Upload
sameerom
View
234
Download
0
Embed Size (px)
Citation preview
7/27/2019 50151779 Ch3 Air Interface
1/19
AIR INTERFACE
By:
Eng\ WALEED ELSAFOURY
7/27/2019 50151779 Ch3 Air Interface
2/19
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
7/27/2019 50151779 Ch3 Air Interface
3/19
AIR INTERFACE
Uplink and downlink
7/27/2019 50151779 Ch3 Air Interface
4/19
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 MHz1805 - 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
AIR INTERFACE
7/27/2019 50151779 Ch3 Air Interface
5/19
AIR INTERFACE
- TDMA in GSM
7/27/2019 50151779 Ch3 Air Interface
6/19
AIR INTERFACE
TDMA frame
7/27/2019 50151779 Ch3 Air Interface
7/19
AIR INTERFACE
TDMA frame structure
3113 57 5726
7/27/2019 50151779 Ch3 Air Interface
8/19
AIR INTERFACE
TDMA frame structure
-Bit rate of the radio carrier is 270.833 Kbps
-Bit duration =1/270.833=3.69 sec
-One time slot =148 bits+8.25 guard bits=156.25 bits-Time slot duration =156.25x3.69 sec= 0.577 msec
-Frame duration=0.577x8= 4.615 msec
7/27/2019 50151779 Ch3 Air Interface
9/19
AIR INTERFACE
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 staggeredby 3 time slots to remove necessity for MS to sendand receive in the same time
4 5 6
1 2 3 4 5 60
0
7/27/2019 50151779 Ch3 Air Interface
10/19
AIR INTERFACE
Types of channels1. Physical channels
2. Logic channels
7/27/2019 50151779 Ch3 Air Interface
11/19
AIR INTERFACE
Types of logic channels1. Traffic channels
2. Control channels
7/27/2019 50151779 Ch3 Air Interface
12/19
AIR INTERFACE
Types of Traffic channels1. Full rate
- 13 Kbps data rate
2. Enhanced full rate
- 12.2 Kbps data rate
3. Half rate
- increase network capacity
13 Kbps 9.8Kbps
12.2 Kbps 10.6Kbps
5.6 Kbps 5.8 Kbps
11.4Kbps
22.8Kbps
Data
Redundancy
7/27/2019 50151779 Ch3 Air Interface
13/19
AIR INTERFACE
Types of control channels
Broadcast control channel
Dedicated control channel
Common control channel
7/27/2019 50151779 Ch3 Air Interface
14/19
AIR INTERFACE
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 anincoming call
Transmits paging
message to indicatean incoming callThe paging messagecontains MS identity.
Listens to PCHregularly.When identifies itsidentity, it responds.
Random AccessCHannel (RACH)
Uplink
Used by MS torequest access to
the network.
Receives requestfrom MS for a
signaling channel.
Answers pagingmessage on RACH byrequesting signaling
channel.
Access GrantCHannel(AGCH)Downlink
Used by BS toinform MS aboutthe channel to use.
Assigns a SDCCHsignaling channel toMS.
Receives a signalingchannel assignmentSDCCH.
7/27/2019 50151779 Ch3 Air Interface
15/19
AIR INTERFACE
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 ControlChannel (SDCCH)
UL+DL
Exchange signalinginformation in uplinkand downlink.
Switches to SDCCH.
Call set-up isperformed in idlemode.BSC assigns a TCH.
Switches to SDCCH.
Call set up is performed.MS receives TCHassignment [carrier +time slot]
Slow Associated
Control Channel(SACCH)UL+DL
Conveys power controland timing informationin downlink.
Conveys link qualityreports in uplink.
Instructs MS about:Transmit powerTime advance.
Sends measurements ofits BTS and neighboringBTSs during a call.
Fast AssociatedControl Channel(FACCH)
UL+DL
Steals TCH to carryhandover and channelreassignment.
Transmits handoverinformation.
Transmits handoverrequest..
7/27/2019 50151779 Ch3 Air Interface
16/19
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
FrequencyCorrectionChannel(FCCH)
Supply MS withreference freq.
Transmits a carrierfrequency.
Identifies BCCHcarrier
synchronizes withthe frequency.
Synch.Channel (SCH)
For TDMA framesynchronization .
Transmits framenumber and BSIC.
Synchronizes withframe structure.
BroadcastControlChannel(BCCH)
Carries parametersneeded to identifyand access network.
It is transmitted atconstant power at alltimes and checked byall MSs.
Broadcasts cellinformation suchas
LAI,maximum output
power needed
Receives LAIMay updateMS sets its outputpower level.
AIR INTERFACE
7/27/2019 50151779 Ch3 Air Interface
17/19
AIR INTERFACE
Adaptive Frame Alignment The BS-MS delay depends on distance. If BSMS 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
7/27/2019 50151779 Ch3 Air Interface
18/19
AIR INTERFACE
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
E E
7/27/2019 50151779 Ch3 Air Interface
19/19
AIR INTERFACE
0 1 5 6432 7
0 1 5 6432 7
0 1 5 6432 7
TDMAframe
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