In today’s high-speed wireless communication standards like LTE, the performance of both base transceiver stations (BTS) and user equipment (UE) transceivers is crucial. LTE supports time-division duplexing (TDD) as well as frequency-division duplexing (FDD). In this post, we look at transmit signal leakage problems that can occur in FDD applications. To do so, we show a numerical analysis using specifications from the Nutaq Radio420X and the standard LTE performance requirements.
Frequency-division duplexing and isolation
FDD implies that the transmitter and receiver operate at different carrier frequencies, allowing for constant and simultaneous transmission and reception. In full-duplex FDD mode, the transmitter signal leakage must be taken into account (this does not apply to TDD or half-duplex modes). The receiver is constantly exposed to this transmit signal leakage and its sensitivity can drop drastically if improper isolation is used. Most of the isolation is obtained with a good PCB layout and shielding, but one will always have to use effective filters/duplexers in order to achieve optimal isolation.
The Radio420X’s receiver has a software-selectable band-pass filter bank. Its filters typically have 40 dB of rejection on either side of the bandwidth. Figure 1 shows a simplified block diagram of the Radio420X transceiver section.
Figure 1 – Simplified Radio420X transceiver block diagram
Transmit signal leakage
Clearly, the fundamental components of the transmit signal can interfere with the received signal, but this is not the only concern. The transmit signal will also generate out-of-band phase noise that falls within the receiver band. This unwanted power affects the receiver sensitivity by raising its noise floor, as shown in Figure 2.
Figure 2 – Out-of-band phase noise effects on sensitivity
Let’s look at a numerical example using the LTE Band 1. It operates within the following frequencies:
- Uplink (UE transmit): 1920 – 1980 MHz
- Downlink (UE receive): 2110 – 2170 MHz
Assume that we want to operate in full-duplex FDD using carrier frequencies 1920 and 2110 MHz for a UE transceiver. The Radio420X’s specifications will be used in the following calculations.
First, we determinate how much power will be leaking into the Rx path when operating at the maximum output power. We know that the fundamental Tx component will be filtered out by 40 dB when it reaches the band-pass filter. However, the first variable amplifier of the Rx chain is placed before the filter and is set to a maximum gain of +18 dB for best sensitivity. Its OP1dB is 20 dBm, so any input signal greater than 2dBm will saturate this amplifier and block the whole receiving process. Thus, we need a minimum of 16 dB Tx/Rx isolation to avoid this situation. Knowing that the PCB traces isolation is better than 55 dB, the only worry is about antenna isolation (the Radio420X uses two antennas instead of a duplexer). At 1960 MHz, 30 dB antenna isolation is achieved with a horizontal separation distance of 12 cm (for a -5 dB gain in the direction of the other antenna), or a vertical separation of 17 cm