Radio Interface

Functions of the Radio Interface

• Speech and user's data • Signaling BTS

BTS-2

BTS-1

Page 1 1

GSM Uses Paired Radio Channels Case of GSM 900

Uplink 890 MHz

Frequency

0

channel #

Downlink 915 MHz 935 MHz

123

0

BTS

Frequency

960 MHz

channel #

123

Example: Channel 48

Duplex spacing = 45 MHz Frequency band spectrum = 2 x 25 MHz Channel spacing = 200 kHz

GSM Band Allocations (MHz) GSM systems

Uplink

Downlink

Band

Duplex Spacing

Duplex channels

P-GSM 900 E-GSM 900 R-GSM 900 GSM 1800 GSM 1900

890-915 880-915 876-915 1710-1785 1850-1910

935-960 925-960 921-960 1805-1880 1930-1990

2x25 2x35 2x39 2x75 2x60

45 45 45 95 80

124 174 194 374 299

ARFCN = n

Frequencies are in MHz

P-GSM 900

Fl(n) = 890 + 0.2*n

1 ≤ n ≤ 124

Fu(n) = Fl(n) + 45

E-GSM 900

Fl(n) = 890 + 0.2*n Fl(n) = 890 + 0.2*(n-1024)

0 ≤ n ≤ 124 975 ≤ n ≤ 1023

Fu(n) = Fl(n) + 45

R-GSM 900

Fl(n) = 890 + 0.2*n Fl(n) = 890 + 0.2*(n-1024)

0 ≤ n ≤ 124 955 ≤ n ≤ 1023

Fu(n) = Fl(n) + 45

GSM 1800

Fl(n) = 1710.2 + 0.2*(n-512)

512 ≤ n ≤ 885

Fu(n) = Fl(n) + 95

Page 2 2

GSM Time Division Multiple Access (TDMA) Frame and Physical Channels Time-slot

TDMA frame

TDMA frame

TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS 0

1

2

3

4

5

6

7

0

1

2

3

4

5

6

7

(frames repeat continuously)

Time 0

4.615 ms

9.23 ms

Physical channel # 2 = recurrence of time-slot # 2

Physical Channel

BTS

time

n+1

TDMAs n TS 0

7

n-1

MS1 MS3 MS2

0

FDMA

1

// 123

ARFCN

Page 3 3

GSM Delays Uplink TDMA Frames BTS side

The start of the uplink TDMA is delayed of three time-slots

TDMA Frame (4.615 ms)

0

1

2

3

4

5

R

R

R

R

R

T

T

T

T

T

6 R

7 R

R

Downlink TDMA BTS

T

T

T Down link

MSs side MS1

R

Up link T

R

T

MS2 Fixed transmit delay of three time-slots

Propagation Delays

MS2

BTS Frame reference

d1>>d2

d2

TS0

TS1

TS2

TS3

TS4

MS1

TS5

TS6

TS7

Propagation Delay τp Bits Overlapping MSs transmit

Page 4 4

Propagation Delay TX BTS RX BTS

CAN

WHAT

HOW yes

TA

RX MS1 TX MS1

CAN

RX MS2 TX MS2 RX MS3 TX MS3

GSM

+3TS

WHEN the

WHAT ms-isdn

yes

WHAT

D

the

GSM

D

ms-isdn

RX MS4 TX MS4

HOW

PROPAGATION DELAY

RX MS5 TX MS5

WHEN

RX MS6 TX MS6

WHAT

RX MS7 TX MS7 RX MS8 TX MS8

Timing Advance TX BTS RX BTS

CAN

RX MS1 TX MS1

CAN

RX MS2 TX MS2 RX MS3 TX MS3

WHAT

GSM

+3TS - TA

WHAT ms-isdn

D

the

GSM ms-isdn

RX MS4 TX MS4 RX MS5 TX MS5

WHEN the

yes

WHAT

D

HOW yes

HOW

PROPAGATION DELAY

WHEN

RX MS6 TX MS6

WHAT

RX MS7 TX MS7 RX MS8 TX MS8

TIMING ADVANCE = 2 * PROPAGATION DELAY

Page 5 5

Logical Channels Traffic and Control Channels TDMA frame

TDMA frame

TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS 0

1

2

3

4

5

6

7

0

...

1

2

3

4

5

6

7

(frames repeat continuously)

...

Voice transmitted over the physical channel #2 defines a logical

traffic channel

Information (e.g. to set up a call) transmitted over the physical channel #1 defines a logical control channel

From Physical Channel to Logical Channels Time Slot

TDMA Frame = 4.615 ms

LOGICAL CHANNELS

Logical channels multiplexing

0 12 345 67 01 234 56 701 23 45 67

...

...

...

...

TS = 577 µs

MESSAGE TYPE

Physical Channel Logical Channel Logical Channel

• TCH • FCCH • SCH • FACCH • SACCH • SDCCH • CBCH • AGCH • PCH • BCCH • RACH

different message types = different logical channels

Page 6 6

Traffic Channels (TCHs)

BSS sp e

BSC MSC

BTS

TC H

ec ho

rd

ata

sp ee ch or d

TC H

ata

Full rate TCH carries: • speech (12.2 kbps) • user’s data (300 bps up to14.4 kbps) Half rate TCH carries: • user’s data (300 bps up to 4.8 kbps)

Control Channels GSM Channels

Traffic Channels (TCHs)

Full rate

Half rate

Control Channels

Broadcast Channels (BCHs)

Common Control Channels (CCCHs)

Downlink

Downlink

Dedicated Control Channels (DCCHs) (down uplink) Uplink Fast

TCH /F

TCH /H FCCH

Traffic Multiframing

SCH

BCCH PCH

AGCH

CBCH

Signaling Multiframing

RACH SDCCH

Slow

FACCH

SACCH

Traffic Multiframing

Page 7 7

The Logical Channels on Radio Interface TS

0123 456 7

BTS

MS FCCH

TCH

SCH

FACCH

Traffic (speech-data)

Frequency correction Synchronization

Associated Signaling

BCCH Broadcast control RACH

SACCH SDCCH

Access request PCH

Dedicated Signaling

CBCH

Subscriber paging Answer to Access request

Radio Measurement + SMS

Broadcast info

AGCH FCCH CBCH

SCH

Broadcast info Dedicated Signaling Sys InFo 5, 6 + SMS

SDCCH

M.S. Pre-synchronization

BCCH

SACCH

RACH Access request PCH

Traffic (speech data)

Subscriber paging

TCH

AGCH

Associated Signaling FACCH

Answer to Access request

Logical Channel Description (1/2) SDCCH MESSAGES

TCH MESSAGES

• Request for a SDCCH assignment

• Full rate speech at 13 kbit/s

• Request for the end of channel

• Half rate speech at 6.5 kbit/s

assignment

• Full rate data at 9.6, 4.8, or 2.4 kbit/s

• Order of commutation from SDCCH to

• Half rate data at 4.8 or 2.4 kbit/s

TCH

• Handover Access message (uplink)

FACCH MESSAGES

SACCH MESSAGES

• Connection establishment from

• System Information 5, 5bis, 5ter and 6

SDCCH to TCH

(connected mode)

• End validation of a SDCCH-TCH

• Measures:

commutation

- power level of the communication - quality level of the communication - level on the beacon frequency of

• Characteristics of the future used BS

after handover

the neighboring cells

• Connection establishment to BS after

handover

• Timing Advance

• Validation of an handover

• Power Control

Page 8 8

Logical Channel Description (2/2) AGCH MESSAGES

FCCH MESSAGES

• For dedicated channel assignment:

• no message is sent (all bits 0)

- frequency number - slot number - frequency hopping description - Timing Advance (1st estimation) - MS identification

SCH MESSAGES • Frame Number • Base Station Identity Code (BSIC)

RACH MESSAGES BCCH MESSAGES

• Service request:

- emergency call - answer to an incoming call - outgoing call - short message - call re-establishment - inscription

• System Information type 1, 2, 2bis,

2ter, 3, 4, 7, 8 (idle mode)

CBCH MESSAGES PCH MESSAGES

• Specific information

• messages containing a mobile

For example: - weather - road information

identity for a call, a short message or an authentication

Traffic Channels Combination Logical Channel Mapping (1/5)

Full Rate - Downlink & Uplink 26 frames = 120 ms

T T T T T T T T T T T T A T T T T T T T T T T T T

time

Half Rate - Downlink & Uplink 26 frames = 120 ms

T0 T1 T0 T1 T0 T1 T0 T1 T0 T1 T0 T1 A0 T1 T0 T1 T0 T1 T0 T1 T0 T1 T0 T1 T0 A1

T : TCH

Ti : TCH

sub-channel n°i

A : SACCH

SACCH Ai :sub-channel n°i

time

: IDLE

Page 9 9

Dedicated Signaling Channels Combination Logical Channel Mapping (2/5) Downlink 51 frames = 235 ms

D0

D1

D2

D3

D4

D5

D6

D7

A0

A1

A2

A3

D0

D1

D2

D3

D4

D5

D6

D7

A4

A5

A6

A7 time

Uplink 51 frames = 235 ms

A5

A6

A7

D0

D1

D2

D3

D4

D5

D6

D7

A1

A2

A3

D0

D1

D2

D3

D4

D5

D6

D7

A0 A4 time

A : SACCH

: IDLE

D : SDCCH

Physical Channel and Logical Channels Common Channels Combination Multiframe m Multiframe m-1

C

Multiframe m+1

51 frames = 235.38 ms

FS

B

C

FS C

C

C

FS

C

FS C

C

FS C

C

FS

B

time

Frames repeat continuously

PCH/AGCH

BTS

Physical Channel ARFCN (n)

Logical Channels

F : FCCH

SCH BCCH FCCH

TS (s)

S : SCH

B : BCCH

C : PCH/AGCH

MS

: IDLE

Page 10 10

Common Channels Combination Logical Channel Mapping (3/5) Downlink 51 frames = 235 ms

FS

B

C

FS

C

C

FS

C

C

FS

C

C

FS

C

C time

Uplink 51 frames = 235 ms RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR RRRRRRRRRRRRRRRRRR time

F : FCCH

S : SCH

B : BCCH

: AGCH

C /PCH

R : RACH

: IDLE

From Speech to Radio Transmission Speech

Speech

Step 1

Digitizing and source coding

Source decoding

Step 2

Channel coding

Channel decoding

Interleaving

De-interleaving

Step 3 Burst deformatting

Burst formatting

Step 4

Ciphering

Step 5

Modulation

Step 6

Transmission

Deciphering

Demodulation equalization

Diversity

Page 11 11

Channel Processing Overview Speech blocks

20 ms

A

20 ms

20 ms

B

C

244 bits

244 bits

244 bits

Source coding Channel coding

A

B

456 bits

A A A A 5 6 7 8

Normal burst

B B B B B B B B 1 2 3 4 5 6 7 8

Interleaving

A5 B1

8 Bursts

C

456 bits

A6 B2

A7 B3

B5 C1

A8 B4

456 bits

8 Sub blocks of 57 bits

B6 C2

B7 C3

C C C C 1 2 3 4

B8 C4

3

57 bits

1

26 bits

1

57 bits

3

Tail

Information

CRL

Training

CRL

Information

Tail

Interleaving: TCH Full Rate 0 1 2 3 4 5 6 7 8 ...

...

452 453 454 455

456 coded bits

Divide 456 bits in 8 sub-blocks

4

5

6

7

1 9 •• • 449

0

2 10 •• • 450

1

2

3 11 •• • 451

3

4 12 •• • 452

4

5 13 •• • 453

5

6

6 14 •• • 454

7

7 15 •• • 455

0

1

57 Rows

0 8 •• • 448

2

reordering & partitioning out

3 diagonal interleaving

bit interleaving burst b0 b1

b56

b0 b1

b56

Page 12 12

Burst Formatting Normal Burst 1 frame: 4.615 ms 0

1

2

DATA

3

3

S

57

1

4

Training S sequence 26

5

6

Guard Band

DATA

1

Burst 148 bits

57

7

3

8.25

Guard

156.25 bits 0.577 ms

Page 13 13