mobile station to the base station).
• 935 - 960 MHz is used for the downlink direction
  (from the base station to the mobile station).
• The channel bandwidth is 200KHz.
• The spacing between the uplink and downlink for
  each channel is 45KHz
• The system is full Duplex.
• 125 carrier frequencies but the first carrier
  frequency is used as a guard band between GSM and
  other services working on lower frequencies.

10
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.
•  

11
Burst Structure

• the burst is the unit in time of a TDMA system.
• Four different types of bursts can be
  distinguished in GSM
• The frequency-correction burst is used on the
  FCCH. It has the same length as the normal burst
  but a different structure.
• The synchronization burst is used on the SCH. It
  has the same length as the normal burst but a
  different structure.
• 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.

12
Burst Structure

• Different types of bursts are shown below

13
Logical Channels

• The traffic channels used to transport speech and
  data information. (TCH).
• Full-rate traffic channels (TCH/F) are defined
  using a group of 26 TDMA frames called a
  26-Multiframe.
• The 26-Multiframe lasts consequently 120 ms.
• The traffic channels for the downlink and uplink
  are separated by 3 bursts.
• The frames that form the 26-Multiframe structure
  have different functions
• 24 frames are reserved to traffic.
• 1 frame is used for the Slow Associated Control
  Channel (SACCH).
• 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

14
Logical Channels

• The control channels used for network management
  messages and some channel maintenance tasks
• 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.
• Common control channels
• The CCCH channels help to establish the
  calls from the mobile station or the network.

15
Logical Channels

• Dedicated control channels
• The DCCH channels are used for message
  exchange between several mobiles or a mobile and
  the network.
• Associated control channels
• The Fast Associated Control Channels
  (FACCH) replaces all or part of a traffic channel
  when urgent signaling information must be
  transmitted.

16
GSM Transmissions and Reception Process
17
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.

18
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.

19
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.

20
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.

21
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

22
Modulation

• GMSK is minimum shift keying (MSK) (continuous
  phase frequency shift keying with its modulation
  index h0.5) with a pre-modulation Gaussian
  filter. Where MSK is also considered as a special
  case of OQPSK.
• The demodulator block diagram is shown below

23
Power Spectral Density for GMSK modulated data

• the power spectral density of GMSK is given by

24
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.

25
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.

26
Probability of bit error (BER) for GMSK modulated
data
27
Probability of bit error (BER) for GMSK modulated
data for fading channel and Rayleigh channel

• BER for GMSK (BbT0.25) with coherent detection

28
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.

29
Multi-path fading

• GSM offers five techniques which combat
  multi-path fading
• 1.  Equalization.
• Diversity.
• Frequency hopping.
• Interleaving.
• Channel coding.

30
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.

31
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

32
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.

33
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