PCFICH mapping

Physical Control Format Indicator Channel (PCFICH)

Introduction

'Channels' are used to differentiate different kinds of traffic on radio path. For example, data channels carry users traffic (Youtube, Skype data etc) and control channels carry signaling traffic to manage radio resources, setting up connection etc

The Physical Control Format Indicator Channel (PCFICH) is one of the control channels that works at physical layer. It is used to dynamically indicate the number of symbols to be used for PDCCH.

Why do we need PCFICH ? 

With the help of PCFICH channel, following scenarios are possible:

- Use less symbols for PDCCH if there are a few users with high data rates. Thus leaving more resource elements to be used for user plane data (PDSCH)

- Use more symbols for PDCCH if there are many users with lower data rates e.g VoIP calls in the cell, thus allowing more users capacity.

Signalled value

PCFICH signalled value depends on channel bandwidth. For channel bandwidth of 3MHz up to 20 Mhz it can carry value of 1, 2 or 3. But for 1.4 Mhz channel bandwidth it can carry value of 2, 3 or 4. Because in case of 1.4 Mhz bandwidth, there are few subcarriers in frequency domain.Therefore, more space is required in time domain to carry PDCCH symbols

Location of PCFICH symbols on Resource Grid

PCFICH occupies 16 resource elements in frequency domain.  These resource elements are divided into groups of four quadruplets distributed within first OFDMA symbol of each 1 ms subframe. The exact position of PCFICH can be measured from Cell ID and bandwidth using formula given in 3GPP spec 36.211 as below

Where

N^RB_SC = Number of frequency carriers per Resource block

N^DL_RB = Number of resource blocks per bandwidth

N^cell_ID  = Physical Cell id

It may look complicated but lets try to understand it with simple example

Lets suppose
Physical Cell id = 20
Bandwidth = 10Mhz  (N^DL_RB = 50)

Then according to 3GPP Formula

k_Bar = (12/2).(20 mod 2*50) = 6*20 = 120

Then the four PCFICH mapping values are below 

120

120 + (50/2)*(12/2) = 270

120 + 2*(50/2)*(12/2)  = 420

120 + 3*(50/2)*(12/2) = 570