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.