PHY - PBCH

PBCH - Physical Broadcast Channel

PDCH is a downlink only channel. After the successful cell search, UE needs to decode the PBCH to know the few vital informations like

1. System Bandwidth   ------ 3 Bits
2. PHICH information ------- 3 Bits
3. System Frame Number (SFN) ---- 8 Bits

So total 14 Bits of Useful content. Along with that 10 reserved bits added upto 24 Bit Data, comprises a MIB in LTE

MIB will always be carried in the 4 OFDM symbols in the 2nd Slot of the every 10ms frame. So lets calculate the RE's allocated in each MIB in every frame.

Consider the following before we go for the calculations of MIB RE'S.

1. LTE is with FDD with Channel Bandwidth of 10 Mhz.

2. 50 Resource Blocks are supported in the 10Mzh channel Bandwidth.

3. Normal cyclic prefix is followed.

As we know very will, in LTE, 

1 Frame -- 10 ms

1 Subframe -- 1 ms

1 Slot - 0.5 ms

In a time domain, we have 14 OFDM symbols per RB

In Frequency domain, we have 12 Subcarriers per RB

A OFDM symbol in a Time domain mapped with a Single Subcarrier comprise a Resource Element, RE.

So Total available RE's in 1 resource block = 12 * 14 = 168 RE's.

So for a 10Mhz BW, total RE's = 168 * 50 = 8400 RE's

Since, MIB is occupying 4 OFDM symbols in the 2nd Slot in a Frame for 6 RB's, the total Resource Elements allocated for MIB is:

Total RE's for MIB = No of RB's * Number of Subcarriers * Number of OFDM symbols.

                                = 6 * 12 * 4

                                = 288 RE's

So in terms of Bits, how do we calculate? As we know PBCH uses QPSK modulation, it needs 2 Bits to transfer a symbol. 

So, Total number of Bits required to Transmit a MIB = Bits used for QPSK * Number of RE's

                                                                                     = 2 * 288 = 576 Bits

  One more Interesting thing about MIB transmission is about the Duplication Transmissions. Why do we need the Duplicate transmissions? If the UE decodes the MIB in the 1st Frame successfully, why the duplicate Transmission is required for the next 3 Frames?

Example of PBCH decoding for MIB: