Tunable Duplexers: Downlink Band Isolation Requirements for LTE User Equipment

Tunable duplexers based on self-interference cancellation present a promising alternative to fixed-frequency duplexers (e.g., surface acoustic wave devices), however the level of transmit-to-receive (Tx-Rx) isolation they provide is typically lower compared to acoustic devices. This letter quantifies the impact of reduced isolation on receiver (Rx) noise figure (NF), and determines minimum isolation requirements for Tx-Rx isolation in cellular handset transceivers. Measurements of Tx noise power spectral density from a cellular handset power amplifier are combined with theoretical analysis and long term evolution (LTE) downlink throughput simulations to assess the impact of isolation on the NF and sensitivity. Although lower duplexer isolation can lead to substantial desensitization for some duplex separation/bandwidth combinations, achieving LTE compliant sensitivity requires only 38 dB of isolation in the Rx band. LTE sensitivity specification compliance is therefore readily achievable with current tunable duplexing technologies.