TDMA Methods, page 1
Access Methods in GSM
1. Fundamentals of Multiple Access
Frequency division multiple access FDMA
Time division multiple access TDMA
Code division multiple access CDMA
2. TDMA in GSM
RF data
TDMA structure in GSM, frames and multiframes
TDMA timers
3. Burst Structures
Information bits
Training sequence
Bit synchronization
Compensation of multipath reception
Guard time
Delay correction
Burst types
Frequency-correction burst
Synchronization burst
Dummy burst
Access burst
4. The Concept of "Channel" in GSM
Physical/logical channel
Physical channels and their definition
Main logical channels and their functions
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TDMA Methods, page 2
1. Fundamentals of Multiple Access
The 200 kHz channel bandwidth of GSM systems seems fairly wide in comparison
with that of conventional systems. This bandwidth is "divided up" using timeslots
which allow one channel to be used by several subscribers (multiple access). The
multiple access methods available and their characteristic features are described in
the following.
Frequency division multiple access FDMA
For analog radio systems, the trend has always been towards a more efficient
utilization of the available frequency spectrum by reducing the channel spacing. The
number of radio channels obtained at a channel spacing of 12.5 kHz is, of course,
twice that obtained with 25 kHz. However, improvements usually have to be traded
off against some drawbacks: the narrower the channel spacing, the higher the
required frequency accuracy. Consequently, the maximum deviation has to be
reduced, which leads to poorer transmission quality due to the lower S/N ratio.
Moreover, the gaps between the channels, which must be several kilohertz wide to
provide a "safety margin", also reduce the available transmission bandwidth.
Fig. 1: Channels in wideband and narrowband systems (fdma.dsf)
Dividing the available frequency spectrum into a number of frequency channels
enables several users to simultaneously access the various frequencies. This form of
multiple access is called frequency division multiple access (FDMA). Consequently,
all radio systems using a spectrum divided into channels are FDMA systems. Today,
the technically useful limit is reached with a channel spacing of 10 to 12.5 kHz. If time
is considered as a third dimension, the following diagram, frequently used in GSM
environments, is obtained:
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TDMA Methods, page 3
Ampl i t ude
Fr equency
Ti me
Channel s
N-1
N
N+ 1
N+ 2
Fig. 2: Frequency division multiple access FDMA
Advantages of FDMA:
- Simultaneous use of a given system bandwidth by many subscribers
- More channels are available thanks to reduced channel spacing
Disadvantages of FDMA:
- Higher frequency accuracy required
- Transmission quality decreases as the channel bandwidth is reduced
- Better selectivity filters required
- One transmitter and also one receiver is required per channel
Time division multiple access TDMA
With TDMA (time division multiple access) systems, the available bandwidth is
divided into considerably fewer and so wider channels than in FDMA systems. It
appears that each channel is simultaneously available to several subscribers but in
fact each subscriber can use the whole channel only for the period of a timeslot. For
the rest of the time, he has no access. This serial access by several subscribers is
repeated over time.
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TDMA Methods, page 4
Amplitude
Frequency
n-1
Time
n
n+1
n+2
Fig. 3: Time division multiple access TDMA
Advantages of TDMA:
- Simultaneous use of a given system bandwidth by many subscribers
- Depending on the number of available timeslots, several subscribers
can be served by one transmitter/receiver unit
- Transmitter and receiver are not permanently on (saves battery power)
- The instrument can perform other tasks in the intervals between
transmission and reception of call (e.g. monitoring the field strength of
neighbouring channels)
- Reduced susceptibility to frequency-selective fading with large channel
bandwidths
Disadvantages of TDMA:
- Accurate time (and frequency) synchronization of intruments required
- Higher processing capacity required
- Broadband modulators required
Code division multiple access CDMA
The advent of powerful, cost-effective signal processors meant that a less
conventional multiple access method could be employed for mass communication
systems. With code division multiple access CDMA, the whole system bandwidth is
available to all subscribers all the time, i.e. all subscribers transmit and receive
simultaneously but each subscriber uses a different code. Logic 1 is represented by a
certain bit sequence, logic 0 is the inverse of this sequence. The different signals are
distinguished in the receiver by cross-correlating the received sum signal, which
contains the different codes of all active subscribers, with the bit sequence of the
subscriber whose transmission the receiver wants to detect. UMTS (Universal Mobile
Telecommunications System), the 3rd generation mobile telephone system uses this
access method.
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TDMA Methods, page 5
Amplitude
Frequency
Time
Fig. 4: Code division multiple access CDMA
Advantages of CDMA:
- Simultaneous use of a given system bandwidth by many subscribers
- Several signals can be received simultaneously by a single RF section
- Reduced susceptibility to frequency-selective fading in the case of large
channel bandwidths
- More subscribers can be served
- Reduced costs for radio network planning
Disadvantages of CDMA:
- Accurate time synchronization of subscribers required
- Fast transmitter power control over a wide dynamic range required
- No mass-market experience available
2. TDMA in GSM
RF data
In spite of the competition from other mobile telephone systems, it was possible to
define common frequency bands for GSM on a worldwide basis. All nations who
signed the GSM-MoU (memorandum of understanding) committed themselves to use
the standardized frequency ranges when they installed their GSM system. The
competition for frequencies mainly affects countries using NMT 900 (Nordic Mobile
Telephone), whose frequency range coincides with the GSM P band. The TACS
(Total Access Communication System) band too has some overlap with the GSM P
band, and the G1 band (extended GSM 900) is completely within the TACS range.
Cordless telephones to the CT1 standard also use the upper end of the GSM P band.
CT1+ telephones which had been assigned a frequency range below the P band
years ago to protect them against GSM are now being ousted by the G1 band.
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GSM 900
Frequency range
Uplink
(MS transmitting)
Downlink
(BTS transmitting)
Duplex spacing
Spectrum
Number of channels
Channel No.s
Channel spacing
Modulation
Data transmission rate
Bit duration
GSM 1800
P band
890 to 915 MHz
G1 band
880 to 890 MHz
1710 to 1785 MHz
935 to 960 MHz
925 to 935 MHz
1805 to 1880 MHz
45 MHz
2 x 35 MHz
124
1 to 124
49
975 to 1023
200 kHz
GMSK with B x T = 0.3
270.833 kbit/s
3.69 µs
95 MHz
2 x 75 MHz
374
512 to 885
Fig. 5: RF data for GSM 900 and GSM 1800
Depending on the resources of the network operator and the technical facilities of the
mobile phone, up to 124 channels of the GSM 900 network are available in the
P band (two frequencies per channel at a spacing of 45 MHz for uplink and downlink), and perhaps another 49 channels in the G1 band (also GSM 900 frequencies),
and probably 374 channels in the E network (GSM 1800, duplex spacing 95 MHz).
Normally, the network operators can use the frequencies (channels) assigned to
them for their base stations as they choose. Each base station requires at least one
channel (C0, also referred to as BCCH carrier) on which it continually sends
synchronization information at full power and - depending on the expected traffic
volume - additional frequency channels (traffic channels which are only used for
actual calls).
Why is GSM referred to as a TDMA system when it uses different frequency
channels?
The key point is that each frequency channel is divided into 8 timeslots. In the first
channel (C0) of every base station, synchronization information is sent in timeslot 0
while the remaining 7 timeslots are used for calls (traffic) or dummy bursts so that
power is always being transmitted. This is also necessary for synchronization and
ensures that a telephone which is switched on in an area for the first time can find its
GSM base station.
To keep the hardware of the telephone as simple as possible and to ensure optimum
utilization, it was decided that transmission and reception should not be simultaneous
(intermittent operation of transmit and receive section). Nevertheless, this seems like
duplex mode to the subscriber because the large amount of speech data transmitted
in compressed form in the timeslots fills all the eight timeslots when expanded.
Operation seen from the mobile:
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TDMA Methods, page 7
RX
RX
RX
* Duplex
spacing
TX
TX
3 time slots
* GSM900: 45 MHz
GSM1800: 95 MHz
Fig. 6: Transmission and reception in "duplex" mode
The receive frequency and transmit frequency are generated by a single synthesizer
in the mobile. The synthesizer lets the receiver "listen" to the base station in a
timeslot and then three timeslots later lets the transmitter transmit (now the base
station should listen). This sequence is repeated after eight timeslots (a frame
consists of eight timeslots). The free slots in between are used, say, for field-strength
measurements on the C0 frequencies of neighbouring base stations. The measured
field strength is the criterion for deciding whether an ongoing call is handed over to
another base station or not.
To simplify timeslot counting, the timeslots of the base station and of the mobile are
counted in the same way. For instance, if the downlink (base station sends) is
assigned to timeslot 0, the mobile station must receive at the same time. The uplink
(mobile station sends) for the phone is also assigned to timeslot 0, but because of the
offset of 3 timeslots described above, the uplink timeslot is delayed by 1.73 ms (for
timing see next section).
TDMA structure in GSM, frames and multiframes
GSM timing is based on 48 periods of a 13 MHz signal (approx. 3.69 ms, which
corresponds to the transmission time of one bit). A certain number of these bits is
combined to form a burst and is transmitted in one timeslot. Eight timeslots form a
frame. A certain number of these frames is combined to give a multiframe. Since
there are several types of multiframe, certain numbers are grouped together to form
standard hyperframes and superframes.
Detailed description:
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Time slot
(0,577 msec)
T=Tailbit, F=Flag, TS=Training Sequence, Guard=guard priod
number of bits:
T Information
F TS
F Information
T Guard
3
1
1
3 8,25
57
26
57
TDMA frame
(4,62 msec)
0
1
2
3
4
5
6
7
Fig. 7: Timeslot in TDMA frame
Two 57-bit information blocks, i.e. 114 bits, are transmitted in every timeslot. A TDMA
frame contains eight of these timeslots and since a call can only use one timeslot per
frame, the raw data transmission rate (coded speech or data signal plus error
correction code) is about
114 bits/4.62 ms = 24.7 kbit/s.
A known bit sequence, referred to as the training sequence, is transmitted between
the information blocks. It is used for synchronization to the bit stream and for
assessing the current transmission characteristics of the radio channel. The training
sequence makes it possible to set channel equalizers in the receiver to considerably
improve decoding. Since the transmission conditions in the radio channel may
change rapidly, the training sequence is sent between the information blocks and
transmitted with each burst.
The guard periode has been inserted to prevent consecutive bursts time overlapping
if signals are not fully time compensated (see further down). It is also required for
ramping up resp. ramping down the transmitter (power ramping). It is certainly
unusual to specify a guard time as a fraction of a bit transmission period (8.25), but
interpreting this information in terms of time (8.25 bit periods x 3.69 µs) has proved to
be useful.
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TDMA Methods, page 9
This "odd" number is obviously due to the maximum signal delays resulting from the
cell size planned in the GSM definition phase.
The flag bit indicates whether the transmitted bits are normal information bits or
whether some of the transmitted bits are used for signalling, e.g. when there is an
urgent need to perform a handover.
Frame numbers are also used on the control channel but the frames of control
channels and traffic channels are numbered separately. This is necessary so that
certain measurements can be performed, e.g. while a call is in progress. 26 TDMA
frames are combined to form a 26-frame multiframe for all timeslots containing a
traffic channel (i.e. voice and data signals and a small amount of signalling data to
keep the link up). All time slots that are exclusively for signalling are counted using
51-frame multiframes.
This method of counting conceals the fact that physical channels (specified in terms
of frequency channels and timeslots) and logical channels (e.g. traffic channels,
TCHs) are handled in a different way. Thinking in terms of logical GSM channels is
an approach that has turned out to be useful. Logical channels have an almost
parallel existence and must, of course, be mapped serially onto the physical channels
by the hardware. The procedure is a bit confusing for a user who is not familiar with
GSM but the approach has proved to be very useful for network operation because
some logical channels are only transmitted when required and can be moved from
one physical channel to another according to the traffic volume. Only very few logical
channels have always to be associated with the same physical channels to allow the
mobile to synchronize to an unknown GSM base station. If the first synchronization
attempt succeeds, the information in a few fixed signalling bursts will be sufficient to
decode the whole data stream.
The TDMA structure provides for hyperframes and superframes above the 26-frame
and 51-frame multiframes. The hyperframes and superframes can be used for both
types. These hyperframes and superframes are used, for instance, for encryption
algorithms. The following structure is obtained:
26-frame multiframe
= 26 frames for timeslots containing traffic channels
"in parallel" with
51-frame multiframe
= 51 frames for timeslots containing control channels
are "combined" in
super frames:
= 51 x 26 frames
(least common multiple,
makes a "combination" possible)
= 1326 frames
= 6.12 s
and
hyper frames:
= 2048 super frames
= 2,715,648 frames
= 3 hours 29 min. 3.5 s
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TDMA timers
The frames within the hyperframe are continuously counted so that the TDMA clock
restarts after approx. 3.5 hours. The frame number, therefore, represents the basic
time unit for the GSM system. As it would be too cumbersome to use just seconds to
measure time, larger units like minutes and hours are defined; the same thing
happens with frame numbers which are expressed in larger units – the various types
of timer. These timers are defined as follows:
T1:
= FN div (26 x 51)
value range:
0 to 2047
T2:
= FN mod 26
value range:
0 to 25
T3:
= FN mod 51
value range:
0 to 50
FN (Frame Number)
value range:
0 to 2715647
FNmax
= 51 x 26 x 2048 - 1
Fig. 9: TDMA timers
The original frame number can be calculated from the three counter readings. For
certain signalling tasks, not all the counters are needed. In other words, this means
that some signalling procedures can be performed without all the counter readings
being known.
The division without remainder function "div" is used to calculate T1; div gives the
whole number obtained when FN is divided by 1326 = 26 x 51. T2 and T3 are equal
to the FN mod 26 and FN mod 51 respectively, the timers are repeatedly counting
from 0 to 25 and 0 to 50.
3. Burst Structures
Information is exchanged between the base station and mobile station in the
timeslots. In each slot a certain amount of information, i.e. a burst, can be
transmitted. Depending on the task to be performed, different types of burst can be
used, although the most frequently used type is the "normal burst" shown in Fig. 10.
It is used for signalling as well as for speech and data transmission.
Rohde & Schwarz Trainingszentrum, V 2.3
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TDMA Methods, page 11
1 time slot(0,577 msec)
number of bits:
T Information
F TS
F Information
T Guard
3
1
1
3 8,25
57
26
57
T=Tailbit, F=Flag, TS=Training Sequence, Guard=guard period
Fig. 10: Normal burst
Each part of the burst serves a specific purpose which will be described below:
Information bits
The normal burst is able to transmit a total of 114 bits so that a maximum data rate of
approx. 24.7 kbit/s is obtained by 2nd generation GSM. The transmission rate within
the system can only be increased when more than one timeslot is used (General
Packet Radio Service GPRS, High-Speed Circuit-Switched Data HSCSD) or another
modulation method (8PSK modulation with EDGE, Enhanced Data Rate for GSM
Evolution).
The bit rate in the control channels is much lower, i.e. the above transmission rate is
only attained by the mobile if a traffic channel has been established. In this case the
mobile and the base station use a control channel in addition to the speech and data
channel, which uses up capacity and carries information on reception quality and
power ramping. Fig. 11 shows the bit rates for the various channels:
Traffic channel:
- Voice (full-rate):
- Data:
Useful data:
Error protection: Total:
13.0 kbit/s
2.4 kbit/s
4.8 kbit/s
9.6 kbit/s
14.4 kbit
9.8 kbit/s
20.4 kbit/s
18.0 kbit/s
13.2 kbit/s
8.4 kbit/s
22.8 kbit/s
Control channel:
Idle frame:
0.95 kbit/s
0.95 kbit/s
Total:
24.7 kbit/s
Fig. 11: Transmission bit rates
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Training sequence
In the middle of the normal burst, a 26-bit training sequence is sent, the bit sequence
being known to the receiver. There are eight different sequences which are referred
to as the training sequence code (TSC). The eight sequences must be stored in all
receivers and at the beginning of a transmission the base station decides on the TSC
to be used. The training sequence performs two main tasks: bit synchronization and
estimating the channel impulse response (instantaneous response of the radio
channel). Using this estimate, the channel equalizers in the receivers can be set for
optimum data stream decoding.
Bit synchronization
Data are transmitted via the air interface in asynchronous mode. The receiver must
be able to regenerate the bit clock from the data stream and needs features in the
data stream to enable it to identify information units (block synchronization).
Conventional data radio therefore uses data telegrams that start with a ...10101010...
sequence so that the receiver can regenerate the bit clock. A predefined bit word tells
the receiver when the actual information (block synchronization) starts. A receiver
synchronized in this way is able to decode the data stream online.
The GSM training sequence is used for fine bit synchronization and for block
synchronization. Since the training sequence is not sent at the beginning of a burst,
the received data stream must be buffered in the receiver and decoded later on.
Synchronization itself makes use of cross-correlation, i.e. the stored data stream is
compared bit-by-bit with the expected training sequence. When the position of the
training sequence is known, the timing of the information bits is also known and fine
tuning of the bit-clock is performed. A burst that does not contain the expected
training sequence cannot be synchronized and decoded.
Compensation of multipath reception
The signal from the transmitter (in the Fig. below the base station is transmitting to
the mobile, but the same explanation still applies, if the mobile is transmitting) arrives
at the receiver not only along the direct path but also on various other paths as a
result of reflections and diffraction caused by obstacles in the signal path. The
propagation conditions on these additional paths are different to those on the direct
path, for instance:
- longer travel time because of longer path
- various receive levels (depending on reflections)
- different Doppler shifts (possibly due to different relative velocities)
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TDMA Methods, page 13
BTS
MS
Fig. 12: Multipath reception due to reflections and diffraction
Because of the different travel times, the signals have different phases at the
receiving antenna. Depending on their phase, components may compensate - i.e.
the total level goes to zero - or reinforce so that a strong signal is received for a short
period of time. RF level variations are random, fading may be up to 40 dB.
In addition to RF level fading, there is another annoying effect which, without
compensation, would make correct signal decoding rather difficult.
Because of the extra length of the indirect paths, the signals at the receiving antenna
not only have different phases but the modulated information arrives at different
times. The sum of all the channel responses to a single pulse is called the CIR
(Channel Impulse Response). If the indirect path is just one kilometer longer, the
GSM echo bit reaches the receiver later than the directly received bit and so
interferes with the next bit to be received. This intersymbol interference may affect
several consecutive bits. With delays greater than 15 µs, identification of the received
signal components and echoes becomes more and more difficult. This problem can
also be solved with training sequences. The echoes on the indirect paths also
contain the echoes of the training sequence. The cross-correlation method used to
find the original training sequence may also be used to find the training sequence
echoes as well as their delay and attenuation. With the aid of this information, the
received signal can be corrected by a channel equalizer.
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Sent pulse
Received signal
t
TD = propagation time
TBit = bit duration
Fig. 13: Channel impulse response
Guard time
Transmission in each time slot is terminated with a guard time of 8.25 bit periods
(8.25 x 3.69 µs ≈ 30 µs) during which no information bits can be sent. During this
time, the burst level must be reduced by up to 70 dB to avoid the next timeslot being
affected. The "owner" of the subsequent timeslot uses this time to increase his
transmitter power to nominal. This means that the guard time is used twice for power
ramping (the transmitter power must be increased and reduced within narrow
tolerances).
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Fig. 14: Guard time at the end of each timeslot
Delay correction
The integrity of a timeslot depends on subscribers transmitting only during the time
assigned to them and otherwise keeping their transmitters off the air. This is only
possible when all subscribers are in strict sync. For practical reasons, the clock signal
is generated by the base station and all the mobiles synchronize to it. There will be
no problems on the downlink, i.e. when (one) base station sends to (several) mobiles.
Mutual interference may, however, occur in the uplink, where up to eight subscribers
must share one radio channel, if the mobiles are not accurately synchronized to the
timeslots.
Where can problems with synchronization occur?
Under the given conditions, radio signals propagate at almost the speed of light.
Even at a speed of 300 000 km/s they still take about 33 µs to cover a distance of
10 km. A mobile station 10 km away from the base station synchronizes to the
received signal which has already travelled for about 33 µs before it arrives at the
mobile. If the mobile station now transmits back to the base station (without any delay
correction), this signal will require the same travel time. The base station, therefore,
receives a signal which is delayed by about 66 µs in its own time frame. Considering
that a timeslot is 577 µs wide (including the guard time of about 30 µs) and cells have
a max. radius of. 35 km (limited by the delay correction factor described in the
following) it is obvious that the neighbouring timeslot will be compromised if there is
no compensation.
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Effect of signal propagation time
Propagation time ~33 µsec
to phone
Propagation time ~33 µsec
to base station
Mobile Station
Base Station
Fig. 15: Signal delay and its effect
The above example with the numbers:
Signal travel time over 10 km is ≈ 33 µs (distance / speed of light)
The (time) sync signals from the base station require this time to arrive at the
antenna of the mobile station, i.e. mobile synchronization is delayed
by 33 µs.
From the point of view of the mobile station, the mobile sends a burst to the
base station with correct timing (without delay correction) but from the
point of view of the base station the signal is sent 33 µs too late.
The signal covers the same distance on its way to the base station antenna
and so is delayed by another 33 µs.
Relative to the base station’s time frame, the
received signal is delayed by 66 µs.
The signal is not sent in the assigned timeslot and compromises
neighbouring timeslots.
The guard time at the end of each burst is only about 30 µs and is certainly not long
enough in the above example (apart from it being required for power ramping of the
transmitters). The greater the distance between mobile station and base station, the
greater the effect of the signal delay. The only way to solve this problem is to make
the mobile send the burst earlier. To do so, the base station has to measure the
signal delays and send the appropriate correction factor to the mobile. A special burst
type (access burst, see further down) is used for this purpose. This burst has a much
wider guard time and is used by the mobile to attach to the base station when it
wants to establish a connection.
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TDMA Methods, page 17
Because of the longer guard time of this burst, interference with neighbouring
timeslots is prevented. The base station can determine a correction factor (referred to
as TA, Timing Advance in GSM, it represents the number of bit periods) and send it
to the mobile station. The mobile station advances the sending time of its burst
accordingly and the signal from the mobile station arrives at the base station in sync.
If the mobile station is moving while a call is in progress, the distance between the
mobile station and the base station generally changes and so also the signal delay.
For this reason, the timing advance is checked about twice every second while a call
is going on.
A few technical limits of the GSM system (GSM 900 and GSM 1800) can be derived
from the timing advance specifications:
The timing advance is transmitted as a 6-bit word. With 6 bits, the numbers 0 to 63
can be represented. Increasing this number by one means that the mobile station
has to advance transmission by one bit duration, i.e. by 3.69 µs. This means a delay
of 63 x 3.69 µs = approx. 232.5 µs can be corrected, which corresponds to a distance
of almost 70 km. Therefore, the mobile station cannot be more then about 35 km
away from the base station. This also means that the distance between the base
stations in the network cannot be more than 70 km. The timing advance also explains
why, for instance, a mobile on a ship near to shore can find a GSM network
(propagation conditions and coverage on water are optimal) but cannot register to the
network because the base station is more than 35 km away. These extreme ambient
conditions make it clear that the timing advance may also be a criterion for call
handover or disconnection. On shore, this scenario is only of theoretical interest
because, due to the traffic volume encountered, none of the base stations has to
cover a cell radius of 35 km.
The timing advance can be used to determine the distance between the mobile and
the base station because each increment in the TA (1 bit duration = 3.69 µs)
corresponds to a signal travel time of 1.1 km. If the base station uses an
omnidirectional antenna, a valid timing advance indicates that the subscriber is on a
550 m wide circle centered on the base station. On the other hand, a valid timing
advance is only available when a link has been set up, i.e. only when a call is in
progress or during a location update. At any other time, the timing advance may be
invalid because the mobile is not transmitting.
Burst types
In addition to the normal burst described previously, which is used in most of the
cases, other burst types are available for special purposes (see Fig. below). All these
burst are exactly 1 timeslot (577 µs) in duration.
Rohde & Schwarz Trainingszentrum, V 2.3
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TDMA Methods, page 18
Normal Burst
T
3
57 data bits
F
26 Bits
1 Training Sequence
F
1
T G
3 8,25
57 data bits
Frequency Correction Burst
T
3
T G
3 8,25
142 fixed Bits
Synchronisation Burst
T
3
64 Bits
Extended Training Sequence
39 data bits
39 data bits
T G
3 8,25
Dummy Burst
T
3
T G
3 8,25
142 fixed Bits
Access Burst
T
8
41 Bits
Training Sequence
36 data bits
T
3
G
68,25
Abbreviations :
T=Tail Bit (3 Bits/ 8 Bits in leading part of Access Burst
F=Flag (1 Bit)
G=Guard Period (8,25 Bits/ 68,25 Bits in Access Burst)
Fig. 16: Burst types (burst_ty.dsf)
Frequency correction burst
The 142 "fixed bits" of the frequency correction burst are all set to logic 0. GMSK
(Gaussian minimum shift keying), the type of modulation used for GSM, produces a
stationary carrier-frequency deviation of approx. 67.7 kHz with this burst. This burst is
sent by the base station only and used by the mobiles for initial synchronization to
the carrier frequency and for compensating any Doppler shift caused by a mobile
moving at speed. It is sent by the base station every 10 frames (i.e. approx. every 46
ms) but only in timeslot 0 and only on carrier C0 (sometimes referred to as the BCCH
carrier).
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Synchronization burst
The synchronization burst is also transmitted in timeslot 0 in the frame after the
frequency correction burst. This burst, too, is only sent by the base station on the C0
carrier. The considerably longer training sequence is one significant difference
between the synchronization burst and the normal burst. Like the 26-bit type, this
training sequence is also used for bit synchronization but, because it is longer,
synchronization is more accurate.
The two 39-bit data blocks contain the timers T1, T2 and T3 in coded form and also a
base station identification code (incl. the training sequence No.). When this "GSM
time" is received, the mobile station is in sync with the base station.
Dummy burst
A base station must continuously transmit at nominal power on its C0 carrier in all
time slots as this carrier is used by the mobile stations to find the nearest base
station and to evaluate reception quality. If normal bursts are not available for
transmission in a timeslot, dummy bursts are sent by the base station instead
because an unmodulated carrier cannot be transmitted. These bursts, too, are used
only by the base station on the C0 carrier but may be sent in any of the timeslots.
Access burst
As already pointed out, the access burst is sent when the mobile station calls the
base station for the first time. The base station uses this burst for a delay
measurement, determines the associated timing advance and informs the mobile
station accordingly. This means that delay correction is performed for the next call
from the mobile which can now use a normal burst with a much shorter guard time.
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TDMA Methods, page 20
4. The Concept of Channel in GSM
Every base station sends on at least one frequency in 8 timeslots. It has become
common practice to refer to physical channels that are defined by frequency and
timeslot. Several "types" of data are sent on these physical channels, e.g. speech,
test reports, instructions, etc. For these data types the term "logical channel" is used.
Logical channels are considered to be "parallel" channels which are serially mapped
by the hardware onto the physical channel (which must not always be the same;
frequency and/or timeslot may be changed as required). For instance, the FCCH
(Frequency Correction Channel), which is used for correcting the frequency of the
mobile station, the SCH (Synchronization Channel) with the initial information on the
base station, the BCCH (Broadcast Control Channel) acting as a kind of "notice
board" with further information, and many other logical channels are transmitted in
timeslot 0 (first time slot) of carrier C0. Some of these logical channels are only
transmitted in specific contexts and their position in the physical data stream is not
always the same.
Physical channels and their definition:
- ARFCN + TN
The number of the carrier frequency channel (absolute radio frequency channel
number) together with the timeslot number defines the simplest version of a physical
channel.
- several ARFCNs + TN + HSN + MAIO
When frequency hopping is activated, the hopping sequence number (HSN) and the
mobile allocation index offset value MAIO must also be specified.
- ARFCN + TN + SSN
If the half-rate speech codec is used for communication, two calls can share a fullrate channel. The subsequence number SSN is used to distinguish the calls. It
indicates whether the half-rate link uses even or odd frame numbers.
- several ARFCNs + TN + HSN + MAIO + SSN
same as above, but the frequency of a half-rate channel is also assigned and
frequency hopping is activated at the same time.
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TDMA Methods, page 21
Main logical channels and their functions
Traffic channels
Traffic channels are used to carry digitized speech or other user data. They are
normally classified according to transmission speed. For voice transmission, the
following is defined:
- Traffic channel using full-rate data transmission, a full-rate channel operating
at 22.8 kbit/s. 13 kbit/s are used for speech transmission, the rest is basically
used for error protection.
- Traffic channel using half-rate transmission, a half-rate channel operating at
11.4 kbit/s. 6.5 kbit/s are available for speech transmission.
The subscriber can also choose between half-rate and full-rate transmission for data.
Available bit rates:
Designation
Explanation
TCH/FS
TCH/HS
TCH/F14.4
TCH/F9.6
TCH/F4.8
TCH/H4.8
TCH/F2.4
TCH/H2.4
Full-rate speech traffic channel
Half-rate speech traffic channel
14.4 kbit/s full-rate data traffic channel
9.6 kbit/s full-rate data traffic channel
4.8 kbit/s full-rate data traffic channel
4.8 kbit/s half-rate data traffic channel
2.4 kbit/s full-rate data traffic channel
2.4 kbit/s half-rate data traffic channel
Fig. 17: GSM traffic channels and their bit rates
Control channels
Even if no call is in progress (traffic channel), the resources required for signalling are
considerable. Information has to be continuously exchanged via the air interface (e.g.
location update). The control channels allow the mobile station to receive information
from the base station any time or to send information to the base station.
There are three main groups of control channels:
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Broadcast Channels
This channel group is used by the base station to send relevant information to all
active mobile stations (unidirectional transmission to mobile)
- Frequency Correction Channel FCCH
for the frequency synchronization already described. It is transmitted in
frames 0, 10, 20, 30, 40 and 50 within the 51-frame multiframe
- Synchronization CHannel SCH
with "GSM time" and a code for base station identification. This channel is
sent in the frame directly after the FCCH.
- Broadcast Control Channel BCCH
with information on the radio channel configuration of the home cell and of
neighbouring cells, on the location area code for a location update and on the
organization of the common control channels CCCH (described below). This
channel also contains other important signalling information. The BCCH
comprises four normal bursts which are sent in frames 2 to 5 of the 51-frame
multiframe.
- Cell Broadcast CHannel CBCH
This is a kind of open information channel and comparable to teletext in TV
broadcasting.
Common Control Channels CCCH
This group is used for information exchange between base station and mobile station
(bidirectional) - mainly for access management
- Paging CHannel PCH,
used by the base station for paging mobile stations, e.g. for a mobileterminated call (a call is made to the mobile station).
- Random Access CHannel RACH
used by the mobile for a first call to the base station to request an exclusive
control channel. In the case of a mobile-originated call (mobile station calls a
subscriber), the mobile sends an access burst on this channel.
- Access Grant CHannel AGCH
is practically the response to the RACH. After having received the access
burst, the base station tells the mobile the traffic channel.
- Notification CHannel NCH
enables the base station to notify incoming group calls.
Dedicated Control Channels
A bidirectional dedicated control channel performs signalling tasks independently or
assigned to a traffic channel.
- Slow Associated Control CHannel SACCH,
a slow, dedicated control channel which is coupled to a traffic channel and
used, for instance, for power control, setting the timing advance and for test
reports (receive field strength and quality). The SACCH uses frame 12 of the
26-frame multiframe and is 4 bursts long. This means that it is sent in 4
consecutive 26-frame multiframes.
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TDMA Methods, page 23
- Fast Associated Control CHannel FACCH
This fast, dedicated control channel is coupled to a traffic channel and
performs signalling tasks that cannot be postponed. Example: preparing a
handover. This channel has to notify the "stealing" of bits for the FACCH via
the two flags of the normal burst (traffic transmission). For this reason, the
two flags for the normal burst immediately before and after the training
sequence are called "stealing flags". The FACCH therefore "steals"
transmission capacity from the traffic channel.
- Stand-alone Dedicated Control CHannel SDCCH
This independent control channel is used for exchanging information
between the base station and the mobile station when no call is in progress.
Example: location update, authentication and link setup up to the point when
a call goes through.
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Mapping of logical channels
a) Traffic Channels
Multiframe Structure (26 MF)
T T T TT T T T T T T T A T T T TT T T T T T TT I
0
12
25
T = Traffic
Mapping for Traffic Channels
A = SACCH
I = Idle
b) Control Channels
Multiframe Structure (51 MF)
TS0
F
TS7TS0
TS7
S
~ 4.62 ms
B
B
Channel C0
(down link)
B
t
0
10
20
F S B B B B C C CC F S C C C C C C C C F S C C
51 - Multiframe
~ 235 ms
Rohde & Schwarz Trainingszentrum, V 2.3
50 0
10
CC I F S B B B B CCCC F S C
Channel C0
Time Slot 0
(down link)
F = FCCH
S = SCH
B = BCCH
C = CCCH
I = Idle
TDMA_system-e.doc
TDMA Methods, page 25
CHANNEL MAPPING (1)
FCH +SCH + BCCH + CCCH
DOWNLINK
0
10
B
F S
C
20
C
F S
C
30
C
F S
C
F S
40
C
C
50
C
F S
C
I
R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R
UPLINK
SDCCH/8(0..7) + SACCH/C8(0..7)
DOWNLINK
0
D0
D1
10
D2
D3
D4
D5
D6
30
D7
A0
A1
A2
D0
D1
D2
D3
D4
D5
D6
D7
A4
A5
A6
A5
A6
10
A7
- - -
D0
20
D1
D2
D3
D4
D5
40
D6
D7
A0
A1
A2
A3
- - -
D0
D1
D2
D3
D4
D5
D6
D7
A4
0
20
40
30
50
A3
A7
- - - - -
50
UPLINK
CHANNEL MAPPING (2)
FCCH +SCH + BCCH + CCCH + SDCCH/4(0..3) + SACCH/4(0..3)
DOWNLINK
0
10
20
30
40
50
F S
B
C
F S
C
C
F S
D0
D1
F S
D2
D3
F S
A0
A1
I
F S
B
C
F S
C
C
F S
D0
D1
F S
D2
D3
F S
A2
A3
I
0
10
20
30
40
50
D3
R R
A2
A3
R R R R R R R R R R R R R R R R R R R R R R R
D0
D1
R R
D2
D3
R R
A0
A1
R R R R R R R R R R R R R R R R R R R R R R R
D0
D1
R R
D2
UPLINK
F = FCCH
S = SCH
B = BCCH
C = CCCH
D = SDCCH
A = SACCH
I = Idle
=
=
=
=
=
=
Frequency Correction Channel
Synchronization Channel
Broadcast Control Channel
Common Control Channel (= PCH + RACH + AGCH)
Stand-alone Dedicated Control Channel
Slow Associated Control Channel
Rohde & Schwarz Trainingszentrum, V 2.3
PCH = Paging Channel
RACH = Random Access Channel
AGCH = Access Grant Channel
TDMA_system-e.doc
TDMA Methods, page 26
List of Abbreviations Used:
BSIC
Base station identification code
BTS
Base transceiver station
BxT
Bandwidth/bit duration product
CDMA
Code division multiple access
CIR
Channel impulse response
DCS1800
Digital Communication System 1800 (new: GSM1800)
FCB
Frequency correction burst
FDMA
Frequency division multiple access
FN
Frame number
GMSK
Gaussian minimum shift keying
GSM
Global system for mobile communications
GSM900
GSM at 900 Mhz
GSM1800
GSM at 1800 MHz
MS
Mobile station
NMT
Nordic mobile telephone
RX
Receiver
SB
Synchronization burst
S/N
Signal/noise ratio
TA
Timing advance
TACS
Total access communication system
TDMA
Time division multiple access
TSC
Training sequence code
TX
Transmitter
Rohde & Schwarz Trainingszentrum, V 2.3
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