The document discusses the GSM air interface and its importance in mobile communication systems, detailing the RF spectrum, GSM specifications, and modulation methods. It explains the use of FDMA and TDMA for channel access, digital voice transmission, channel coding, interleaving, and various types of burst formats used in communication. Additionally, it covers the operational aspects of user access, synchronization, and features like discontinuous transmission and diversity to enhance connection quality.

1. GSM Air Interface
(𝑈 𝑚)
PRESENTED BY:
NAVEEN JAKHAR, ITS
ABHISHEK SINGH, ITS

2. Introduction
 The Air-interface is the central interface of every
mobile system and typically the only one to which a
customer is exposed.
 The physical characteristics of the Air-interface are
particularly important for the quality and success of
a new mobile standard.

3. RF Spectrum
 GSM 900
 Mobile to BS (UP-LINK) -890 to 915 MHz
 BS to Mobile (DOWN -LINK) -935 to 960 MHz
 Bandwidth - 25 MHz
 GSM 1800 or PCS 1800
 Mobile to Cell (UP-LINK) -1710 to 1785 MHz
 Cell to Mobile (DOWN -LINK) -1805 to 1880 MHz
 Bandwidth - 75 MHz

4. GSM SPECIFICATIONS
 Carrier Separation - 200 kHz
 Duplex Distance - 45 MHz
 No. of RF Carriers - 124
 Access Method - TDMA/FDMA
 Modulation Method - GMSK
 Transmission Rate - 270.833 Kbps
 Speech Coding - Full rate 13 Kbps
Half rate 6.5 Kbps

5. GSM Air Interface structure
(FDMA/TDMA)
 GSM utilizes a combination of frequency division
multiple access (FDMA) and time division multiple
access (TDMA) on the Air-interface. That results in a
two-dimensional channel structure,

6. WHY FDMA and TDMA?
 In a pure FDMA system, one specific frequency is
allocated for every user during a call. That quickly
leads to overload situations in cases of high
demand.
 In fullrate configuration, eight time slots (TSs) are
mapped on every frequency; in a halfrate
configuration there are 16 TSs per frequency.

7.  in a TDMA system, each user sends an impulse like
signal only periodically, while a user in a FDMA
system sends the signal permanently.
 The difference between the two is illustrated in
Figure

8. DIGITAL VOICE TRANSMISSION
 Speech Coding
 In GSM speech coding a block of 20 ms is
encoded in one set of 260 bits.
 This calculates as 50X 260 = 13 kbps. Thus GSM
speech coder produces a bit rate of 13 kbps per
subscriber.
 This provides speech quality which is acceptable for
mobile telephony and comparable with wire-line
PSTN phones.

9. CHANNEL CODING

10. Interleaving
 The process of interleaving
smaller packages of 456 bits
over a larger time period,
that is, distributing them in
separate TSs.
 How the packets are spread
depends on the type of
application the bits
represent.
 Signalling traffic and packets
of data traffic are spread
more than voice traffic.
 The whole process is referred
to as interleaving.

11.  The goal of interleaving is to
minimize the impact of the
peculiarities of the Air-interface
that account for rapid, short-
term changes of the quality of
the transmission channel. It is
possible that a particular
channel is corrupted for a very
short period of time and all the
data sent during that time are
lost.
 Interleaving does not prevent
loss of bits, and if there is a loss,
the same number of bits are
lost.
 However, because of

12. BURST FORMATTING
 Adds training sequence that helps in the working
equalizer to assist in countering the effects of radio
channel on the signal.
 Total of 136 bits added, bringing overall total to 592
bits.

13.  Each TS of TDMA frame is 0.577 ms long and during
this time 156.25 bits are transmitted.
 One burst contains only 148 bits. Rest of the space,
8.25 bits time, is empty and is called Guard Period (
GP ).
 GP enables MS/BTS to “ramp up” and “ ramp
down”.

14. GMSK
 The modulation method in GSM is GMSK which
facilitates the use of narrow bandwidth and
coherent detection capability.
 Rectangular pulses are passed through a Gaussian
filter prior to their passing through a modulator.
 The modulation scheme almost satisfies the
adjacent channel power spectrum density
requirements of -60dB specified by CCIR.

16. FRAME HIERARCHY

17.  The frame hierarchy is used for
synchronization between BTS and MS,
channel mapping, and ciphering.
 Every BTS permanently broadcasts the
current frame number over the
synchronization channel (SCH) and
thereby forms an internal clock of the
BTS. There is no coordination between
BTSs; all have an independent clock,
except for synchronized BTSs.
 That information is very important,

18. Physical Versus Logical
Channels
 Physical channels are all the available TSs of a BTS,
whereas every TS corresponds to a physical
channel. Two types of channels need to be
distinguished, the halfrate channel and the fullrate
channel.
 Logical channels are piggybacked on the physical
channels. Logical channels are, so to speak, laid
over the grid of physical channels. Each logical
channel performs a specific task.

19. LOGICAL CHANNELS

21. Frequency Correction Burst
 This burst format is used by FCCH channel only.
 The whole data space (142 bits) is used for
unmodulated carrier (pure sinusoid) or carrier
modulated with all zero bits.
 This pure carrier is the ‘identity’ of a beacon
frequency (also called BCCH-frequency or base-
frequency) and FCCH slot

22. Synchronization Burst
 This burst format is used by SCH channel only.
 This channel makes a mobile station time-
synchronized with the base station clock. That is why
the synchronization training sequence is very large
for this burst comparing to other burst types.
 Only one training sequence is defined for this burst.

23. Access Burst
 This burst format is used by RACH and AGCH channels.
 When a mobile station sends an RACH message and receives
an AGCH reply, neither MS nor the BTS does have the timing-
advance information.
 For that reason, the actual message is relatively short and
have a long guard band (GB) in order to make sure that there
will be no overlap with the next burst.
 The length of the guard band in the access burst (68.25 bits x
3.69 = 251.16 ms) is equivalent to 37.5 km propagation delay.
 The GSM allows a cell radius up to of 35 km.
 That is, an RACH message from an MS at a distance of up to 35
km from the base station can reach to the base station
antenna without overlapping the next burst.
 The FACCH channel uses this burst during handover operation

24. Normal Burst
 This burst format is used by all other channels
(except FCCH, SCH, RACH and AGCH).
 This normal burst is used by TCH, SDCCH, SACCH,
FACCH, BCCH and PCH.
 A few important features of the burst is stated
below.
 Maximum 57 x 2 = 114 bits of voice/data per burst
 Flag bit is to indicate if the channel is carrying user
traffic (Flag = 0) or control message bits (Flag = 1).
 That is the flag is 0 for TCH and 1 for others.

25. Dummy Burst
 This is like normal burst but has no meaning of its
payload bits

26. CONTROL MULTIFRAME

27. TRAFFIC MULTIFRAME (FULL
RATE)
 This example shows 2 users using
full rate voice traffic channels.
(One user uses the slot 2 at every
frame and the other user use the
slot 4 at every frame).
 At the center of the 26-frame
traffic channel multiframe (i.e,
Frame 12) is the Slow Associated
Control Channel (SACCH) which
carries link control information to
and from the MS–BTS. At the last
frame is 1 idle frame. All the
remaining frame are allocated
for Traffic. There is no dedicated
FACCH frame or slots. FACCH
steals TCH whenever it needs.

28. TRAFFIC MULTIFRAME (HALF
RATE)
 This example shows 26-
multiframe structure for
TCH/HR, showing 2 users
using HR voice traffic
channel. In this example,
the two users shares the slot
2 of every frame in
alternating fashion. You
would notice that there are
two Frames for SACCH. The
Frame 12 is for SACCH
of user 1 and Frame 25 is for
SACCH of user 2.

29. CONTROL MULTIFRAME
(BROADCAST CHANNELS) An example of control channel
multiframe structure for Broadcast
Channel (Base Control Channel)
which is made up of FCCH, SCH,
BCCH, CCCH. It is for Downlink
multiframe structure.
 In Uplink, every frame is for single
channel - RACH. Some key facts
about DL Base Control Channel are
 There are five FCCH equally spaced
within the 51 multiframe.
 Each FCCH is followed by a SCH,
meaning that there are five SCH as
well.
 Four frames (Frame 2-5) are
allocated for BCCH.
 The last frame (Frame 50) is allocated
for Idle.
 All the remaining Frames are
allocated for CCCH(e.g, PCH or
AGCH).

30. Mapping of Logical Channels Onto
Physical Channels

31. OPERATIONAL ASPECTS
 Subscribers are not allocated dedicated channels
 TCH allocated to users only when needed
 Hence IDLE MODE & DEDICATED MODE

32. IDLE MODE
 When MS is powered on (active) without being in
dedicated mode
 MS stays continuously in touch with BS
 Listens to transmissions from BS to intercept Paging
Messages ( for incoming calls)
• Monitors Radio Environment in order to evaluate
Channel Quality & choose the most suitable BS
• Listens to BS to avail short message broadcast
service

33. ACCESS PROCEDURE
 Access to system ( switch over from IDLE to
DEDICATED Mode)
 MS indicates to BS that it needs a connection
 BS accepts the request & indicates which traffic
channel it may use
 For above purpose specific transmission is done over
“ Common Channels”

34. OTHER FEATURES
 Discontinuous Transmission
 Synchronization between uplink and downlink
 Diversity

35. Discontinuous Transmission
 Speech activity only 40% of time.
 Needs Voice activity detection.
 Annoying clicks/inefficient DTX.
 Generation of Comfort Noise at receiver to avoid
the feeling of the set being dead.

36. SYNCHRONIZATION BETWEEN
UPLINK AND DOWNLINK
 For technical reasons, it is necessary that the MS and
the BTS do not transmit simultaneously. Therefore,
the MS Is transmitting three timeslots after the BTS.
 The time between sending and receiving data is
used by the MS to perform various measurements on
the signal quality of the receivable neighbour cells.

37.  Depending on the
distance between the
two, a considerable
propagation delay
needs to be taken into
account.
 That propagation
delay, known as timing
advance (TA), requires
the MS to transmit its
data a little earlier as
determined by the

38. DIVERSITY
 Space Diversity
 Mounting two receiver antenna physically
separated a distance.
- Probability of both of them being affected by a
deep fading dip at same time is low.

39. THANK YOU!