LTE PBCH Channel-LTE Physical Broadcast Channel

 LTE PBCH or LTE Physical Broadcast Channel with respect to LTE system. It is used to broadcast MIB using BCH as transport and BCCH as logical channel.

The PBCH broadcasts a limited number of parameters essential for initial access of the cell such as downlink system bandwidth, the Physical Hybrid ARQ Indicator Channel structure, and the most significant eight-bits of the System Frame Number.

As shown in the figure, PBCH is downlink only channel. It occupies 72 subcarriers belong to first 4 OFDMA symbols of second slot of every 10ms radio frame. Pls. note that reference signal resource elements(REs) are excluded from the PBCH allocation. Hence PBCH will occupy about (72 x 4) - 48 = 240 REs. As PBCH uses QPSK modulation, it leads to about 480bits per 240 REs.

•  These parameters are carried in - a Master Information Block (MIB) (14 bits long). This information is required to decode other physical channels.

•  The PBCH is designed to be detectable without prior knowledge of system bandwidth and to be accessible at the cell edge also.

•  The MIB is coded at a very low coding rate and mapped to the 72 center sub-carriers (6 RBs, each RBs = 12 sub carriers, so 6x12 = 72 subcarriers) of the OFDM structure.

•  PBCH transmission is spread over four 10 ms frames (over subframe 0) to span a 40 ms period.

•  Each subframe is self decodable which reduces latency and UE battery drain in case of good signal quality, otherwise, the UE would soft-combine multiple transmissions until the PBCH is decoded.

After the successful execution of the cell-search procedure, UE decodes the PBCH (MIB/SIBs).

Overhead Generated By LTE PBCH Channel

LTE PBCH represents an overhead which reduces the number of REs available for user plane data. Overhead is less for larger bandwidth and more for extended CP(cyclic prefix). As mentioned in the table below LTE PBCH overhead is in-significant for larger channel BW and significant for small channel BW.

LTE Bandwidth	1.4 MHz	3 MHz	5 MHz	10 MHz	15 MHz	20 MHz
PBCH Resource Elements per radio Frame	240	240	240	240	240	240
Overhead(normal CP)	240/ 10080 = 2.4%	240/ 25200 = 1.0%	240/ 42000 = 0.6%	240/ 84000 = 0.3%	240/ 126000 = 0.2%	240/ 168000 = 0.1%
Overhead(extended CP)	240/ 8640 = 2.8%	240/ 21600 = 1.1%	240/ 36000 = 0.7%	240/ 72000= 0.3%	240/ 108000 = 0.2%	240/ 144000 = 0.2%

LTE PBCH channel REFERENCE: document 3GPP TS 36.211