Sunday, February 16, 2014

BCCH Frequency planning

Following Figure will show how the frequency band is separated for BCCH and TCH channels. For each site consists of three sectors we need to have three BCCH frequencies to be allocated. Direction of a sector is given by its Azimuth angle (horizontal direction measured from North in clockwise rotation). Standard azimuths are 0 °, 120 ° and 240 ° (sector1, sector 2 and sector3). But in practical implementation this will be different with the requirement. Generally in there is a minimum number of two TREs per a sector (1 or 2). Maximum we can go is four TREs per a sector per a band. Functionality of TREs and BTSs are discussed in the Network Operations section. Each channel has a bandwidth of 200 kHz. We can configure TREs in a sector in different ways. I will explain a one way of doing that. We can allocate a BCCH frequency in one of the TREs and for the remaining TREs we need to allocate frequency using Frequency Hopping technique. For a one TRE there are eight Time Slots (TS). We can configure them to be TCH, BCCH, SDCCH and PDCH (for packet data). Also we can configure them to be static or dynamic. BCCH frequency needs to be allocated in a way that it will not interfere with neighboring sites. This will become a difficult task when there are large number of sites nearby.

Frequency Hopping

Dynamically change the frequencies to avoid nearby sites having the same frequency at the same time. Could uses Slow Frequency Hopping (SFH) and channels from x1-xn are to be used. SFH changes its frequency after a TDM multi frame. Therefore hopping frequency is 216.6 Hz (TDM multi frame = 4.616 ms). There are two ways of Hopping is used and they are explained below.

Base Band Hopping (BBH)

Each TRE is given a frequency in a way they avoid interferences. BCCH frequency is also defined for first timeslot of TRE1. Now assigned frequencies are hopped (changed) among each TRE. See Figure: BBH for more details.

Radio Frequency Hopping (RFH)

Using hopping channels, we can obtain sixty three pseudo random sequences. Those sequences are numbered and they are called as Hopping Sequence Number (HSN). Now these Sequences are used instead of direct single frequencies.  RFH causes quality degrades of voice calls. More hopping channels used, higher will be the quality. Hopping channels used in RFH are given a number called Mobile Allocation Index (MAI).  Following Table shows an example of allocating MAI.  Hopping sequences are formed using MAIs. Consider a situation where there are four sectors in sector and we need to allocate frequencies for that. First we may assign BCCH frequency for TRE1. Other TREs are assigned the chosen HSN with an offset in MAI. This offset is called as Mobile Allocation Index Offset (MAIO). Two TREs at same site should have at least offset difference of two. See Figure: RFH Example for more details.

RFH Example

 Further explanation into MAIO

Consider there are four frequencies F1, F2, F3 and F4. See table 2.3.

Now let’s see how MAIO sequences are defined.
MAIO = 0 >> F1 F2 F3 F4
MAIO = 1 >> F2 F3 F4 F1
MAIO = 2 >> F3 F4 F1 F2
MAIO = 3 >> F4 F1 F2 F3

Also it is evident that why I say that two TREs of the same site should have at least an offset of two. That means we can use MAIO 0 and 4 in the same site. Because they don’t interfere each other. When MAIO 0 transmits F1 MAIO 2 transmits F3. They apart by one channel. If we uses MAIO 0 and 1, during the first time interval F1 and F2 will interfere.

HSN and MAIO allocation of three nearby site

There are three sites in this example. TRE configurations of them are as follows.
Site1 – 2, 3, 3
Site2 – 4, 4, 4
Site3 – 4, 4, 4
Sites with number of TREs included in them

 HSN and MAIO allocation is given in the following table.

Frequency Planning

     In mobile telecommunication customers are directly exposed to the air interface. All the weather phenomena and human activities are going on in the same interface. Therefore it is not an easy task to provide and maintain services with higher quality. GSM technology uses Time Division Multiple Access (TDMA)/ Frequency Division Multiple Access (FDMA) in the air interface. Modulation scheme of GSM is Gaussian Minimum Shift Keying (GMSK). Following are the allocated standard frequency bands for GSM telecommunication purposes.

Uplink (UL): Link from MS to BTS
Downlink (DL): Link from BTS to MS

      The low frequency range is allocated for UL because the attenuation of the signals with lower frequencies is lower. Therefore MS can transmit the signal with lower power. And the high frequency range is allocated for the DL. Attenuation does not become a bottleneck for the DL because the BTS can transmit the signal at a higher power level. 

GSM 900 Band

      As shown in the Figure: GSM 900 Band there are 124 channels are available for both UL and DL. Similarly DCS 1800 consists of 374 channels for both UL and DL. For convenience we use set of numbers corresponds to actual frequencies. This set of numbers are known as Absolute Radio Frequency Channel Number (ARFCN). It means there are 124 and 374 ARFCNs for GSM 900 and DCS 1800 respectively. ARFCN ranges:
  • GSM 900: 1-124
  • DCS 1800: 512-885
Example on ARFCN calculation is as follows (similar calculation for DCS as well).

                                ARFCN 65 in UL = 890 + [(65-1)*0.2] + 0.1 = 902.9 MHz

                                ARFCN 70 in DL = 935 + [(70-1)*0.2] + 0.1 = 948.9 MHz

      There are five mobile operators in Sri Lanka. Telecommunication Regulatory Commission Sri Lanka (TRCSL) has allocated each operator a distinct subset of ARFCNs. Given channels are not enough to cater a large customer base. Therefore frequencies needed to be reused effectively. When we reuse the frequencies, need to consider co-channel interference (interference created by two carriers which have the same frequency. Interference margin for two channels is -9 dBm.) and adjacent channel interference (interference created by two carriers which have adjacent frequencies. i.e. channel 60 and 62 are adjacent to channel 61. interference margin for two channels is 9 dBm.).

       A carrier is identified in the air interface by Broadcast Control Channel (BCCH) and Base Station Identification Code (BSIC). BCCH channel is used for the signaling phase of the air interface. BSIC is composed with two parts Base-station Color Code (BCC) and Network Color Code (NCC). For Etisalat Lanka permitted values for NCC are x and y, permitted values for BCC are a to n (actual values not mentioned here). BSIC is used to distinguish between the BTS using same BCCH frequency. Therefore co-BCCH and co-BSIC combinations should be avoided in cells influencing each other.