PART 4 GSM – Radio Interface
Lecture 4.1 Physical channels
Giuseppe Giuseppe Bianchi Bianchi
GSM Radio Spectrum Frequency [MHz] 2 x 25 Mhz Mhz band band Ö Duplex spacing: 45 MHz Î 124 carriers x band Ö 200 KHz channels Ö Suggested use: only 122 Î
960 DOWNLINK BS Æ MS
tional guard ÆUse top & bottom as additional
935
Î
8 TDMA slots x carrier Kbps Ö full rate calls – 13 Kbps Ö If half-rate used, 16 calls at 6.5 kbps
915
1 2 3 4 5 6 7 8
890.4 UPLINK MS Æ BS
890.2
Fuplink (n) = [890.2+ 0.2(n − 1)] MHz “guard “guard band”
890
Fdwlink (n) = [935.2+ 0.2(n− 1)] MHz
Giuseppe Giuseppe Bianchi Bianchi
1
Adjacent channels (due to GMSK)
60dB
35dB
Specif Specifiication: cation: 9dB I n practice, due to power control and and shadowing, dowing, adjacent channels channels Cannot annot be use used d withi within n the sam same cell… Giuseppe Giuseppe Bianchi Bianchi
Physical channel Î Î
200 KHz bandwidth + GMSK modulation Ö 1625/6 kbps gross channel rate (270.8333 kbps) 1 time slot = 625/4 bits Ö 156.25 bits Ö 15/26 ms = 576.9 µs
time slot 0
time slot 7
time
577 µs 1 frame frame=60/13 ms =4.615 =4.615 ms 26 frame frames =120 =120 ms (this this is is the key number) Giuseppe Giuseppe Bianchi Bianchi
2
Hybrid FDMA-TDMA physical channel = (time slot, frequency)
frequency
Tota Total n. n. of channels: ls: 992
200 200 KHz KH z 200 200 KHz KH z 200 200 KHz KH z 200 200 KHz KH z 200 200 KHz KH z 200 200 KHz KH z
slot
200 200 KHz KH z 200 200 KHz KH z 200 200 KHz KH z 577us 577us 577us 577us 577us 577us
577us 577us
time Giuseppe Giuseppe Bianchi Bianchi
DCS 1800 radio spectrum Î
Greater bandwidth available Ö EUROPE: 75 MHz band Æ1710-1785 MHz uplink; 1805-1880 MHz downlink
Ö
ITALY: 45 MHz band from 2005 Æ1740-1785 MHz uplink; 1835-1880 MHz downlink
Î
Same GSM specification Ö 200 KHz carriers ÆA total of
Î
374 carri carriers ers (versus124 (versus124 in GSM )
DCS 1800 operators Ö Common rule in most of the countries: and seco second nd opera operators @ 900 MHz; Hz; Thir T hird d etc @1800 @1800 MHz MHz ÆFirst and DCS 1800 deployme deployment (1996+): ÆDCS » 15 MHz (=75 carriers) carriers) to Wind Wind;; 7.5 (=37 carriers) carriers) to first fi rst and second econd operator (plus existing 27 GSM 900 carriers)
Giuseppe Giuseppe Bianchi Bianchi
3
Other GSM bands Î Extended
GSM (E-GSM) band Ö Uplink: 880-915 MHz Ö Downlink: 925-960 MHz Î Other bands: Ö 450 MHz Æ (450.4-457.6 up; 460.4-467.6 down) Ö 480 MHz Æ (478.8-486 up; 488.8-496 down) Ö 1900 MHz Æ (1850-1910 up; 1930-1990 MHz)
Giuseppe Bianchi
Duplexing - MS uses SAME slot number on uplink and downlink - Uplink and downlink carriers always have a 45 MHz separation -I.e. if uplink carrier is 894.2 Æ downlink is 919.2 -3 slot delay shift!! 0
1
2
3
UPLINK
0
4
1
5
6
2
3
7
4
DOWNLINK
5
6
7
MS: no need to transmit and receive in the same time on two different frequencies! Giuseppe Bianchi
4
Slow Frequency hopping (optional procedure within individual cell) f7 f6 f5 f4 f3 f2 f1
Hopping sequence(example): … Æ f1 Æ f2 Æ f5 Æ f6 Æ f3 Æ f7 Æ f4 Æ f1… Slow =on a per-frame basis - 1 hop per frame (4.615 ms) =217 hops/second Physical motivation: - combat frequency-selective fading - combat Co-Channel Interference next slot may not interferere with adjacent cell slot (different hopping sequence)
- improvements: acceptable quality with 9 dB SNR versus 11 dB Giuseppe Bianchi
Structure of a TDMA slot Normal burst TB 3
DATA 57
S 1
Training sequence 26
S 1
Data 57
TB GP 3 8.25
148 bit burst 156.25 bit (15/26 ms = 0.577 ms)
Symmetric structure Î DATA: 2 x 57 data bits Ö 114 data bits per burst Ö “gross” bits (error-protected; channel coded) Ö “gross” rate: 24 traffic burst every 26 frames (120 ms) Î
Æ22.8 kbps gross rate Æ13 kbps net rate!
Î
S: 2 x 1 stealing bit Ö Also called stealing flags, toggle bits Ö Needed to grab slot for FACCH (other signalling possible) Giuseppe Bianchi
5
Tail & training bits Î
2 x TB = 3 tail bits set to 000 Ö At start and end of frame Ö Leave time available for transmission power ramp-up/down Ö Assures that Viterbi decoding starts and ends at known state
Î
26 bit training sequence Ö Known bit pattern (8 Training Sequence Code available) Ö for channel estimation and synchronization Ö Why in the middle? ÆBecause channel estimate reliable ONLY
when the radio channel “sounding” is taken! ÆMultipath fading rapidly changes the channel impulse response…
Giuseppe Bianchi
Training sequences
Different codes used in adjacent cells! Avoids training sequence Disruption because of co-channel interference. Giuseppe Bianchi
6
Power mask for Normal Burst
7.6 bits
4.9 bits 156.75 bits; 162.2 bits
Giuseppe Bianchi
Guard Period rationale BTS
d
Î
Î
Assume the following synchro mechanism: Ö BTS transmits at time 0 Ö MS receives at time d/c Ö MS transmits at time 3+d/c Ö BTS receives at time 3+ 2d/c Offset depending on d! Giuseppe Bianchi
BTS downlink tx MS downlink rx
1
2 1
3 2
4 3
4
MS uplink rx
1
BTS uplink rx
1
1
Expected RX time!
7
Guard period sizing …
dwlink slot 1 dwlink slot 1 MS time
dwlink slot 4
…
dwlink slot 4
…
uplink slot 1
…
BTS time
uplink slot 1
Maximum cell radius:
GTbits 2d = Crate c
→ d=
c⋅ GTbits 300000⋅ 8.25 ≈ ≈ 4.5Km 2Crate 2⋅ 270833
I s there something wrong? (GSM says that cells go up to 35 km) Giuseppe Bianchi
Frame synchronization TA (transmitted in the SACCH) BTS
Î
Timing Advance (TA) Ö Parameter periodically transmitted by BTS during MS activity Ö 6 bits = 0-63 Ö Meaning: anticipate transmission of TA bits BTS time Ö TA=0: no advance
dwlink slot 4 dwlink slot 4 TA
MS time
uplink slot 1
Æ I.e. transmit after 468.75 bits
after downlink slot Ö
TA=63: Æ Transmit after 405.75 bits time
uplink slot 1 uplink slot 1
TA avoids collision! Giuseppe Bianchi
8
Timing Advance analysis Î
Downlink propagation delay: Ö d/c
Î
Uplink propagation delay: Ö d/c
Uplink delay with TA: Ö d/c-TA Î Perfect resynchronization occurs when Ö TA = 2d/c Î
Î
Maximum cell size for perfect resync:
TA 31.5[bits] ⋅c= ⋅ 300000[km/ s] = 34.89 [km] d= 2 270833[bits / s] 8.25 bits Guard time additionally available for imperfect sync (+/- error) Giuseppe Bianchi
And when the user is not connected? But wants to connect… Solution: USE A DIFFERENT BURST FORMAT Access Burst: much longer Guard Period available drawback: much less space for useful information Ac ces s b ur st TB 8
Training sequence 41
Data 36
TB 3
GP 68.25
88 bit burst 156.25 bit (0.577 ms)
No collision with subsequent slot for distances up to 37.8 km Giuseppe Bianchi
9
Other burst formats in GSM Î5
different bursts available
Ö Normal Burst Ö Access Burst
Ö Frequency Correction Burst Ö Synchronization Burst Ö Dummy Burst
Giuseppe Bianchi
Frequency Correction Burst Frequency Correction Burs t TB 3
Fixed bit pattern (all 0s) 142
TB GP 3 8.25
Î All
0s burst (TB=0, too) Î After GMSK modulation: Ö Sine wave at f reference+1625/24 kHz (67.7083 kHz) Î Acts as a “beacon” Ö When an MS is searching to detect the presence of a carrier Î Allows an MS to keep in sync with reference frequency
Giuseppe Bianchi
10
Dummy Burst Dummy Burst TB 3
Fixed bit pattern 58
Training sequence 26
Fixed bit pattern 58
TB GP 3 8.25
Î Used
to fill inactive bursts on the BCCH Î Guarantees more power on BCCH than that on other channels Ö Important, when MS needs to find BCCH
Giuseppe Bianchi
Synchronization Burst Synchronization Burs t TB 3
Sync data 39
Training sequence 64
Sync data 39
TB GP 3 8.25
Î Longer
training sequence Ö It is the first burst an MS needs to demodulate! Ö 1 single training sequence Î Data field: Ö Contains all the information to synchronize the frame Æi.e. synchronize frame counter
Ö Contains the BSIC (Base Station Identity Code, 6 bits) Æ3 bits network code (operator)
» Important at international boundary, where same frequencies can beshared by different operators Æ3 bits color code Æ To avoid listening a signal
comes from the actual one!
from another cell, thinking it
Giuseppe Bianchi
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PART 4 GSM – Radio Interface
Lecture 4.2 logical channels
Giuseppe Bianchi
Logical vs Physical channels Logical channels (traffic channels, signalling (=control) channels) Physical channels (FDMA/TDMA) Î
Physical channels Ö Time slots @ given frequencies Ö Issues: modulation, slot synchronization, multiple access techniques, duplexing, frequency hopping, etc
Î
Logical channels Ö Built on top of phy channels Ö Issue: which information is exchanged between MS and BSS Giuseppe Bianchi
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Logical – physical mapping Physical Channel: data rate r, time slot i frequency
Logical Channel Mapping: Different channels may share a same physical channel frequency Frame 8
Frame 9
Frame 10
Frame 11
Frame 12
Logical channel A: data rate r/3, time slot i, frame 3k Logical channel B: data rate 2r/3, time slot i, frame 3k+1, 3k+2 Giuseppe Bianchi
GSM logical channels Traffic channel (TCH )
TCH /F
TCH full rate
MSÅÆBSS
TCH /H
TCH half Rate
MSÅÆBSS
Br oadcast channel
BCCH
Broadcast control
BSSÆMS
(same information to all MS in a cell)
FCCH
Frequency Correction
BSSÆMS
SCH
Synchronization
BSSÆMS
Common Control channel (CCCH)
RACH
Random Access
MSÆBSS
(point to multipoint channels)
AGCH
Access Grant
BSSÆMS
(used for access management)
PCH
Paging
BSSÆMS
Dedicated Control channel (DCCH)
SDCCH
Stand-alone Dedicated control MSÅÆBSS
(point-to-point signalling channels)
SACCH
Slow associated control
MSÅÆBSS
(dedicated to a specific MS)
FACCH Fast associated control
MSÅÆBSS
Additional logical channels available for special purposes (SMS, group calls, …) Giuseppe Bianchi
13
An example procedure involving signalling
Setup for an incoming call (call arriving from fixed network part MS responds to a call) Steps: - paging for MS - MS responds on RACH - MS granted an SDCCH - authentication & ciphering on SDCCH - MS granted a TS (TCH/FA CCH) - connection completed on FACCH - Data transmitted on TCH
Giuseppe Bianchi
PART 4 GSM – Radio Interface
Lecture 4.3 Traffic channels and associated signalling channels
Giuseppe Bianchi
14
Traffic channels (TCH/F) Periodic pattern of 26 frames (120 ms =15/26 ms/TS * 8 TS/frame* 26 frame)
1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23 24
24 TCH frames over 26
Same TS in every frame 0 1 2 345 67 01 2 3 45 67 01 2 3 45 67 01 2 34 56 7 Theoretical rate: 1/8 channel rate: r=33.85 kbps 2 signalling frames: r Æ 31.25 kbps Burst overhead(114 bitsover 156.25): rÆ 22.8 kbps
TB 3
DATA 57
S Training S 1 Seq. (26) 1
Data 57
TB GP 3 8.25
148 bit burst 156.25 bit (0.577 ms)
Giuseppe Bianchi
Speech coding
Analog voice
A/D conversion 8000 samples/s 13 bit/sample
160 voice samples (20 ms) (2080 bits)
Speech CODER (8:1 compression)
Digital voice 104 kbps
260 bits block 13 kbps
Giuseppe Bianchi
15
Discontinuous transmission Î
Speech coder implements Voice Activity Detection (VAD) Ö Voice activity: idle for about 40% of the time Ö To avoid clipping: hangover period (80ms)
talking
listening
talking time
Î
Î
When IDLE, do not transmit Ö Save battery consumption Ö Reduces interference Receiver side: silence is disturbing! Ö Missing received frames replaced with “comfort noise” Ö Comfort noise spectral density evaluated by TX decoder Ö And periodically (480ms) transmitted in special frame (SID= Silence Descriptor) Giuseppe Bianchi
Channel Coding Coding: needed to move from 10-1 to 10-3 radio channel native BER down to acceptablerange(10-5 to10-6) BER 260 bits 182 bits
78 bits
Parity bits Tail bits(0000)
50 bits 3
132 bits
260 bits block divided into -Class I: important bits (182) -Class Ia: Most important 50 -Class Ib: Less important 132 -Class II: low importance bits (78)
4
Convolutional coding, r=1/2
378 bits
78 bits
456 bits First step: block codingfor error detection in class Ia (error Ædiscard frame) Second step: convolutional coding for error correction Giuseppe Bianchi
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Block interleaving 8 0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
16 17 18 19 20 21 22 23
.. .. .. ..
.. .. .. ..
.. .. .. ..
.. .. .. ..
.. .. .. ..
.. .. .. ..
.. .. .. ..
.. .. .. ..
57
448 449 450 451 452 453 454 455
B1B2B3B4B5B6B7B8
8 blocks, each with 57 bits
Giuseppe Bianchi
Diagonal Interleaving Block n
Block n-1 B1 B2 B3B4B5B6 B7B8
B5n−1 / B1n
…
B1 B2 B3 B4B5 B6 B7B8
…
B8n−1 / B4n B5n / B1n+1
Block n+1 B1B2B3B4B5B6B7B8
…
…
B8n / B4n+1
PRICE TO PAY : delay!! (block spreaded over 8 bursts Æ 37 ms)
Inter-burst Interleaving = Bxn−1 TB 3
= Bxn− 4 Training
S S sequence 1 1 26
TB GP 3 8.25
Giuseppe Bianchi
17
Bad Frame Indicator ÎWhen
errors cannot be corrected: ÖFrame must be discarded ÎIn substitution, transmit speech frame Öpredictively calculated ÖNo more than 16 consecutive BFI ÆIf
this happens, receiver muted
Giuseppe Bianchi
Slow Associated Control Channel Î Always
associated to instaurated call on TCH (TCH + SACCH = TA CH)
Î On the same Time Slot Î
Periodic (order of ½ second) time-scale information for radio link control
TCH/F(0…7)
TCH/F(0…7)
1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23 24
SACCH(0…7)
IDLE frame
SACCH-0SACCH-1SACCH-2SACCH-3SACCH-4SACCH-5SACCH-6SACCH-7
1 SACCH burst (per TCH) every 26 frames (120 ms) Giuseppe Bianchi
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SACCH block Î 184
signalling bits
ÆBlock coding adds 40 bits (=224) Æ4 tail (zero) bits (=228) Æ1/2Convolutional
encoding (=456 bits)
Î Interleaving:
B1 B2 B3 B4 B5 B6 B7 B8
Ö
8 blocks of 57 bits;
Ö
spreadedinto four consecutive bursts
Ö 4 bursts = 1 and only 1 SACCH block! Î SACCH
rate: Ö 184 bits/480 ms = 383.3 bit/s
Odd/even interleaving
104 frames =480 ms Giuseppe Bianchi
SACCH contents Î
184 bits = 23 bytes
0 1 2 3 4 5 6 Power level Î Timing advance Î Measurement reports for link quality Î Measurement reports for handover management Î
Î
free free
7
Power level Timing advance
(21 bytes – datalink layer) Includes measurement reports
When available space: SMS Ö When call in progress!
Giuseppe Bianchi
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Power control MS has ability to reduce/increase power Maximumpower Ö Up to its power class maximum (defined by class) Î Maximum one 2 dB step every 60 ms Î uplink power measures taken by BTS 2 dB steps;Î notified back to MS via SACCH Ö Power level values: 0-15 Î
Æ0
=43dBm(20 W) Æ15 =13dBm(20 mW)
Minimum power (13 dBmfor GSM) (0 dBm for DCS 1800)
Î
algorithm: manifacturer specific Ö runs on BSC
Î
power control applied also on downlink
Giuseppe Bianchi
Measurement values Î RXLEV
Power level Ö Present channel + neighbohr cell) Ö
RX signal level
From To (dBm) (dBm)
Î RXQUAL Ö
Bit Error Rate (raw)
Bit error Ratio
From (%)
To (%)
RXQUAL_0
-
0.2
RXLEV_0
-
-110
RXQUAL_1
0.2
0.4
RXLEV_1
-110
-109
RXQUAL_2
0.4
0.8
RXLEV_2
-109
-108
RXLEV_3
-108
-107
RXQUAL_3
0.8
1.6
…
…
…
RXQUAL_4
1.6
3.2
…
…
…
RXQUAL_5
3.2
6.4
RXLEV_62
-49
-48
RXQUAL_6
6.4
12.8
RXLEV_63
-48
-
RXQUAL_7
12.8
-
Averaged over 1 SACCH block (480ms =104 frames) Giuseppe Bianchi
20
Fast Associated Control Channel Î
FACCH: urgent signalling Ö Used when several signalling information needs to be transmitted ÆCall setup ÆHandover
FACCH block = 184 Ö 456 after coding Î Interleaved as voice block Ö Spreaded on 8 bursts Î Replaces a voice block (20 ms) on the TCH Ö Via stealing bits Ö Voice block(s) discarded Î Maximum FACCH bit rate Ö 184*6/120 [bits/ms] = 9.2 kbps (vs 383 bps of SACCH!) Î
Giuseppe Bianchi
FACCH insertion in TCH Via Stealing bits - upper bit =odd bits stolen - lower bit =even bits stolen - both bits=all burst stolen
Figure: shows example of 2 FACCH blocks stealing a TCH (note begin and end behavior due to interleaving)
time Giuseppe Bianchi
21
Half -rate traffic channels (TCH/H) Î Support
for 5.6 kbps voice codecs Î Specification in 1995 Î 228 bits block Ö 112 bits of compressed 20ms voice Æ95 bits class I
» +3 parity +6 tail +convolutional coding 104/211 Æ17 bits class 2
Î Interleaving: Ö Same as voice (block + diagonal + odd/even) Ö But on 4 bursts Î Framing: TCH/H 0…7 TCH/H 8…15 [subchannel 0] [subchannel 1]
1
2 3 4 5 Giuseppe Bianchi
SACCH 0…7
6
7
SACCH 8…15
8
9
10
11
12
Other traffic channels ÎTCH
data ÖBasic speed: 9.6 kbps (data,fax) ÖOther speeds: <2.4; 4.8; 9.6; 14.4; ÎDifferent coding details ÖSee text(s) ÎMajor difference with voice: ÖInterleaving with depth = 19 (!!!) Öcomplete fading of a burst is recoverable (unlike voice) Giuseppe Bianchi
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PART 4 GSM – Radio Interface
Lecture 4.4 Broadcast Carrier and Channels
Giuseppe Bianchi
Broadcast Channel (usual) organization 51 frame structure vs 26 Ö 235.38 ms period (vs 120 ms) Î Sub-blocks with 10 frames Ö Starting with Frequency Correction Channel (FCCH) Ö Immediately followed by Synchronization Channel (SCH) Î Other frames (numbered from #0 to #50) : Ö #50 idle Ö #2,3,4,5 BCCH Ö Remaining: Paging (PCH) / Access Grant (AGCH) [=PAGCH] Î
51 frame structure - downlink F SBBBBPPPPF SPPPPPPPPF SPPPPPPPPF SPPPPPPPPF SPPPPPPPP
10 frame sub-block Giuseppe Bianchi
23
BCCH carrier placement Î On
Downlink Ö Corresponding uplink dedicated to Random Access Channel 51 frame structure - uplink
RRR RR RR RRRRRRRRRRRRRRRRRRRRRRRRRR RR RR RR RR RR RR RR RR RR
Î On
one frequency per cell (beacon) Î MUST BE on Time Slot #0 Î Other Time slots may be used by TCH Provided that: • All empty slots are filled with DUMMY bursts • Downlink power control must bedisabled Giuseppe Bianchi
MS powering up
First operation when MS turned ON: spectrum analysis (either on list of up to 32 Radio Frequency Channel Numbers of current network) (or on whole 124 carriers spectrum) Giuseppe Bianchi
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tuning Î MS
listens on strongest beacon for a pure sine wave (FCCH) Ö Coarse bit synchronization Ö Fine tuning of oscillator Î Immediately follows SCH burst Ö Fine tuning of synchronization (64 bits training sequence) Ö Read burst content for synchronization data Æ25 bits (+10 parity +4 tail +½convolutional
coding =78
bits) Æ6 bits: BSIC Æ19 bits: Frame Number (reduced) Î Finally,
MS can read BCCH
Giuseppe Bianchi
Multi-framing structure 8 TS, 4.615 ms 0
1
2
3
4
1 multiframe=26 TDMA frames (120 ms) T T T T T T T T T T T T S T T T T T T T T T T T T
6
7
1 multiframe=51 TDM A frames(235.38 ms) F S B B B B P P P P F S P P P P PP P P F S P P P P PP P P F S P P P P PP P P F S P P P P PP P P
……… … Multiframe 1
Multiframe 0 Multiframe 1 Multiframe 2 Multiframe 0
5
Multiframe 49Multiframe 50 Multiframe 25
1 superframe=51 x 26-multiframe or 26 x 51-multiframe (1326 TDMA frames, 6.12 s)
superframe0
superframe1
superframe2
superframe2046 superframe2047
1 hyperframe =2048 superframe (2715648 TDMA frames, 3h28m53s.76)
Giuseppe Bianchi
25
Why frame number? Superframe #
FN =
Multiframe#
frame#
Î Frame
# Ö Distinguishes logical channels in the same physical channel Î Multiframe # Ö Determines how BCCH is constructed ÆI.e. which specific information transmitted on BCCH
during a
givenmultiframe Î Superframe
# Ö Used as input parameter by encyphering algorithm Giuseppe Bianchi
BCCH contents Î
184 bits Ö Coded in 456 bits and interleafed in 4 bursts Ö same coding and interleaving as SACCH Ö BCCH capacity Æ184 bits / (51*8*15/26 ms) ~782 bps
Î
Information provided Ö Details of the control channel configuration Ö Parameters to be used in the cell ÆRandomaccess backoff
values Maximum power an MS may access (MS_TXPWR_MAX_CCCH) Æ ÆMinimumreceived power at MS (RXLEV_A CCESS_MIN) ÆIs cell allowed? (CELL_BA R_A CCESS) ÆEtc. Ö
List of carriers used in the cell ÆNeeded if frequency hopping is applied
Ö
List of BCCH carriers and BSIC of neighboring cells
Giuseppe Bianchi
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control channel alternative organization 51 frame structure – small capacity cell DOWN
F S B B B B P P P P F S P P P P P P P P F S D D D D D D D D F S D D D D D D D DF S D D D D D D D D
UP
DD D DRRD D D DD D D DR RR RR RR RR RR RR RR R RR R R R R RD D D DD D D DR RD D D D
Integrates SDCCH in same channel as other control information Leaves additional TS all available for TCH
51 frame structure – large capacity cell DOWN UP
BBBBPPPP
PPPPPPPP
PPPPPPPP
PPPPPPPP
PPPPPPPP
RRRRRRR RR RRRR RR RR RR RR RR RR RR RRRRRRRRRRRRRRRRRRRRRRRR
Used in TS 2 (and, eventually, 4 and 6) of beacon carrier Provides additional paging and RACH channels Giuseppe Bianchi
Why 26 and 51? Last frame (idle) in TCH multiframe(Frame #25) used as “search frame”! T T T T T T T T T T T T S T T T T T T T T T T T T
- An active call transmits/receive in 25 frames, except the last one. - in this last frame, it can monitor the BCCH of this (and neighbor) cell - this particular numbering allows to scan all BCCH slots during asuperframe - important slots while call is active: frequency correction FCCH and sync SCH! - needed for handover -Worst case: at most every 11 TCH multiframes (1.32 s), there will be a frequency correction burst of a neighboring cell
Giuseppe Bianchi
27
PART 4 GSM – Radio Interface
Lecture 4.5 Paging, Random Access, dedicated signaling
Giuseppe Bianchi
Why paging Ö Channel assignment: Æonly upon explicit request from MS
Ö Paging Æneeded to “wake-up” MS from IDLE
state when incoming call
arrives to MS Ö MS accesses on RACH to ask for a channel ÆGenerally SDCCH (but immediate TCH assignment is possible)
1) paging MS
2) Randomaccess
BSS/MSC
3) Channel assignment Paging channel: PCH Access Grant Channel: AGCH RandomAccess Channel: RACH
PAGCH
CCCH Common Control CHannel
Giuseppe Bianchi
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Paging Î Paging
message generated by MSC Ö Which receives incoming call Î Transferred to subset of BSC Ö Paging limited to user’s location area Ö Paging message contains: ÆList of cells where paging should be performed ÆIdentity of paged user (IMSI
or TMSI)
Î Paging
message coded in 4 consecutive bursts over the air interface Ö Same coding/interleaving structure of SACCH (184Æ456 bits) Î Paging for more MSs may be joined in one unique paging message Giuseppe Bianchi
Discontinuous Reception (DRX) MS in idle mode should listen to paging channel Î To save battery, PCH divided into sub-channels Ö Subdivision based on last 3 digits of IMSI Ö User listens only to relevant sub-channel Î
ÆSwitches off otherwise
Î
PCH sub-channels pattern communicated on BCCH Ö Up to 9 sub-channels
51 frame BCCH structure- downlink FSBBBBPPPPF SPPPPPPPPF SPPPPPPPPF SPPPPPPPPF SPPPPPPPP
Giuseppe Bianchi
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Access procedure Always activated by MS Ö In response to paging (incoming call) Ö When location update needs to be performed Ö When new call is generated by MS Î Based on access burst sent on RACH Ö Access burst coding: 8 payload bits (channel_req) Æ 36 coded bits Î
Æ+6 parity; +4 tail; +½convolutional
TB 8
Training Seq. (41)
100: response to radio call 101: emergency call 110: new establishment of call 111: supplementary service (SMS, etc) … 000: other case
coding
Ac ces s b ur st Data TB 36 3
GP 68.25
Channel_req message
Establishment Random discr iminator Cause (3 bits) (5 bits)
Random discriminator: (0…31) value randomly generated by MS Giuseppe Bianchi
RACH operation MS-C
A,B
C,B
MS-A
C
A,B
A
B
MS-B Multiple Access Techniquefor simultaneous access Collision resolution based on - randomretrial period Same thing..! - “permission” probability (SLOTTED ALOHA protocol)
Giuseppe Bianchi
30
RACH performance ÎN
stations Î Each transmits with probability p Ö Retries, in average, every 1/p slots Î relevant probabilities:
idle: success:
(1− p)N N −1 Np(1− p) ⎛ N ⎞ k ⎜⎜ ⎟⎟ p (1− p)N −k ∑ k= 2 ⎝ k ⎠ N
collision:
Maximumefficiency: when p=1/N Giuseppe Bianchi
RACH performance control Î BCCH
broadcasts Ö Backoff time Æuniform distribution; max value: 3 to 50
Ö Maximum number of retransmission attempts ÆNever greater than 7
Î May
also specify: Ö time interval for a retry after failure (default=5s) Ö RACH group access control ÆMSs divided into 10 groups, depending on SIM-related
information ÆBTS may block selected groups ÆAllows to reduce RACH load down to as low as 10% » Emergency calls bypass this rule… Giuseppe Bianchi
31
Access signaling - 1 MS
BTS
BSC
MSC
Channel_request Channel_required
rnd number
rnd&frame number, Delay (TA estimate)
Channel_activation Ch_activation_ack Immediate_assignment rnd&frame number, channel description, Initial TA , initial maxpower
Giuseppe Bianchi
Immediate Assignment message Î on
paging channel Ö I.e. AGCH is a dynamically mapped channel – name PAGCH is perhaps better…
Î Sent
“as soon as possible” Ö MS continues accessing the RACH Ö Message scheduling is an implementation dependent issue ÆI.e. which messageto send in case of many messages, and on
which paging slot (4 bursts) Ö MS must disable DRX Æ To monitor PAGCH
for Immediate Assignment message
detection Î Immediate
assignment reject possible
Giuseppe Bianchi
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If same random discriminator? Î
two MSs may have same random discriminator Ö Likely in heavy load, with only 5 bits
And transmit in the same frame Î Only one wins (the other is faded) Î contention resolved via explicit MS identification MS1 MS2 BTS Ö On SDCCH ID1 Î
ID2
ID1 leave continues Giuseppe Bianchi
Access signaling - 2 MS
…
…
BTS
…
…
BSC
MSC
Immediate_assignment Initial_message
MS ID (IMSI or TMSI), MS capabilities (=classmark), establishment cause
Initial_message_ack (UA)
Establishment_indication
Copy of Initial message(including MS ID)
Further signaling: MSC to MS
Giuseppe Bianchi
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