Frequency Allocation in GSM

Frequency Allocation and its Relation

      

  GSM frequency bands or frequency ranges are the cellular frequencies designated by the ITU for the operation of GSM mobile phones.

·        In India Spectral Allocation is GSM 900 and GSM 1800.

·        GSM Spectral allocation are four type :-

o   P-GSM Spectral (Primary GSM)

o   E-GSM Spectral (Extended GSM)

o   DCS-1800 Spectral

o   PCS-1900 Spectral        

a) P-GSM Spectral (Primary GSM)

·        The initial allocation of spectrum for GSM provided 124 carriers with Frequency Division Duplex for uplink and downlink:

·        Duplex sub bands of width 25 MHz - duplex spacing 45 MHz

·        Uplink sub band: 890 MHz to 915 MHz

·        Downlink sub band: 935 MHz to 960 MHz

·        Frequency spacing between carriers is 200 kHz (0.2 MHz)

·        One carrier is used for guard bands.

·        Total number of carriers (ARFCNs) = (25 – 0.2) / 0.2 = 124

       ARFCNs = Absolute radio frequency channel number.

b) E-GSM Spectral (Extended GSM)

·        E-GSM allocated extra carriers at the low end of the spectrum. The ARFCN numbers of P-GSM were retained (with 0 now included) and new ARFCNs introduced for the lower end, numbered 975 – 1023.

·        Duplex sub bands of width 35 MHz - duplex spacing 45 MHz.

·        Uplink sub band: 880 MHz to 915 MHz

·        Downlink sub band: 925 MHz to 960 MHz.

·         One carrier used to provide guard bands

c)   DCS-1800 Spectral

• —       Digital Communication System – 1800 MHz introduced a further spectrum range for GSM, typically used for smaller micro cells overlaid over existing macro cells.
• —  Duplex sub bands of width 75 MHz - duplex spacing 95 MHz
• —  Uplink sub band: 1710 MHz to 1785 MHz
• —  Downlink sub band: 1805 MHz to 1880 MHz
• —  Frequency spacing of 200 kHz
• —  One carrier used to provide guard bands

     d)PCS-1900 Spectrum

• —  Personal Communication System – 1900 MHz is used in USA and Central America to provide a service similar to GSM.
• —  Duplex sub bands of width 60 MHz - duplex spacing 80 MHz
• —  Uplink sub band: 1850 MHz to 1910 MHz
• —  Downlink sub band: 1930 MHz to 1990 MHz
• —  Frequency spacing of 200 kHz
• —  One carrier used to provide guard bands.

FOR  IDEA

IDEA uses 1800 MHz band. It has duplex sub bands of width 4.4 MHz- duplex spacing 95 MHz. Uplink sub band is from 1740-1744.4 MHz and downlink sub band is from 1835- 1839.4 MHz. Frequency spacing is of 200 kHz. So total number of carriers= 4.4 MHz/ 200 kHz

            = 22 carriers.

Range of ARFCN is 707-719 (13 carriers; BCCH ARFCN), 777-785 (9 carriers; TCH ARFCN). An absolute radio-frequency channel number (ARFCN) is a code that specifies a pair of physical radio carriers used for transmission and reception in a land mobile radio system, one for the uplink signal and one for the downlink signal.  There are also other variants of the ARFCN numbering scheme that are in use for other systems that are not GSM. One such example is the TETRA systems that have 25 kHz channel spacing and use different base frequencies for

Numbering.

 GSM FRAME STRUCTURE:-

 The method chosen by GSM is a combination of Time- and Frequency-Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart (for P-GSM). One or more carrier frequencies are assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame. 8 full-rate or 16 half-rate speech channels per 200kHz channel. There are eight radio timeslots within a radio frame. Half rate channels use alternate frames in the same timeslot. The channel data rate is 270.833 Kbit/s, and the frame duration is 4.615 ms TDMA/FDMA scheme. On top of this frame structure there is multiframe,

superframe and hyperframe structures.

Time division multiple access (TDMA) is a channel access method for shared medium (usually radio) networks. It allows several users to share the same frequency channel by dividing the signal into different timeslots. The users transmit in rapid succession, one after the other, each using his own timeslot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only the part of its bandwidth they require. TDMA is a type of Time-division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters.

 TRX:-

A transmission/reception facility is called as a transceiver (TRX).Typically, a cell has several TRXs, and one frequency is allocated to each TRX. The capacity of a cell can therefore be measured in the number of TRXs. A time slot that carries user traffic is also called as Traffic Channel (TCH). For user speech service 1 time slot/radio frame is reserved in TCH/F and 1 time slot in alternating radio frames in TCH/H. For administrative purposes, e.g. providing mobile terminals with control information, there is also one BCCH in each cell. This channel consumes one time slot (slot #0), and is operated by one TRX in each cell.

A TRX has 8 time slots. Out of which TS0 and TS1 is used for BCCH and signaling data respectively. We suppose there is a sectorised antenna with 3 sectors. The person planning a sector has to observe few rules while doing so, for example, same or adjacent frequencies cannot be given to the next sector as it may cause interference of nearby frequencies. Since there are only 9 carriers for TCH traffic  so the number of TRX’s for TCH in each sector (there are total 3 sectors) can be (2 2 1). TCH frequencies used at each sector are given below:-

§  Sector 1

(777, 783)

§  Sector 2

(779, 785)

§  Sector 3

(781)

Taking into account the TRX’s for BCCH and signaling data also, each sector can have (3 3 2) TRX. BCCH TRX can have any frequency in the ARFCN range 709-719. Now let us calculate the total number of calls that a sector can handle:

3 TRX= 24 time slots.

Time slots available for TCH traffic= 24-2 ( 2 used for BCCH and signaling data)

                                                       = 22 time slots.

Each time slot can handle 2 calls ( half-rate).

Total no of calls that can be handled by 3 TRX = 22x2= 44 calls.

Similarly, for 2 TRX, time slots for TCH= 16-2= 14 time slots.

                Total no of calls that can be handled by 2 TRX = 14x2= 28 calls.

Thus a sector with TRX ( 3 3 2) can engage (44+44+28=) 116 calls at a single time.

As the number of subscribers increases, the call traffic increases and the existing sectors may not be well equipped for taking huge number of calls due to unavailability of frequency associated with each TRX. In such a situation we normally come across what is known as frequency crunch. To overcome this, the network must buy more spectrum.

For instance, when IDEA faced a similar situation, that is, frequency/ capacity crunch it bought 1.25 MHz additional spectrum. Thus the new capacity of each sector became (4 4 3) including TCH TRX and BCCH TRX.

                                                                   

         

Number of calls that can be handled by 4 TRX’s= 32-2 = 30 timeslots x 2calls/ timeslot

                                                                                     = 60 calls

Total number of calls that a (4 4 3) sector can engage at a time= (60+60+44)

                                                                                                        =164 calls.           

* This info was gathered from Internet and Telecom Sources