The Radio Interface

The Radio interface

The most important interface in GSM technology. i.e.

The GSM technology

Why is it so important..?? • It is universal media for transmission and reception of signals. • Key element to enable mobility and wireless access and provide roaming facility. • It is limited resource shared by all users. • It must be well defined to obtain compatibility between MS and BTS.

The GSM technology

The frequency spectrum is very congested, with only narrow slots of bandwidth allocated for cellular communications. Standard GSM has a total of 124 frequencies available for use in a network. Most network providers are unlikely to be able to use all of these frequencies and are generally allocated a small subset of the 124.The spectral efficiency depends on radio interface and transmission. The spectrum efficiency can be increased by decreasing the interference . It includes : -Frequency reuse The GSM technology -Sectorization.

Frequency re-use Three types of frequency reuse patterns • 7 Cell reuse pattern • 4 cell reuse pattern • 3 cell reuse pattern • Frequency re-use increases the capacity and hence spectrum efficiency.

2 7 1 6

3 4

5

D Cell Dia = R

7/21 cell cluster

Sectorization • Omni directional antennas radiates same frequency in every direction, thereby increasing the chances of interference and no. of cells in a particular area too. • Sectorization splits a site into number of cells ,which increases the capacity of the system. • Directional antennas are used which ensures that radio from one sector is concentrated in a particular direction which : --increases the signal strength. --makes frequency reuse pattern to be used efficiently.

OMNI CELL 1 ANTENNA

b2 b1

b3

120O CELLS 3 ANTENNAS

Access methods • Since radio frequency is limited, so there is no dedicated channel to individuals but it is provided on demand. Radio channels are shared by all users. • Both FDMA and TDMA methods helps to provide radio access to subscribers and hence provides mobility.

Tdma technology Time division multiple access is a digital technology that allows a number of users to access a single radio frequency channel without interference by allocating a unique time slots to each user within the channel. – Each carrier frequency is subdivided in time domain into 8 time slots – Each mobile transmits data in a frequency, in its particular time slot - Burst period = 0.577 milli secs. – 8 time slots called a TDMA frame. Period is .577 * 8 = 4.616 milli secs 0.577 ms

0

1

2

3

4

4.616 ms

5

6

7

How it works…? Let us consider that four different conversation is going on simultaneously. A. Marry had a little lamb. TDMA divides a single B. Ramu is a good boy. RF carrier into 8 slots C. There was an old man who lived and provides each in a shoe. D.Jack and Jill went up the hill conversation a single slot for very short duration. After the four slots conversation has been transmitted, process is repeated again.

RF Marry had Ramu is a There was Jack and channel a an Freq. 1 Slot 1 Slot 2 Slot 3` Slot 4

Tdma advantages • It can easily adapt to transmission of data as well as voice communication. • ability to carry 6 kbps to 120 Mbps of data rates which allows services like fax, voice band data, and SMS as well as multimedia too. • Since TDMA technology separates users according to time, it ensures that there will be no interference from simultaneous transmissions. • It provides users with an extended battery life, since it transmits only portion of the time during conversations. • most cost effective technology to convert an analog system to digital.

Tdma disadvantages • One major disadvantage using TDMA technology is that the users has a predefined time slot. When moving from one cell site to other, if all the time slots in this cell are full the user might be disconnected. • Likewise, if all the time slots in the cell in which the user is currently in are already occupied, the user will not receive a dial tone. • Another problem in TDMA is that it is subjected to multipath distortion. A signal coming from a tower to a handset might come from any one of several directions. It might have bounced off several different buildings before arriving which can cause interference. to overcome this distortion, a time limit can be used on the system. The system will be designed to receive ,treat and process that signal within a certain time limit. Once the time limit is expired the signal is ignored.

960 MHz

124

959.8MHz

TS: Time slot

123

DOWNLIN K 200KHz

935.2 Mhz

GSM utilizes twoDownlink bands of (TDMA 25 MHz.frame) 890-915 MHz = 8 TS band is used for uplink while the 935-960 MHz is used for downlink.

……. ……

0 1

2

935 MHz

915 MHz

45 MHz

UPLINK

124 123

200KHz

……. ……

890.2 MHz

4 5 6 7

The frequency bands are divided into 200 KHz wide channels called ARFCNs (Absolute Radio Frequency Channel Numbers) there are 125 Data burst = 156.25 bit periods =i.e. 576.9µ s ARFCNs out of which only 124 are used.

1

914.8 MHz

2 3

Each ARFCN supports 8 users with each user transmitting / receiving on a particular time slot (TS). 0 1 2 3 4 5 6 7

Uplink (TDMA frame)

2 1

890 MHz

Therefore 1 TDMA frame = 156.25 x 8 = 1250 bits and has a duration of 576.92µ s x 8 = 4.615 ms

frequency division multiple access (fdma) • Divide available frequency spectrum into channels each of the same bandwidth • Channel separation achieved by filters: • Good selectivity • Guard bands between channels • Signaling channel required to allocate a traffic channel to a user • Only one user per frequency channel at any time • Used in analog systems, such as AMPS, TACS • Limitations on: • frequency re-use • number of subscribers per area

Uplink and downlink frequency distribution

Uplink 890 MHz Frequency

0

channel #

Downlink 915 MHz935 MHz

124 0 Example: Channel 48

BTS

Frequency

channel #

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

960 MHz

124

Contd…

The bandwidth of 25 MHz is divided into equal parts of 200 kHz and this band of 200kHz is called as GUARD BAND.  The single radio frequency is called as ABSOLUTE RADIO FREQUENCY CHANNEL NUMBER (ARFCN). Frequency allotment is always done in pairs i.e. if 890.2 MHz is allocated as transmission frequency then 935.2 MHz will be allocated to the same operator as receiving frequency. The spacing between trans and receive frequency is called as DUPLEX SPACING and it is equal to 45 MHz

From Speech to Radio Transmission Blah... Blah... Blah...

Blah… Blah… Blah...

Digitizing and Source Coding

Source Decoding

Channel Coding

Channel Decoding

Interleaving

De-interleaving

Ciphering Burst Formatting Modulating

Deciphering

Burst De-formatting Demodulating

The GSM technology

channels

There are two types of channels- physical and logical channels.

Physical channels Each timeslot on a TDMA frame is called a physical channel. Therefore, there are 8 physical channels per carrier frequency in GSM. Physical channels can be used to transmit data, speech or signaling information. 890MHz

0

915 MHz

1

2

3

4

5

Physical channels

6

7

Logical channels Logical channels are multiplexed into physical channels. They are laid over the grid of physical channels. Each logical channel performs a specific function. It is of two types: + traffic channels (TCHs) + control channels (CCHs)

Traffic channels carry user information such as encoded speech or user data. Two general forms are defined: Full rate traffic channel at a gross bit rate of 22.8kbps. Half rate traffic channel at a gross bit rate of 11.4kbps. Control channels carry system signalling and synchronisation data for control procedures Such as location registration mobile station synchronisation, paging, random access etc. between base station and mobile Station . It is of three types: Broadcast Common dedicated

LOGICAL CHANNELS

TRAFFIC

FULL RATE Bm 22.8 Kb/S

CONTROL

HALF RATE Lm 11.4 Kb/S

FCCH

SCH

BROADCAST

COMMON CONTROL DEDICATED CONTROL

BCCH PCH

CCH -- FREQUENCY CORRECTION CHANNEL CH -- SYNCHRONISATION CHANNEL CCH -- BROADCAST CONTROL CHANNEL CH -- PAGING CHANNEL ACH -- RANDOM ACCESS CHANNEL GCH -- ACCESS GRANTED CHANNEL DCCH -- STAND ALONE DEDICATED CONTROL CHANNEL ACCH -- SLOW ASSOCIATED CONTROL CHANNEL ACCH -- FAST ASSOCIATED CONTROL CHANNEL

RACH

AGCH

SDCCH

SACCH

DOWN LINK ONLY UPLINK ONLY

FACCH

BOTH UP & DOWNLINKS

Broadcast Channel BCH

• Broadcast control channel (BCCH) is a base to mobile channel which provides general information about the network, the cell in which the mobile is currently located and the adjacent cells. • Frequency correction channel (FCCH) is a base to mobile channel which provides information for carrier synchronization. • Synchronization channel (SCH) is a base to mobile channel which carries information for frame synchronization and identification of the base station transceiver

Common control Channel CCH • Paging channel (PCH) is a base to mobile channel used to alert a mobile to a call originating from the network. • Random access channel (RACH) is a mobile to base channel used to request for dedicated resources. • Access grant channel (AGCH) is a base to mobile which is used to assign dedicated resources (SDCCH or TCH)

Dedicated Control Channel DCCH • Slow associated control channel (SACCH) is a bi-directional channel used for exchanging control information between base and a mobile during the progress of a call set up procedure. The SACCH is associated with a particular traffic channel or stand alone dedicated control channel • Fast associated control channel (FACCH) is a bi-directional channel which is used for exchange of time critical information between mobile and base station during the progress of a call. The FACCH transmits control information by stealing capacity from the associated TCH

Location update from the mobile RACH send channel request AGCH receive SDCCH SDCCH request for location updating SDCCH authenticate SDCCH authenticate response SDCCH switch to cipher mode SDCCH cipher mode acknowledge SDCCH allocate TMSI

MS

SDCCH acknowledge new TMSI SDCCH switch idle update mode

BTS

Call establishment from the mobile RACH send channel request AGCH receive SDCCH SDCCH send call establishment request SDCCH do the authentication and TMSI allocation SDCCH send the setup message and desired number SDCCH require traffic channel assignment FACCH switch to traffic chnl & send back (steal bits) FACCH receive alert signal ringing sound

MS

FACCH receive connect message

FACCH acknowledge connect message and use TCH TCH conversation continues

BTS

Call establishment to a mobile Mobile receives paging message on PCH Generate Channel Request on RACH Receive signaling channel SDCCH on AGCH Answer paging message on SDCCH Receive authentication request on SDCCH Authenticate on SDCCH Receive setup message on SDCCH Receive traffic channel assignment on SDCCH FACCH switch to traffic chnel & send back (steal bits)

MS

BTS

Receive alert signal and generate ringing on FACCH Receive connect message on FACCH FACCH acknowledge message and switch to TCH