GSM

GSM Channels

GSM Access Scheme and Channel Structure

• GSM uses FDMA and TDMA to transmit voice and data

• the uplink channel between the cell phone and the BTS uses FDMA and a specific frequency band

• the downlink channel between the BTS and the cell phone uses a different frequency band and the TDMA technique

• there is sufficient frequency separation between the uplink freq. band and the downlink freq. band to avoid interference

• each uplink and downlink frequency bands is further split up as Control Channel (used to set up and manage calls) and Traffic Channel (used to carry voice)

GSMThe European TDMA Digital Cellular Standard

• GSM Radio Aspects : Uplink(Mobile to base)

• 890-915 MHz (Total 25 MHz)

Downlink ( Base to Mobile)• 935-960 MHz (Total 25 MHz)

Total 45 MHz spacing for duplex operation GSM uses TDMA and FDMA

GSM uplink/downlink frequency bands used

GSM Frequency band

Uplink/BTS Transmit

Downlink/BTS Receive

900 MHz 935-960 MHz 890-915 MHz

1800 MHz 1805-1880 MHz 1710-1785 MHz

1900 MHz 1930-1990 MHz 1850-1910 MHz

GSMThe European TDMA Digital Cellular Standard

• GSM Using FDMA

Uplink Freq 890MHz 915Mhz

200KHz

1 2 3 4 5 124

Total Frequency range(Uplink)=25Mhz

Spacing between two carriers= 200kHz

No. of Carriers=25MHz/200KHz = 124

GSMThe European TDMA Digital Cellular Standard

• GSM Using TDMA– TDMA Frame is divided into 8 time slots.

0 71 2 4 5

0 33 3 5 6 7

Time

Freq

Down Link carrier

Up Link carrier

45MHz

200 KHz

Time slots

4.6 ms0.57ms

3 6

5421

GSMThe European TDMA Digital Cellular Standard

• Classification of channels in GSM

• Two types– Traffic channels (TCH)– Control channels (CCH)

Channels are used to carry speech , data and control information.

GSMThe European TDMA Digital Cellular Standard

• Classification of Channels• 1 Traffic (TCH)

– Speech• Full rate 22.8 kb/s• Half rate 11.4 kb/s

– Data• 9.6kb/s• 4.8kb/s• 2.4kb/s

GSMThe European TDMA Digital Cellular Standard

• 2 Control (CCH)– Broadcast (BCCH)

• Freq correction (FCCH)• Synchronization (SCH)

– Common (CCH)• Paging (PCH)• Access grant (AGCH)• Random Access (RACH)

– Dedicated (DCCH)• Fast Associative (FACCH)• Slow Associative (SACCH)• Stand alone (SDCCH)

GSM Control Channel

• is used to communicate management data (setting up calls, location) between BTS and the cell phone within a GSM cell

• only data is exchanged through the control channel (no voice)

• a specific frequency from the frequency band allocated to a cell and a specific time slot are allocated for the control channel (beacon frequency); a single control channel for a cell

• GSM control channels can have the following types:– broadcast channel– common control channel– dedicated control channel

Broadcast Channel

• type of control channel used for the initial synchronization between the cell phone and the BTS

• is composed from:– Frequency Correction Channel (FCCH) – is composed from a sequence

of 148 zeros transmitted by the BTS

– Synchronization Channel (SCH) – follows the FCCH and contains BTS identification and location information

– Broadcast Control Channel (BCCH) – contains the frequency allocation information used by cell phones to adjust their frequency to that of the network; is continuously broadcasted by the BTS

Common Control Channels

• type of control chan. used for call initiation

• is composed of:– Paging Channel (PCH) – the BTS uses this channel to inform the cell

phone about an incoming call; the cell phone periodically monitors this channel

– Random Access Channel (RACH) – is an uplink channel used by the cell phone to initiate a call; the cell phone uses this channel only when required; if 2 phones try to access the RACH at the same time, they cause interference and will wait a random time before they try again; once a cell phone correctly accesses the RACH, BTS send an acknowledgement

– Access Grant Channel (AGCH) – channel used to set up a call; once the cell phone has used PCH or RACH to receive or initiate a call, it uses AGCH to communicate to the BTS

Dedicated Control Channels

• control channel sed to manage calls

• is comprised from:– Standalone Dedicated Control Channel (SDCCH) – used along with

SACCH to send and receive messages; relays signalling information

– Slow Associated Control Channel (SACCH) – on the downlink BTS broadcasts messages of the beacon frequency of neighboring cells to the cell phones; on the uplink BTS receives acknowledgement messages from the cell phone

– Fast Associated Control Channel (FACCH) – used to transmit unscheduled urgent messages; FACCH is faster than SACCH as it can carry 50 messages per second, while SACCH an caryy only 4.

Traffic Channel

• is used to carry voice data

• based on the TDMA the traffic (voice channel) is divided in 8 different time slots numbered from 0 to 7

• the BTS sends signals to a particular cell phone in a specific time slot (from those 8 time slots) and the cell phone replies in a different time slot

Channel Allocation

Introduction

• Allocation of channels to a cellular system is important from a performance point of view.

• Why????

– Whenever a MS want to make a new call, it must submit a request for a channel to a BS.

– BS can grant such an access provided that channel is readily available.

– The probability of a new call to be dropped or blocking probability for calls originated in a cell can be minimized.

Introduction (cont.)

• How to solve the dropped or blocking probability for calls problems????

– Increase the number of channels per cell. But????

Introduction (cont.)

• We have a limited frequency to allocate for wireless communication – there is a limit to maximum number of channels – restricting availability of channel in each cell.

• Channel Allocation – a given spectrum is to be divided into a set of disjoint channels, which can be used simultaneously while minimizing interference in adjacent channels.

• Interference can be avoid using appropriate reuse distance.

• One simplistic approach – divide the channels equally among the cells.

Introduction (cont.)

• If Stotal is the total number of channels and N is the size of reuse cluster, then

S = the number of channels per cell = Stotal /N

If we reducing N, we can get more channels…but

It will against the philosophy of frequency reuse.

Why????

Introduction (cont.)

• Any reduction in the reuse distance increase the interference.

• Any other option?

– Allocate channel to different cell as per their load.– However, it hard to predict the instantaneous

traffic.

Introduction (cont.)

• An ideal situation:– All parameters are assumed to be the same:

• Traffic

• External condition

• Etc.

Static vs Dynamic Allocation

• Two ways the traffic channels can be allocated to different cells in cellular system: static and Dynamic.

• In static allocation: a fixed number of channels is allocated to each cells.

• In dynamic allocation: channels are allocated as needed dynamically, possibly from a central pool.

• In brief, channel allocation schemes can be divided as:– Fixed Channel Allocation (FCA) schemes.

– Dynamic Channel Allocation (DCA) schemes.

– Hybrid Channel Allocation (HCA) schemes.

Fixed Channel Allocation Schemes

• A set of channel is permanently allocated to each cell of the system.

• If the total number of available channels in the system is divided into sets, the minimum number of channel sets N required to serve the entire coverage area:

Where, D - frequency Reuse distance R - radius

RDN3

Fixed Channel Allocation Schemes(cont.)

• How to address increased traffic of new originating and handoff call in a cell in FCAS ???

• Borrow free channels from neighboring cells.– The borrowing process must make sure that there is no

interference.

• There are many possible channel borrowing schemes:– Simple Borrowing Schemes

– Complex Borrowing Schemes

Fixed Channel Allocation Schemes(cont.)

Simple Borrowing Schemes– If all channels allocated to a cell are being used,

then additional channels can be borrowed from any cell that has some free unused channels.

– The selection process:1. Select a donor cell from among adjacent cells that

has the largest number of free channels.

2. Select the first free channel found for borrowing when the search follows a predefined sequence.

Fixed Channel Allocation Schemes(cont.)

• Complex Borrowing Schemes– Basic strategy:

• Divide the channels into two groups: one group assigned to each cell permanently and the second group kept reserved as donors.

• The ratio between groups can be determines a priori or based on the estimated traffic in the system.

– Alternative strategy:• Borrowing with channel ordering – assign priorities to

all channels of each cell. Lowest priority channel for borrowing.

Fixed Channel Allocation Schemes(cont.)– Alternative strategy (cont.):

• Borrowing with directional locking – borrow the available nearby cochannel cells with minimizing interference.

– The basic sectoring technique can be used to allocate channels temporarily.

– There are relative advantages and disadvantages of different complex schemes in term of: Total channel utilization,Total carried traffic, Allocation complexity.

– In channel borrowing schemes, there are two step of verifying potential interference and potential prohibition:

• Checking the reuse distance with other nearby clusters using those borrowed channels

• Looking of sector directions of all cells not satisfying the reuse distance

Dynamic Channel Allocation Schemes

• Channels are allocated dynamically as new calls arrive.• All free channels are kept in a central pool.• When call completed, channel will be returned to central

pool.• Free channel can be allocated to any cell, as long as

interference constraints in that cell can be satisfied.• The control can be centralized or distributed.• DCAS can be classified into two : centralized schemes

and distributed schemes.

Dynamic Channel Allocation Schemes (cont.)

• Centralized Dynamic Channel Allocation Schemes– Channel is selected for a new call from a central pool of free

channel and a specific characterizing function is used to select one among candidate free channel.

– The schemes:• Select the first available free channel that can satisfy the reuse

distance – the simplest.

• Select a free channel that can minimize the future blocking probability

• Channel reuse optimization, maximizes the use of every channel

Dynamic Channel Allocation Schemes (cont.)

• Distributed Dynamic Channel Allocation Schemes– The allocation is primarily based on one of these three

parameters: cochannel distance, signal strength measurement and signal-to-noise interference ratio.

– In cell-based distributed schemes:• Use a table to indicates if other cochannel cells in the neighborhood

are not use and select one of them.• Cochannel interference scheme• Adjacent channel interference scheme

– Signal strength measurement-based distributed schemes:• Channels are allocated to a new call if the anticipated CIR(Carrier-

to-Interference Ratio) above a threshold.

Other Channel Allocation Schemes

• Hybrid Channel Allocation Scheme– Combination of fixed and dynamic channel allocation scheme

where channels are divided into fixed and dynamic sets.

• Flexible Channel Allocation Scheme– Similar to HCAS but the channels are divided into fixed and

flexible (emergency) sets.– Flexible scheme require central control with up-to-date traffic

pattern.– With two different strategies:

• Scheduled – a priori estimate the variation traffic needed to schedule emergency channel at predetermined traffic change

• Predictive – traffic intensity and blocking probability is monitored in each cell all the time for assigning flexible channels.

Allocation in Specialized System Structure

• Channel allocation also depends on the inherent characteristics of the system structure.– Example: schemes for freeway.

• Channel Allocation in One Dimensional Systems– Can be used for a highway, where handoff and forced call

termination do occur frequently due to small size of cell and speed of MSs located in fast moving cars .

• Reuse Partitioning Based Channel Allocation– Cell is divided into multiple concentric, equal-size zones.– Basic idea: inner zone require lesser power to attain a desired CIR or

SIR. The net effect – lower value of reuse distance for inner zones, as compared to outer zones

Allocation in Specialized System Structure (cont.)

• Overlapped Cells-Based Allocation– Cells are split into seven microcells with

separate BS and microwave tower for each microcells.

– Logical structure can be changed dynamically, starting with only the main cell for low traffic

– As traffic increase, one or more microcell will turned on.

– Another possibility to have an overlap of cell area between two adjacent cells.