ch3 air interface
DESCRIPTION
telecomTRANSCRIPT
AIR INTERFACE
ContentContent1.1.What is TDMA ?What is TDMA ?
2.2.Types of channelsTypes of channels
3.3.Types of Traffic channelsTypes of Traffic channels
4.4.Types of control channelsTypes of control channels
5.5.Types of BurstsTypes of Bursts
AIR INTERFACE
AIR INTERFACE
GSM 900
0.8 0.9 1.0 1.1 1.6 1.7 1.8 1.9 2.0 GHz
GSM 1800
GSM 1900
200 kHz
f
Carrier nCarrier (n+1)
• GSM bands
AIR INTERFACE
Uplink and downlink
• GSM Frequency Bands
System P-GSM 900 E-GSM 900GSM(DCS)
1800GSM(PCS) 1900
Uplink (MS BS)
Downlink(BS MS)
890 – 915 MHz935 – 960 MHz
880 – 915 MHz925 - 960 MHz
1710 – 1785 MHz
1805 - 1880 MHz
1850 – 1910 MHz1930 - 1990 MHz
Wavelength 33 cm 33 cm 17 cm 16 cm
Bandwidth 25 MHz 35 MHz 75 MHz 60 MHz
Duplex distance 45 MHz 45 MHz 95 MHz 80 MHz
Carrier separation
200 kHz 200 kHz 200 kHz 200 kHz
No. of carriers 124 174 374 299
Channel rate 270.8 kbps 270.8 kbps 270.8 kbps 270.8 kbps
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- TDMA in GSM
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• TDMA frame
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• TDMA frame structure
3113 57 5726
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• TDMA frame structure
-Bit rate of the radio carrier is 270.833 Kbps
-Bit duration =1/270.833=3.69 µsec
-One time slot =148 bits+8.25 guard bits=156.25 bits
-Time slot duration =156.25x3.69 µsec= 0.577 msec
-Frame duration=0.557x8= 4.615 msec
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• Guard band is used for :
- burst alignment errors
- time dispersion on the propagation path
- Time required for smooth switch on-off in transmitter
1 2 3
• The numbering scheme of the time slots is staggered by 3 time slots to remove necessity for MS to send and receive in the same time
4 5 6
1 2 3 4 5 60
0
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• Types of channels
1. Physical channels
2. Logic channels
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• Types of logic channels
1. Traffic channels
2. Control channels
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Channel
Logical Physical
FR EFR HR
Traffic
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• Types of Traffic channels
1. Full rate
- 13 Kbps data rate
2. Enhanced full rate
- 12.2 Kbps data rate
3. Half rate
- increase network capacity
13 Kbps 9.8 Kbps
12.2 Kbps 10.6 Kbps
5.6 Kbps 5.8 Kbps
11.4 Kbps
22.8 Kbps
Data
Redundancy
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• Types of control channels
• Broadcast control channel
• Dedicated control channel
• Common control channel
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Channel
Logical Physical
FR EFR HR
BCCH CCCH DCCH
Traffic Control
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Channel
Logical Physical
FR EFR HR
CCCH
PCH RACH AGCH
Traffic Control
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Common Control Channels (CCCHs)Transfer control information between all MSs and BTS.Necessary for call origination and paging.
Logical ch. Function BTS MS
Paging CHannel
(PCH)Downlink
•Alert the MS of an incoming call
Transmits paging message to indicate
an incoming callThe paging message contains MS identity.
Listens to PCH regularly.
When identifies its identity, it responds.
Random Access
CHannel (RACH)Uplink
•Used by MS to request access to
the network.
Receives request from MS for a
signaling channel.
Answers paging message on RACH
by requesting signaling channel.
Access Grant CHannel (AGCH)
Downlink
•Used by BS to inform MS about the channel to use.
Assigns a SDCCH signaling channel to MS.
Receives a signaling channel assignment SDCCH.
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Channel
Logical Physical
FR EFR HR
CCCH DCCH
PCH RACH AGCH SDCCH SACCH FACCH
Traffic Control
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Dedicated Control Channels (DCCHs)Carry messages between MS and network.SDCCH is used for call setup, update, authentication
Logical ch. Function BTS MS
Stand-alone Dedicated Control Channel (SDCCH)
UL+DL
•Exchange signaling information in uplink
and downlink.
Switches to SDCCH.Call set-up is
performed in idle mode.
BSC assigns a TCH.
Switches to SDCCH.Call set up is
performed.MS receives TCH
assignment [carrier + time slot]
Slow Associated Control Channel (SACCH)
UL+DL
•Conveys power control and timing
information in downlink.
•Conveys link quality reports in uplink.
Instructs MS about:Transmit powerTime advance.
Sends measurements of its BTS and neighboring BTSs during a call.
Fast Associated Control Channel (FACCH)
UL+DL
•Steals TCH to carry handover and channel reassignment.
Transmits handover information.
Transmits handover request..
AIR INTERFACE
Channel
Logical Physical
FR EFR HR
BCCH CCCH DCCH
BCH FCCH SCH PCH RACH AGCH SDCCH SACCH FACCH
Traffic Control
The Broadcast Channels (BCHs) Is transmitted by BS all the time. Monitored by MSs periodically (every 30 sec). All BCHs are downlink.
Logical channel
Function BTS MS
Frequency Correction Channel (FCCH)
Supply MS with reference freq.
Transmits a carrier frequency.
Identifies BCCH carrier synchronizes with the
frequency.
Synch. Channel (SCH)
For TDMA frame synchronization .
Transmits frame number and BSIC.
Synchronizes with frame structure.
Broadcast Control Channel (BCCH)
•Carries parameters needed to identify and access network.
•It is transmitted at constant power at all times and checked by all MSs.
Broadcasts cell information such as
LAI, maximum output
power needed
Receives LAI May update MS sets its output power level.
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• Adaptive Frame Alignment The BS-MS delay depends on distance. If BS–MS separation= 10km, Propagation delay=33 sec. Long guard interval is inefficient. GSM uses adaptive time alignment system whereby BS
transmits timing advance at the MS. This instructs the mis-aligned MS to transmit earlier
TDMA frame
Propagation delay
TS0 TS2TS1
B: Far away from BS
BA
A: Close to BS
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Rayleigh fading is frequency-selective.
BS&MS hop from one frequency to other.
Frequency hopping is optional.
Rate of hopping =1/4.615msec=217 hop/sec.
Slow Frequency Hopping
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0 1 5 6432 7
0 1 5 6432 7
0 1 5 6432 7
TDMA frame
3 4210 765
3 4210 765
3 4210 765
Frame 1Carrier 1
slot 3
Frame 2Carrier 3
slot 3
2 310
2 310
2 310
Frame 3Carrier 2
slot 3