LTE PCFICH channel -Physical Control Format Indicator Channel
This page describes LTE PCFICH channel with respect to LTE system. The Physical Control Format Indicator Channel (PCFICH) is used at the starting of each 1ms subframe. It provides information about number of symbols used for PDCCH transmission. The signalling values for PCFICH depends upon channel bandwidth. The same is mentioned in the following table-1 for different LTE channel bandwidths.
| Channel Bandwidth | ||||||
|---|---|---|---|---|---|---|
| 1.4MHz | 3MHz | 5MHz | 10MHz | 15MHz | 20MHz | |
| PCFICH values | 2,3,4 | 1,2,3 | ||||
As mentioned 1.4MHz requires more time domain symbols compare to other channel bandwidths due to less carriers in frequency domain. Signalling value depends on eNodeB RRM(Radio Resource Management). It depends on number of active connections. Hence PDCCH signalling increases as per increase in number of active connections.
As shown in the figure, LTE PCFICH channel occupies 16 REs(Resource Elements) in first OFDMA symbol of each 1ms frame. PCFICH uses QPSK modulation and hence 16 REs will occupy 32 bits. This 16REs are divided into 4 quadruplets. The position of which in first OFDMA symbol depends on Channel BW and Physical layer cell identity.
As mentioned each quadruplet is mapped to REG(Resource Element Group) with subcarrier index k = k' and is as
per following equation.
k'= (Nsc per RB /2) * (NCellID mod2 NDL-RB )
The rest of three quadruplets are mapped to REGs spaces at intervals of (NDL-RB/2) * (Nsc per RB /2)
from the first quadruplet and each other. This way LTE PCFICH channel information is spread across entire subframe as shown.
The PCFICH carries CFI(Control Format Indicator) which has a value ranging from 1 to 3.
This CFI is coded to occupy complete PCFICH capacity of 32 bits.
Actual value = signalled value + 1 (for 1.4 MHz BW)
Actual value = signalled value (for all the channel BWs)
Overhead due to LTE PCFICH channel
Like LTE PBCH, PCFICH also introduces overhead which reduces number of resource elements needed for user plane data. As mentioned in table-2 overhead is less for higher bandwidth and more for extended CP.
| 1.4MHz | 3MHz | 5MHz | 10MHz | 15MHz | 20MHz | |
|---|---|---|---|---|---|---|
| PCFICH REs per 1ms subframe | 16 | |||||
| Normal CP | ||||||
| Total REs per 1ms subframe | 1008 | 2520 | 4200 | 8400 | 12600 | 16800 |
| Overhead | 1.6% | 0.6% | 0.4% | 0.2% | 0.1% | 0.1% |
| Extended CP | ||||||
| Total REs per 1ms subframe | 864 | 2160 | 3600 | 7200 | 10800 | 14400 |
| Overhead | 1.9% | 0.7% | 0.4% | 0.2% | 0.2% | 0.1% |
LTE PCFICH channel Reference: Document 3GPP TS 36.211
LTE RELATED LINKS
Difference between SC-FDMA and OFDMA
LTE Cyclic Delay Diversity
LTE eNodeB Physical Layer Measurements
LTE EPC Network Inerfaces
LTE UE Physical Layer Measurements
LTE Bearer types
LTE UE Event Measurement Reporting
LTE RSRP vs RSRQ
LTE Signalling radio bearers
LTE QoS
LTE EPS mobility management states
LTE EPS connection management states
LTE UE cell selection
LTE UE PLMN selection
LTE UE Paging Procedure