08-GSM Radio Interface

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Radio Interface

Modulation

MGSK (Minimum Gaussian Shift Keying)

Channel Coding

Convolutional

with several different rates

Equalization

known bit sequences transmitted in each physical burst

Equalization filter synthetized at the receiver

Voice Coding

13 Kbit/s (PRE codec - full rate)

12.2 Kbit/s (CELP - enhanced full rate)

Physical Layer Management

Frequency Hopping

changes frequency slot by solt in order to reduce the impact of fading on some carriers.

Power Control

MS power is controlled by BTS.

BTS power (traffic carriers only) is controlled by MS.

The power control mechanism is based on a closed loop scheme, with up/down commands send in both directions.

Synchronization

Carrier

board;channel

Slot

time advance

Frame

frame number

Base station

optional

Time Advance

(0~233μs)

(0~63bit)

(0~70Km)

Time Advance

GSM is designed for cells with a radius of up to Rmax =37.8Km, normally is 35Km.

The guard time is 2τ=2×35/c=233μs

Eeuivalent 63bits (3.7μs/bit×63bits×c/2=35Km)

3.7μs/bit is the time of per bit

Using delay compensation, guard time of normal slots in GSM is 8.25bits

Physical Block (Brust)

The burst is the physical layer PDU transmitted in a time slot

Due to TDMA scheme each burst is an autonomous transmission with its own power profile

Normal Brust

Access Brust

Channels

Logical Channels

They in GSM identify different types of Information transmitted over through the air interface.

Signaling

Traffic data

They are divided into

Traffic Channels and Control Channels

Common Channels and Dedicated Channels

Logical Channels

Common Channels

Broaddcast Control Channels

FCCH: Frequency Correction Channel

SCH: Synchronisation Channel

BCCH: Broadcast Control Channel

Common Control Channels

PCH: Paging Channel

RACH: Random Access Channel

AGCH: Access Grant Channel

Dedicated Channels

Dedicated Control Channels

SDCCH: Stand-alone Dedicated Control Channel

SACCH:Slow Associated Control Channel

FACCH: Fast Associated Control Channel

Traffic Channels

TCH/F: Traffic Channel FUll rate

TCH/H: Traffic CHannel Half rate

TCH/E:Traffic Channel Enhanced

Traffic Channels

Channels transporting user data (voice and data)

full rate channel is 22.8Kb/s and half is 11.4Kb/s

Control Channels

Used for transporting signaling of different types (14 types of control channels )

Broaddcast Control Channels

System information transmitted in the downlink

Common Control Channels

Shared channels for connection initialization

Dedicated Control Channels

Signaling channels dedicated to specific traffic connections

Mapping

Mapping of Logical channel into Physical channels(TDMA structure)

Using different rates for different logical channels the mapping into physical channels is based on a mult-frame.

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