RFLTE PBCH Channel Explained: Physical Broadcast Channel
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This page describes the LTE PBCH, or LTE Physical Broadcast Channel, within the LTE system. It’s used to broadcast the Master Information Block (MIB) using the Broadcast Channel (BCH) as the transport channel and the Broadcast Control Channel (BCCH) as the logical channel.
The PBCH broadcasts crucial parameters needed for initial cell access. These include the downlink system bandwidth, the Physical Hybrid ARQ Indicator Channel (PHICH) structure, and the most significant eight bits of the System Frame Number (SFN).
Image alt: LTE PBCH Channel
As illustrated in the figure, the PBCH is a downlink-only channel. It occupies 72 subcarriers within the first 4 OFDMA symbols of the second slot of every 10ms radio frame. It is important to note that reference signal resource elements (REs) are excluded from the PBCH allocation. Therefore, the PBCH occupies approximately (72 x 4) - 48 = 240 REs.
Since PBCH utilizes QPSK modulation, this translates to about 480 bits per 240 REs.
Key aspects of the PBCH:
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These parameters are conveyed in the Master Information Block (MIB), which is 14 bits long. This information is crucial for decoding other physical channels.
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The PBCH is designed to be detectable without prior knowledge of the system bandwidth and to be accessible even at the cell edge.
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The MIB is coded at a very low coding rate and mapped to the 72 center sub-carriers (6 Resource Blocks (RBs), where each RB comprises 12 subcarriers, hence 6 x 12 = 72 subcarriers) of the OFDM structure.
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PBCH transmission is spread across four 10 ms frames (occurring in subframe 0) to cover a 40 ms period.
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Each subframe is self-decodable. This reduces latency and conserves UE battery in cases of good signal quality. Otherwise, the UE can soft-combine multiple transmissions until the PBCH is successfully decoded.
Following successful cell-search execution, the UE decodes the PBCH (MIB/SIBs).
Overhead Generated by LTE PBCH Channel
The LTE PBCH introduces overhead, reducing the number of REs available for user plane data. The overhead is less pronounced for larger bandwidths and more significant for extended Cyclic Prefixes (CP). As demonstrated in the table below, the LTE PBCH overhead is insignificant for wider channel bandwidths but more noticeable for smaller ones.
| 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
