Minimum Downlink Band Duplex Isolation Requirements for LTE User Equipment

Abstract

Duplexers based on self-interference cancellation can provide substantial transmit-to-receive (Tx-Rx) isolation, whilst being tunable over wide frequency ranges, presenting a promising alternative to fixed-frequency acoustic duplexers commonly used in today's mobile devices. However, the level of Tx-Rx isolation provided by tunable duplexers is typically lower compared to surface acoustic wave and bulk acoustic wave devices. This paper investigates the impact of reduced isolation on receiver (Rx) noise figure (NF), and quantifies the minimum requirement for Tx-Rx isolation in a long term evolution (LTE) mobile terminal. Tx noise in the Rx band is quantified through measurements taken from a cellular handset power amplifier, for a range of duplex separations and uplink bandwidths, and combined with a simple linear model to calculate the desensitized Rx NF as a function of Tx-Rx isolation. LTE downlink throughput simulations are used to assess the impact of isolation on LTE sensitivity, and establish the minimum isolation required for LTE sensitivity specification compliance. Results show that reduced duplexer isolation leads to substantial desensitization for some duplex separation/bandwidth combinations, however, to achieve the minimum LTE sensitivity requires only 38 dB of isolation in the Rx band; this is achievable using current tunable duplexing technologies.