What is Transmit Diversity?
Transmit diversity is a technique that combats signal fading by transmitting the same information from two or more independent antennas, ensuring that if one transmission path experiences deep fading, the other paths are likely to maintain adequate signal strength. It is the simplest multi-antenna technique and provides reliability improvement (diversity gain) without requiring multiple antennas at the receiver. In 5G NR, transmit diversity is used for broadcast channels and control channels where reliability is more important than throughput.
How Does Transmit Diversity Work?
The most common transmit diversity scheme is SFBC (Space-Frequency Block Coding), based on Alamouti coding adapted for OFDM. With 2-antenna SFBC, the gNB transmits a pair of symbols [s₁, s₂] from antenna 1 and [-s₂*, s₁*] from antenna 2 simultaneously. The receiver, knowing the channel from each antenna, can decode both symbols with full diversity gain using a simple linear combiner. In 5G NR, transmit diversity is specified for PBCH (Physical Broadcast Channel), PDCCH (control channel), and PDSCH (data channel) when the UE reports rank 1 and low SINR. 4-antenna transmit diversity uses frequency-switched transmit diversity or SFBC+FSTD combinations. Transmit diversity provides approximately 3 dB gain per doubling of transmit antennas at the same error rate.
Use Cases
5G NR broadcast channel transmission (PBCH, SIB), control channel reliability (PDCCH), cell-edge coverage enhancement, IoT device coverage extension, and fallback mode for MIMO when channel conditions are poor.
3GPP / Standards Reference
3GPP TS 38.211 (NR SFBC transmit diversity), 3GPP TS 38.214 (NR transmit diversity selection)
Related Terms
MIMO | SU-MIMO | Beamforming | gNB | DL
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