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GBC_002_E0_0 GSM Air interfaceTechnology
Course Objectives:
Describe GSM Voice Service Process Procedure
State GSM key technology
Grasp physical and logical channel in air interface
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Contents
1 GSM Speech Processing ............................................................................................................................ 1
1.1 GSM Speech Processing ................................................................................................................... 1
1.2 Voice encoding .................................................................................................................................. 1
1.3 Channel Encoding............................................................................................................................. 2
1.4 Interlacing Technology...................................................................................................................... 3
1.5 Encryption/Decryption...................................................................................................................... 7
1.6 Modulation and Demodulation ......................................................................................................... 7
2 GSM Key Technologies.............................................................................................................................. 9
2.1 Diversity Reception........................................................................................................................... 9
2.2 Discontinuous Transmission ........................................................................................................... 10
2.3 Power Control ................................................................................................................................. 11
2.4 Timing Advance .............................................................................................................................. 14
2.5 Frequency Hopping......................................................................................................................... 15
3 Frame Structure and Radio Channels.................................................................................................... 19
3.1 Radio Frame Structure .................................................................................................................... 19
3.2 Physical Channel............................................................................................................................. 20
3.3 Logical Channels............................................................................................................................. 21
3.3.1 Common Channel................................................................................................................. 22
3.3.2 Dedicated Channel ............................................................................................................... 23
3.3.3 Channel Combination........................................................................................................... 23
3.4 Mapping between Logical and Physical Channels.......................................................................... 25
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1 GSM Speech Processing
1.1 GSM Speech Processing
In the GSM system, the MS processes voice signals on wireless interfaces as shown in
Fig 1.1-1.
A/D
D/A
Voice
codingChannel
coding
Interle
avingEncryption
Burst pulse
formingModulation
Voice
decoding
Channel
decodingDeinterleaving Decryption
Burst pulse
disassembleDemodulation
260bit/20ms 456bit/20ms 33.8kbit/s 270kbit/s
Fig 1.1-1 Voice Processing in the GSM System
The process of sending voice signals is as follows: for analog voice signals, first make
A/D conversion before doing voice coding to output 13Kbit/s digital voice signals. To
control errors in the process of transmission, channel coding and interlacing processing
shall be conducted on digital voice signals, which are then encrypted according to the
input/output bit stream of 1:1. These bits are grouped into 8 1/2 burst pulse sequences
(corresponding to voice signals/20ms segment) before they are transmitted at about
270Kbit/s in the appropriate timeslots.
The process of receiving voice signals is as follows: for the wireless signals sent by
BTS, first do demodulation before decomposing and decrypting burst pulses. After
every 8 1/2 burst pulse sequences are received, they are subjected to interlacing
processing and re-assembled into 456 bit information. After that, do channel decoding
and detect and correct the errors that occur in the middle of transmission before finally
conducting voice decoding of the bit stream generated by the decoder and converting it
analog voices.
1.2 Voice encoding
Given below is a brief introduction to the voice coding process of the GSM system
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using full-rate voice coding as an example.
Currently, what the GSM system uses is 13kb/s voice coding scheme, known asRPE-LTP (Rule Pulse Excitation-Long Term Prediction). The aim of this scheme is to
produce near-PSTN voice quality when no error occurs.
It first divides the voice into voice blocks by 20ms and samples it with 8kHz frequency
to get 160 sample values. Then each sample value is quantified to generate 16bit digital
voice signals. The 128Kbit/s data stream is obtained this way. As the rate is too high to
be transmitted on the wireless path, it needs to be compressed by a coder. If a full-rate
coder is used, each voice block will be compressed into 260bits to generate 13Kbit/s
source code rate in the end. The process of processing other signals such as channelcoding comes after that.
On the BTS side, BTS can recover 13Kbit/s source rate, but to generate 16Kbit/s rate so
that it can be transmitted on the Abis interface, it is necessary to add 3Kbit/s signaling so as
to control the operation of the remote TC. On the TC side, to accommodate 64Kbit/s
transmission rate of A interface, it is also necessary to conduct rate conversion between
13Kbit/s and 64Kbit/s.
1.3 Channel Encoding
Channel coding serves to improve transmission quality and overcome the negative
impact of interferences on signals.
Using specialized redundancy technology, channel coding inserts redundancy bits in
certain regularity at the transmitting end for coding while the receiving end in the
process of decoding detects error codes and corrects errors as many as possible using
these redundancy bits to recover the originally transmitted information.
The coding schemes as used in GSM are convolutional code and packet code which areused in a combinational way in actual applications.
Convolutional code: compiles k information bits into n bits. Both k and n are very
small so that they are suitable for transmission in a serial port manner. Besides they
also show very little delay. The coded n code elements are not only related to k
information code elements of this packet, but also to information code elements in the
preceding (N-1), where N is called constraint length. The convolutional code is
generally represented as (n, k, N). The error tolerance of the convolutional codes
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1 GSM Speech Processing
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increases as N increases while its error rate decreases as N increases. The convolutional
code is mainly designed for error correction. When the demodulator uses the maximum
likelihood estimation method, it can generate very effective error correction results.
Convolution code can be expressed as (n, k, N). The error-correction capability in
convolution encoding grows stronger with the rise of N, while the error probability
decreases exponentially as N rises. The convolution code is used to correct errors, and
it is effective when the decoder works in the maximum likelihood estimate mode.
Packet code: This is a kind of shortened loop code, which gets the redundancy bits by
increasing the exclusive-or algorithm of information bits and maps k input information
bits to no output binary code elements (n>k) through exclusive-or algorithm. The
packet code is designed mainly to detect and correct error codes in groups and it is
used in a mixed way with the convolutional code. The packet code is used for detecting
and correcting errors in groups. It is generally used along with the convolution code.
1.4 Interlacing Technology
The occurrence of burst error codes in wireless communication is usually caused by
fading that lasts a long time. It is not enough to detect and correct errors in the
above-mentioned channel coding scheme. To better address the issue of error codes, theinterlacing technology is introduced to the system. The interleaving technology is
adopted in channels to better solve the error problems.
Interlacing is in fact to send separately the original continuous bits of a message block
in certain regularity. In other words, the original continuous block in the middle of
transmission becomes discontinuous and creates a group of interlaced transmission
message blocks. At the receiving end, this kind of interlacing message blocks is
restored (de-interlaced) to original message blocks. To control the operations and
sessions, the TCAP are classified into two layers, CSL and TSL. The CSL is used tomanage the operations and the TSL is used to manage the transactions (sessions), as
shown in Fig 1.4-1.
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Packet
Interleaving
Packet after
interleaving
Error code
11 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
11 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4
11 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
Fig 1.4-1 Interleaving Technology
After the interlacing technology is applied, if a message is lost in the middle of
transmission, it is in fact part of each message block that is lost, but the whole part of it.
The missing messages can be recovered easily with the coding technology.
In the GSM, different coding and interleaving modes are used in different types of
channels. See Table 1.4-1 for details.
Table 1.4-1 Coding and Interweaving of Circuit Logical Channels
Code
Channel Type
Input
Rate
(Kbit/s)
Input Code
Block bits Check Bit Tail BitConvolutional
Code Rate
Output
Code
Block bits
Interleaving Depth
Ia 13 50Parity
check, 3
Ib 13 132
4 1/2TCH/F
S
II 13 78
456 On eight 1/2 bursts
Ia 5.6 22Parity
check, 3
Ib 5.6 73
6 1/3TCH/
HSII 5.6 17
228 On four 1/2 bursts
TCH/F9.6
TCH/H4.8
12
6240 4
1/2, one bit is
removed
every 15 bits.
456Combine on 22
unequal bursts
TCH/F4.8 6 120 32 1/3 456 Ditto
TCH/F2.4 3.6 72 4 1/6 456 On eight 1/2 bursts
TCH/H2.4 3.6 144 8 1/3 456Combine on 22
unequal bursts
SCH 25 Parity 4 1/2 78 Combine on one SB
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1 GSM Speech Processing
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Code
Channel Type
Input
Rate
(Kbit/s)
Input Code
Block bits Check Bit Tail BitConvolutional
Code Rate
Output
Code
Block bits
Interleaving Depth
check, 10 burst
RACH 8Parity
check, 64 1/2 36
Combine on one AB
burst
FACCH 184Packet
coding, 404 1/2 456 On eight 1/2 bursts
SACCH
BCCH
SDCCH
AGCH
PCH
184Packet
coding, 404 1/2 456 On four whole bursts
The voice input rate on TCH/FS is 13 Kbit/s, that is, each speech frame lasts 20 ms and
contains 260 bits. According to the interference of different bits on voice, the 260 bits
are divided into I category (182 bits in total) and II category (78 bits in total). The I
category is further divided into Ia and Ib. The Ia bits are very important bits. If any of
them is incorrect, the subscriber will hear a loud noise in 20 ms voice interval. There
are 50 Ia bits and 132 Ib bits. That is, the 260 bits in a speech frame (20 ms) is { d (0),
d (1),…, d (181), d (182), …, d (259)}. The part with a single line is I category, and
that with a double-line is II category. It is similar to the TCH/HS.
Table 1.4-1 gives the coding and interleaving adopted in different types of transmission.
The first column lists the channels and the related transmission mode. The Input Code
Block column gives the size of the data block (bits) before channel coding. The Output
Code Block column gives the size of the data block (bits) after channel coding. In Code,
the parameters are listed in the same sequence as the coding sequence. The tail bit is
"0". The decoding is in the reverse order.
Following is description of channel coding and interweaving, taking voicecommunication for example.
In the GSM, the voice input rate on TCH/FS is 13kb/s, that is, 260 bits are transmitted
every 20ms. The 260 bits are protected by means of segmented coding.
Among the 260 bits, 182 bits adopts 1/2 convolutional coding, and the remaining 78
bits are not protected. Among the 182 bits, 50 bits are performed with parity check and
then with 1/2 convolutional coding. Three information bits are added. Those 50 bits are
called Ia bits. The other 132 bits, called Ib bits, are performed with 1/2 convolutional
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coding directly.
Fig 1.4-2 shows the interleaving algorithm of voice signals on TCH/F. After channelcoding, 456 bits are carried in every 20ms. Those bits are divided into eight groups,
with the 57 bits in each group carried in different burst pulses (eight BPs in total). To
maximize irrelevancy between the bit sequences, the bits should be arranged as
described in Table 1.4-2.
0 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
. . . . . . . .
. . . . . . . .
. . . . . . . .
1 2 3 4 5 6 7 8
456bits
0 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
. . . . . . . .
. . . . . . . .
. . . . . . . .
456bits
0 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
. . . . . . . .
. . . . . . . .
. . . . . . . .
456bits
0 1 2 3 4 5 6 7
8 9 10 11 12 13 14 15
. . . . . . . .
. . . . . . . .
. . . . . . . .
456bits
57 1 57 1 57 1 57 1 57 1 57 1 57 1 57 1
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
A block
Odd bits
B block
Even bits116 bit 116 bit 116 bit
Fig 1.4-2 Interleaving of Cells
Table 1.4-2 Full-rate speech interleaving algorithm
No. Item Note
1 0, 8, …, 448 Even bits (B block) in BP (N)
2 1, 9, …, 449 Even bits (B block) of BP (N 1)
3 2, 10, …, 450 Even bits (B block) of BP (N 2)
4 3, 11, …, 451 Even bits (B block) of BP (N + 3)
5 4, 12, …, 52 Odd bits (A block) v BP (N 4)
6 5, 13, …, 453 Odd bits (A block) v BP (N 5)
7 6, 14, …, 454 Odd bits (A block) v BP (N 6)
8 7, 15, …, 455 Odd bits (A block) v BP (N + 7)
456 bits are divided into eight groups (rows). Each group has 57 bits (columns),
occupying Block A or Block B of BP (N) to BP (N+7). After interleaving, a BP carries
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1 GSM Speech Processing
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114 bits of information plus 2 bits of stolen frame (116 bits in total). The 114 bits
contain 57 bits (odd bits) of information block A and 57 bits (even bits) of information
block B. The remaining two bits indicate respectively whether the first half BP (odd bit)
and the last half BP (even bit) are subscriber data or fast channel associated signaling.
1.5 Encryption/Decryption
There are encryption measures available in the GSM system. They are applicable to
voice, data and signaling. They are independent of the data type and work for normal
bursts only. Encryption is accomplished by exclusive or operation of an encryption
sequence (computed by A5 encryption algorithm via key Kc and frame number) and114 information bits on a normal burst.
The original transmission data can be obtained by using the same sequence at the
receiving end to conduct exclusive-or operation with the encryption sequence.
1.6 Modulation and Demodulation
Modulation and demodulation are the last step in signal processing. Using GMSK
modulation mode at a rate of 270.833 k Baud, GSM usually conducts demodulation
with Viterbi algorithm (with a balanced demodulation method). Demodulation is the
reverse process of modulation.
GMSK is a special digital FM modulation mode. The modulation rate is 270.833
kilobauds. The Frequency Shift Keying (FSK) modulation with bit rate four times of
frequency offset is called MSK (Minimum Shift-frequency Keying). In GSM, the
Gaussian demodulation filter is used to further reduce the modulation spectrum. It can
cut the frequency conversion speed.
The GMSK can be expressed by a I/Q diagram. If there is no Gaussian filter, when aseries of constant 1s are sent, the MSK signal will be kept in the state that is higher
than the center frequency 67.708 kHz of the carrier. If the center frequency of the
carrier serves as the fixed phase reference, the signal 67.708 kHz will cause steady
increment of phase. The phase rotates 360° at 67,708 times per second. In a bit period
(1/270.833 kHz), the phase moves 1/4 a circle in the I/G diagram, that is, 90°. The data
1 can be looked as 90° plus the phase. Two 1s makes a phase increment by 180°, three
1s makes a increment by 270°, and so on. The data 0 indicates the same phase change
in the reverse direction.
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The actual phase track is strictly controlled. In the GSM, digital filter and 1/Q or digital
FM modulator are used to generate correct phase track accurately. The Root Mean
Square (RMS) between the actual track and the ideal track allowed by GSM
specifications cannot exceed 5°, and the peak deviation cannot exceed 20°.
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2 GSM Key Technologies
2.1 Diversity Reception
The diversity reception technology is usually introduced to the GSM system to receive
on several tributaries the signals with little relativity but carrying the same information
and then output the signals after they are combined. In this way, the impact of fading
on the stability of receiving signals can be played down.
There are ways of diversity as follows: space diversity, frequency diversity, time
diversity and polarization diversity.
1. Space Diversity
Two receiving antennas are set in the space to receive independently the same
signals before combining and outputting them. In this way, the degree of fading
can be dramatically lowered. This is the so-called space diversity. The space
diversity is based on the fact that the field strength varies randomly with the
space. The longer the distance, the more variant the multi-path transmission will
be, and the less relevant the receiving filed strength will be. The relevancy refersto the similarity between the signals. Therefore, the necessary distance must be
determined. According to the test and statistics, CCIR suggests the spacing
between two antennas should be larger than 0.6 wavelength, namely d>0.6λ, to
achieve a satisfactory diversity result and that it should be better to near the odd
number multiplication of λ /4. Even if the distance between antennas is shortened
to be λ /4, good diversity effect can be achieved.
2. Time Diversity
Time diversity is to send the same message with some delays or part of the
message in different time within the delay range tolerable by the system. In the
GSM system, time diversity is achieved by the interlacing technology. In the
GSM, interleaving technology is adopted to implement the time diversity.
3. Frequency Diversity
Frequency diversity means more than two frequencies send a signal concurrently.
The receiving end combines the signals of different frequencies and reduces or
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eliminates the impact of fading with different paths of the wireless carrier waves
of varied frequencies. The frequency diversity is effective and requires one set
of antenna only. Frequency diversity in GSM is implemented by frequency
hopping technology.
4. Polarization Diversity
Polarization diversity is to receive signals by making two pairs of receiving
antennas with polarization direction into some angles against each other, which
can generate a good diversity result. The two sets of polarized antennae in
polarity diversity can be integrated in one set of antenna. Thus, only one
receiving antenna and one transmitter antenna are required in a cell. If duplexeris adopted, only one transceiving antenna is required. It saves antennas greatly.
2.2 Discontinuous Transmission
There are two voice transmission modes. One is continuous voice coding (one speech
frame every 20ms) no matter whether the subscriber is talking or not. Another is
discontinuous transmission (DTX) with 13kb/s coding in voice activation period and
500b/s coding in non voice activation period. In addition, a comfort noise frame (20ms
per frame) is transmitted every 480ms, as shown in Fig 2.2-1.
There are two purposes of employing the DTX mode: one is to lower the general
interference level in the air; the second is to save the power of transmitters. However,
the DTX may slightly lower the transmission quality. Therefore, the DTX mode and
common mode are optional.
TRAU BTS
BTS MS
Comfort
noise frame
Speech frame
480 ms
Fig 2.2-1 Speech Frame Transmission in DTX Mode
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2.3 Power Control
Power control means to control the actual transmitting power (keep it as low as
possible) of MS or BS in radio propagation, so as to reduce the power consumption of
MS/BS and the interference of the entire GSM network. Needless to say, the
prerequisite of power control is to ensure the good communication quality of the
ongoing calls. The power control process is simply illustrated in Fig 2.3-1.
A B
Fig 2.3-1 Power Control
As shown in Fig. 1.5-16, the MS at point A is far from the BS antenna. Because the
propagation loss of electric wave in air is in direct proportion to n power of the
distance, the MS at A needs higher transmit power to ensure good communication
quality. Comparatively, point B is closer to the BS transmission antenna, hence smaller
transmission loss; therefore, to obtain similar communication quality, a mobile phone
at point B can use lower transmission power during communication. When a mobile
phone in communication is moving from point A towards point B, the power control
can reduce its transmitting power gradually. On the contrary, if it is moving from point
B towards point A, the power control can increase its transmitting power gradually.
The power control is classified as uplink power control and downlink power control,
they function separately. By uplink power control, it means to control the MS
transmitting power, while downlink power control means to control the BS transmitting
power. No matter uplink power control or downlink power control, the uplink or
downlink interference is suppressed as the transmit power is reduced. Meanwhile the
power consumption of the MS or base station is reduced. The most obvious benefits are
the average conversation quality of the whole GSM network is greatly increased, and
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the MS standby time is prolonged.
1. Power control process
The original information used for decision making during a power control
process is obtained from the measurement data of the MS and BS and
corresponding control decision can be made after processing and analyzing of
the original data. Similar to the handover control process, the whole power
control process is shown in Fig 2.3-2.
Measurement data saving
Average measurement data
processing
Power control decision
making
Power control command
sending
Measurement data correction
Fig 2.3-2 Power Control Process
1) Measurement data saving
The measurement data related to power control includes uplink signal level,
uplink signal quality, downlink signal level, and downlink signal quality.
2) Average measurement data processing
To reduce the influence of complex radio transmission on the measurement
values, the smooth processing of the measurement data usually adopts the
forward averaging method. That is, the average value of multiple measurement
values is used to make a power control decision. The parameter setting in
averaging calculation may vary with the types of the measurement data, i.e.,
quantity of the measurement data to be used may be different.
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3) Power control decision making
In the decision making of power control, there are three parameters: a threshold,an N value, and a P value. Among the latest N average values, if there are P
parameters exceed the threshold, the signal level is too high or the signal quality
is good; if there are P parameters are lower than the threshold, the signal level is
too low or the signal quality is poor.
According to the condition of the signal level or quality, the mobile phone or BS
can judge how to control the transmitting power, and the increase or decrease
amplitudes are determined by the pre-configured values.
4) Power control command sending
According to the power control decision, the corresponding control command is
sent to the BS, which will then execute the command or transfer it to MS.
5) Measurement data correction
After power control, the original measurement data and average values are
useless. If the useless information is still kept, it may cause incorrect power
control decision. Therefore, it is necessary to discard the outdated data or update
it for later use.
The fastest power control can be performed once every 480 ms, which is the
highest speed that the measurement data is reported. In other words, an entire
power control process is executed once in at least 480ms.
2. High-speed power control
The control extent of the power control process recommended by ETSI is fixed
as 2dB or 4dB normally. However, in most practical cases the fixed power
control extent is unable to achieve optimal effects, for a simple example:
When an MS initiates a call at a location very near to the BS antenna, its start
transmitting power is the max. transmitting power of the MS in the system
message broadcast in the cell BCCH (MS_TXPWR_MAX_CCH). It’s obvious
that at this time as the MS is quite close to the MS antenna, the power control
process is supposed to reduce its transmitting power as fast as possible. However,
it can hardly be achieved by the power control process recommended by the
ETSI specifications, because only 2dB or 4dB is decreased each time. In
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addition, there is an interval between every two power control processes
(because enough new measurement data need be collected). Therefore, it takes a
long time to reduce the transmit power of the MS to a proper value. It is the
same in the downlink direction. Obviously this is disadvantageous in terms of
reducing interference to the whole GSM network. To improve this, the power
control extent each time should be increased, which is the core idea of the
high-speed power control.
The high-speed power control can, according to the actual signal strength and
quality, work out the power control extent to be realized, without the limitation
of the fixed extent, thus solving the power control problem without much effort
when the MS makes the initial access. Of course its functions are not limited to
this situation. It can work in many cases e.g. fast moving mobile phones,
sudden interference or obstacles. Whenever large extent power control is
required, the high-speed power control process is the ideal solution.
2.4 Timing Advance
In the GSM, because TDMA is adopted in the air interface, the MS must employ the
TSs allocated to it only, and remain inactive in other time. Otherwise, it may affect theMSs using other TSs on the same carrier.
In the GSM, the MS requires three intervals between timeslots when receiving or
transmitting signals. See Fig 2.4-1.
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0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1Offset
Downlink:
Uplink:
Sent by the BTS Sent by the MS
TDMA frame number
TDMA frame number
Fig 2.4-1 Uplink and Downlink Offset of TCH
Suppose an MS occupies TS2 and moves away from the base station, the messages sent
from the base station will be delayed further and further in reaching the MS.
Meanwhile, the response returned by the MS will also be delayed further and further in
reaching the base station. If nothing is done to solve the problem, the message sent bythe MS from TS2 will eventually overlap with another calling message received by the
base station in TS3. Therefore, it is important to monitor the time when a call reaches
the base station. As the distance between the MS and the base station changes, the
system issues instructions to the MS, notifying it of the time advanced. This process is
the adjustment of timing advance.
After a specific connection is established, the BTS measures the time offset between
the pulse TSs and the received MS TSs. Based on the value measured, the BTS
calculates the timing advance and notifies the MS of it through the SACCH at a certainfrequency.
2.5 Frequency Hopping
In the digital mobile communication system, to enhance the anti-jamming capability of
the system, the spread spectrum technology is usually introduced. There are two modes:
direct spread mode and frequency hopping mode, which is used by the GSM system.
There are two reasons for why frequency hopping is used. First, based on the principle
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of frequency diversity, this technique is used to counteract Raileigh fading. Rayleigh
fading refers to the short-term change in amplitude that mobile radio transmission
suffers inevitably in case of any obstacle. Different frequencies will suffer different
degrees of fading, which becomes more independent with the increase in frequency
difference. By means of FH, the BPs will not be damaged by Rayleigh fading in the
same way. Second, it is used on the basis of anti-jamming feature. In the area where
traffic is heavy, the cellular system is liable to be restricted by the interference from
frequency reuse, and the C/I may change a lot during the call. C depends on the
position of the MS relative to the BTS. I depends on whether the frequency is used in
the adjacent cell. FH enables it to scatter interference among many calls that may
interfere with the cell instead of one call.
FH refers to hopping of the carrier frequency within a wide frequency band at a certain
sequence. The control and information data are modulated into base band signals,
which are then sent into the carrier for modulation. Afterwards, the carrier frequency
changes under the control of pseudo-random codes, the sequence of which is the FH
sequence. Finally, the signals are sent via the RF filter to the antenna for transmission.
The receiver determines the receiving frequency according to FH synchronization
signals and FH sequence, receives corresponding signals after FH for demodulation.
The basic structure of FH is illustrated in Fig 2.5-1.
Synchronization
circuit
Frequency
modulationsequence
generator
Variablefrequency
synthesizer
Message
modulation
Up
converter
Send
Message
demodulationDown converter
Receive
Fig 2.5-1 Basic Structure of FH
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2 GSM Key Technologies
17
Features of frequency hopping technology: The frequency hopping technology can be
employed to increase the working band of the system so as to enhance the
anti-jamming and anti-jamming capability of the communication system. Frequency
hopping can help improve and protect the pulse of the effective information part from
the impact of Rayleigh fading in the communication environment. After frequency
hopping is done, the original data are recovered by means of channel decoding. The
times of frequency hopping are increased to boost frequency hopping gains so as to
enhance the anti-jamming and anti-fading capability of the system.
The frequency hopping technology is actually to avoid external interferences so that
they cannot follow the changes of frequencies, thus avoiding or markedly lowering
same-channel interference and frequency selective fading. The reason to increase the
number of hoppings is that the gain of frequency hopping system is equal to the ratio of
frequency hopping system bandwidth to N minimum frequency hopping intervals.
Usually, the FH number should be greater than three. If frequency diversity is also
available for the FH system, and a message is transmitted by several groups of
frequency hopping simultaneously and then judged by the law of large numbers, more
subscribers can use services at the same time with least mutual interference.
The frequency hopping comprises baseband hopping and RF hopping.
The baseband hopping enables the transmit and receive frequencies of each
carrier unit to remain unchanged. At different frame number (FN) moment, the
frame unit sends data to different carrier units.
RF FH is to control the frequency synthesizer of each transceiver, making it hop
according to different schemes in different timeslots.
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3 Frame Structure and Radio Channels
GSM air interface uses TDMA based frame structure. Communication services are
obtained by transmission of information using logical channels on physical channels.
Mapping between the logical channel and physical channel is the process that arranges
the information to be sent to the suitable TDMA frames and timeslots.
3.1 Radio Frame Structure
Five levels of GSM radio frame structure are timeslot, TDMA frame, multiframe,
superframe and hyperframe.
Timeslot is the basic unit of a physical channel.
TDMA frame consists of eight timeslots. It is a basic unit occupying carrier
bandwidth. Each carrier has eight timeslots.
There are two types of multiframes:
One type of multiframe consists of 26 TDMA frames. This type of multiframe is
used in TCH, SACCH, and FACCH.
The other type of multiframe consists of 51 TDMA frames. This type of
multiframe is used in BCCH, CCCH, and SDCCH.
The superframe is a consecutive 51 x 26 TDMA frame. It consists of 51
26-multiframes or 26 51-multiframes.
The hyperframe consists of 2,048 superframes.
Fig 3.1-1 shows GSM frame structure.
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0
TDMA frame
00 01
1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
02
0 1 2 3 4 5 6 7
0 1 2 3 4 22232425
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1 2 3 4 4748 4950
0 1 2 3 47 48 49 50
0 1 2524
2042 2043204420452046 20476543210
1 26-multiframe = 26 TDMA frames (120 ms) 1 51-multiframe = 51 TDMA frames (3036/13 ms)
1 superframe = 1326 TDMA frames (6.12s)= 51 26-multiframe or 26 51-multiframes
1 hyperframe = 2048 superframes = 2715648 TDMA frames
Fig 3.1-1 GSM Frame Structure
3.2 Physical Channel
GSM adopts mixed technology of Frequency Division Multiple Access (FDMA) and
Time Division Multiple Access (TDMA). GSM features high frequency utilization.
FDMA - enables 124 carrier frequencies (carriers for short) to be assigned to the
uplink (from the MS to the BTS) 890 MHz – 915 MHz or downlink (from the BTS to
the MS) 935 MHz – 960 MHz in GSM900 band. Interval between carriers is 200 kHz.
Carriers in the uplink and downlink are in pairs called duplex communication mode.
Interval between duplex receiving and transmitting carrier pair is 45 MHz.
TDMA - enables each carrier of GSM900 band to be divided into eight time segments.
Each time segment is called a timeslot. See Fig 3.2-1.
This type of timeslot is called a channel or a physical channel. Eight consecutive
timeslots on a carrier constitute a TDMA frame, that is, a carrier of GSM provideseight physical channels.
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3 Frame Structure and Radio Channels
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16/25 ms
200 kHz
Timeslot
Time
Frequency
Fig 3.2-1 Time-Frequency Structure of Physical Channel
Eight timeslots in TDMA frame are called physical channels.
3.3 Logical Channels
Each physical channel is time multiplexed with different logical channels. Logical
channels carry various signaling or traffic information based on user and network
requirements. To provide signaling traffic control, logical channels map on physical
channels.
Logical channels are classified into Common Channel and Dedicated Channel.
EnhancedFull-ratechannel
LogicalChannels
Fast AssociatedControl Channel
(FACCH)
Slow AssociatedControl Channel
(SACCH)
Stand-aloneDedicated ControlChannel (SDCCH)
FrequencyCorrection
Channel (FCCH)
Common ControlChannel (CCCH)
DedicatedChannel
BroadcastChannel (BCH)
DedicatedControl Channel
(DCCH)
Traffic Channel(TCH)
Broadcast ControlChannel (BCCH)
Paging Channel(PCH)
Random AccessChannel (RACH)
Access GrantChannel (AGCH)
CommonChannel
Half-ratechannel(TCH/H)
Full-rateChannel(TCH/F)
Synchroniza-tion Channel
(SCH)
Fig 3.3-1 GSM Logical Channels
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3.3.1 Common Channel
Common Channel is classified in two main types:
Broadcast Channel (BCH): BCH transmits broadcast messages from base station to
MS. Broadcast Channel is unidirectional channel from base station to MS. It is of three
types:
Frequency Correction Channel (FCCH): It carries the information used to correct
the MS frequency. MS receives frequency correction information through FCCH and
corrects its time base frequency.
Synchronization Channel (SCH): It carries frame synchronization (TDMA frame
number) information and Base Station Identity Code (BSIC) to MS.
Broadcast Control Channel (BCCH): It broadcasts general information of BTS. For
example, broadcasts the local cell and neighboring cell information, and
synchronization (time and frequency) information. MS listens to BCCH periodically
to obtain the information transmitted on it, such as the Local Area Identity, List of
Neighboring Cell, frequency table used in local cell, cell identity, power control
indication, intermittent transmission permission, access control, and CBCH description.
BCCH carrier is transmitted by base station at a fixed power, and its signal strength is
measured by all MSs.
Common Control Channel (CCCH): CCCH is point-to-multipoint bi-directional
channel. It carries signals required to set up a connection between base station and MS.
It is of three types:
Paging Channel (PCH): It broadcasts paging messages from base station to MS. It is a
downlink channel.
Random Access Channel (RACH): MS sends information to base station through this
channel when accessing the network at random. The information sent includes response
to the paging message of base station and access of mobile-originated call. MS also
applies for a Stand-alone Dedicated Control Channel (SDCCH) from base station
through this channel. RACH is an uplink channel.
Access Grant Channel: The base station sends the assigned SDCCH to the MS that
accesses the network successfully through this channel. The AGCH is a downlink
channel.
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3.3.2 Dedicated Channel Dedicated channel is a traffic channel which carries voice and data. Some types of
dedicated channel are used for the control purpose.
Dedicated Channel is classified in two main types:
Dedicated Control Channel (DCCH): DCCH is a point-to-point bi-directional
channel between base station and MS. It is of three types:
Stand-alone Dedicated Control Channel (SDCCH): It carries signaling and
channel information between base station and MS, such as the authentication
and registration signaling messages. During the establishment of a call, SDCCH
supports bi-directional data transmission and short messages transfer.
Slow Associated Control Channel (SACCH): Through this channel, base
station sends power control message and frame adjustment message to MS, and
receives signal strength report and link quality report from MS.
Fast Associated Control Channel (FACCH): It carries inter-cell handover
signaling messages between base station and MS.
Traffic Channel (TCH): TCH carries voice and data. According to switching mode,
TCH can be divided into circuit-switched channel and data-switched channel.
According to transmission rate, TCH can be divided into full-rate channels and
half-rate channels.
Rate of the GSM full-rate channel is 13 kbps, and that of the GSM half-rate channel is
6.5 kbps. In addition, the enhanced full-rate channel has same rate as the full-rate
channel, which is 13 kbps. However, it has better compressed coding scheme than
full-rate channel. That is why enhanced full-rate channel provides better voice quality.
3.3.3 Channel Combination
In actual application, different types of logical channels are mapped on the same
physical channel. This is called channel combination.
Following are nine GSM channel combinations:
Full-rate traffic channel (TCHFull): TCH/F + FACCH/F + SACCH/TF
Half-rate traffic channel (TCHHalf): TCH/H (0, 1) + FACCH/H(0, 1) +
SACCH/TH (0, 1)
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Half-rate1 traffic channel (TCHHalf2): TCH/H (0, 0) + FACCH/H (0, 1)
+SACCH/TH (0, 1) + TCH/H (1, 1)
SDCCH: SDCCH/8 (0,…, 7) + SACCH/C8 (0,…, 7)
Main broadcast control channel (MainBCCH): FCCH + SCH + BCCH + CCCH
Combined broadcast control channel (BCCHCombined): FCCH + SCH +
BCCH + CCCH + SDCCH/4 (0,…,3) + SACCH/C4 (0,…, 3)
Broadcast channel (BCH): FCCH + SCH + BCCH
Cell broadcast channel (BCCHwithCBCH): FCCH + SCH + BCCH + CCCH +
SDCCH/4 (0,…, 3) + SACCH/C4 (0,…, 3) + CBCH
Slow dedicated control channel (SDCCHwithCBCH): SDCCH + SACCH +
CBCH
Among the above channel combinations, CCCH = PCH + RACH + AGCH. As a
downlink channel, only CBCH carries cell broadcast information and shares the
physical channel with SDCCH.
Each cell broadcasts FCCH and SCH. The basic combination in the downlink direction
includes FCCH, SCH, BCCH and CCCH (PCH + AGCH). It is allocated to TN0 of
BCCH carrier configured for a cell, as shown in Fig 3.3-2.
SF B C
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
51 frames
SF C C SF C C SF C C I
R R R R R R R R R R
D0 D1 D2D3
D4 D5 D6 D7 A0 A1 A2 A3
SF C C
R R R R R R R R R R
III
D0 D1 D2D3
D4 D5 D6 D7 A4 A5 A6 A7 III
A1 A2 A3 III
A5 A6 A7 III
D0 D1 D2 D3D4
D5 D6 D7 A0
D0 D1 D2 D3 D4 D5 D6 D7 A4
SF B C SF C C SFD0
D1
SFD2
D3
ISFA0
A1
SF B C SF C C SFD0
D1
SFD2
D3
ISFA2
A3
D3
D3
R R
R R
A2 A3
A0 A1
D2D2
SF
SF
D0
D1
D0
D1
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
F: FCCH S: SCH
B: BCCH C: CCCH (CCCH=PCH+AGCH+RACH)
R: RACH D: SDCCH
A: SACCH/C I: Idle
BCCH+CCCH
Downlink
BCCH+CCCHUplink
8 SDCCH/8
Downlink
8 SDCCH/8Uplink
BCCH+CCCH+4SDCCH/4
Downlink
BCCH+CCCH
+4SDCCH/4Uplink
(a) FCCH+SCH+BCCH+CCCH
(b) SDCCH/8(0,...,7)+SACCH/C8(0,...,7)
(c) FCCH+SCH+CCCH+SDCCH/4(0,...,3)+SACCH/C4(0,...,3)
Fig 3.3-2 Frame Channel Structure
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For half-rate voice channel combination, each timeslot has two half-rate sub-channels
and corresponding SACCH, with 26 TDMA frames as a multiframe.
Fig 3.3-3 shows the frame structure.
H
0
H
0
S
1
S
0
H
1
H
0
H
0
H
0
H
0
H
0
H
0
H
0
H
0
H
0
H
0
H
1
H
1
H
1
H
1
H
1
H
1
H
1
H
1
H
1
H
1
H
1
26 frames
Fig 3.3-3 Half-Rate Voice Channel Frame Structure
3.4 Mapping between Logical and Physical ChannelsLogical channels in GSM are much more than the eight physical channels that a GSM
carrier can provide. If each logical channel is configured with a physical channel, the
eight physical channels provided by a carrier are not enough.
In such case, extra carriers must be added. However, the communication in this way is
not highly effective. The way to solve this problem is to multiplex the CCCH, that is,
multiplex the CCCH on one or two physical channels.
Mapping between physical channels and logical channels in GSM is as follows:
Base station has N carriers, and each carrier has eight timeslots. Define the carriers as
f 0, f 1, f 2 … Downlink starts from timeslot 0 (TS0) of f 0. TS0 is used to map with
control channel only. f 0 is also called broadcast control channel (BCCH).
Fig 3.4-1 shows BCCH and CCCH on TS0 multiplexing.
012 7012 701
FS B C FS C C FS C C FS C C FS C C I
TDMAframe
BCCH+CCCH
Downlink
F (FCCH): MS synchronizes its frequency through it.S (SYCH): MS reads TDMA frame number and Base Station Identity Code (BSIC)through it.B (BCCH): MS reads the general inforamtion of the cell through it.I (IDLE): Idle frame, containing no information. It serves as the end flag of themulti-frame.
Fig 3.4-1 Multiplexing of BCCH and CCCH on TSO
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BCCH and CCCH occupy total 51 TS0s. Although only the TS0 of each frame is
occupied, the total length is 51 TDMA frames in terms of time. Each time when an idle
frame appears, the multiframe ends. After that, a new multiframe starts from F and S.
Repeat like this, and TDMA multiframe is constructed.
When there is no paging or call connected, the base station always transmits on f 0. This
enables MS to detect the signal strength of the base station to determine the cell to be
used.
For the uplink, the TS0 on f 0 does not include the above channels. It is used for the MS
access only, that is, it is used as the RACH.
Fig 3.4-2 shows the TS0 of 51 consecutive TDMA frames.
012 7012 701
RR
TDMAframe
RACH
UplinkRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR
Fig 3.4-2 Multiplexing of RACH on TSO
BCCH, FCCH, SCH, PCH, AGCH, and RACH are all mapped on TS0. RACH is
mapped on uplink, and the rest are mapped on downlink.
TS1 on downlink f 0 is used to map DCCH to physical channel.
Fig 3.4-3 shows the mapping relationship.
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012 7012 701
D0 I
TDMAframe
SDCCH+SA CCCH
Downlink
D1 D2 D3 D4 D5 D6 D7 A0 A1 A2 A3 I I
D0 ID1 D2 D3 D4 D5 D6 D7 A4 A5 A6 A7 I I
Fig 3.4-3 Multiplexing of SDCCH and SACCH on TS1 (Downlink)
Since the bit rate in call setup and registration is quite low, eight dedicated control
channels can be placed on one timeslot to improve the multiplexing ratio of the
timeslot.
SDCCH and SACCH have 102 timeslots in total, that is, 102 time division
multiplexing (TDM) frames.
DX (D0, D1 …) of SDCCH is used in the early time when a call is set up. When the
MS transfers to the TCH, and the subscriber starts the conversation or the release is
triggered after registration, the DX is used by other MSs.
AX (A0, A1 …) of the SACCH transfers unimportant control information, such as
radio measurement data, that is TS0 of 51 consecutive TDMA frames.
TS1 on the uplink f 0 has the same structure with the TS1 on the downlink f 0. They have
an offset in time, which means bi-directional connection can be performed at the same
time for an MS.
Fig 3.4-4 shows the multiplexing of the SDCCH and SACCH on TS1 of the uplink f 0.
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012 7012 701
A5
TDMAframe
SDCCH+ SACCCH
Uplink
A6 A7 D0 D1 D2 D3 D4 D5 D6 D7
A1 A2 A3 D0 D1 D2 D3 D4 D5 D6 D7
I I I
I I I
A0
A4
DX: same as uplink AX: Same as downlink
Fig 3.4-4 Multiplexing of SDCCH and SACCH on TS1 (Uplink)
Uplink and downlink TS0 and TS1 on f 0 are used by the logical control channel, while
other six physical channels (TS2 to TS7) are used by TCH.
Fig 3.4-5 shows the mapping from TCH to physical channel.
0 1 2 7 0 1 2 7 0 1
T T
TDMAframe
T CH
DownlinkT T T T T T T T T T AT T T T T T T T T TT T I
2
Note: There are 26 t imeslots in total.The seque ncestarts from the begining after the idle timeslot.
T=TCH A=SACCH I=Idle
Fig 3.4-5 TCH Multiplexing
Fig 3.4-5 shows TS2 time division multiplexing.
TCH carries voice or data. SACCH carries control commands such as the command to
change the output power.
Idle I does not contain any information but is used in measurement.
TDM is implemented on TS2 with 26 timeslots as a cycle.
The idle timeslot I serves as the beginning or end of the repeated sequence.
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Uplink TCH is of the same structure with the downlink TCH. They only have a time
offset, which is three timeslots. That is, the TS2 of the uplink and that of the downlink
do not appear simultaneously, which means that the MS does not send or receive data
at the same time.
Fig 3.4-6 shows the offset between the uplink and downlink of the TCH.
0
TDMA frame number
Uplink C0
00 01
From BTS to MS
From MS to BTS
Downlink C0
45MHz (GSM900)
95MHz (DCS1800)
Offset
1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0
TDMA frame number
00 011 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Fig 3.4-6 Offset between Uplink and Downlink of the TCH
The conclusion is that on carrier f 0:
TS0: a logical control channel, with repeat cycle of 51 timeslots.
TS1: a logical control channel, with repeat cycle of 102 timeslots.
TS2: a logical traffic channel, with repeat cycle of 26 timeslots.
TS3 to TS7: logical traffic channels, with repeat cycle of 26 timeslots.
The TS0 to TS7 of other f 0 – f N are all traffic channels.