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 , which is easily realized.
The second concern about transmit signal leakage is its out-of-band phase noise. This power can enter the Rx band and affect its sensitivity. The Radio420X shows a typical phase noise of -140 dBm/Hz at a 20 MHz offset with a 2000 MHz carrier, measured with 0 dBm of output power. Assuming that the phase noise remains constant at greater offsets, -122 dBm/Hz (for 18 dBm of output power) of the transmitted signal noise spectral power density reaches the Rx band. The receiver sensitivity, -103 dBm, is measured within a 200 kHz bandwidth with a 5 dB signal-to-noise ratio (SNR). In order to allow the transmitter to affect sensitivity by no more than 0.5 dB, the transmitter noise power needs to be 9 dB below the noise floor, which correspond to -117 dBm. The corresponding phase noise power for a maximum power output of +18 dBm is -69 dBm (-122dBm/Hz + 10log(200kHz)), which is within the LTE specification of -50 dBm for maximum emission from the UE transmitter in its own receive band.
Finally, to get to the -117 dBm target, we need to isolate the antennas by 48 dB. This can be performed easily with an external low-cost ceramic duplexer. However, for a dual separate TX-RX antenna setup, this requires a horizontal and vertical spacing of about 68 cm and 41 cm respectively . Keep in mind that these requirements only have to be met when the transmitter is set to maximum output power in order to not affect the receiver sensitivity.
The worked-out example shows that the main concern regarding transmit signal leakage, in typical conditions, is the transmitter phase noise. The out-of-band noise power will enter into the receiver band and affect the whole Rx path, degrading its sensitivity. This demonstrates how different specifications can critically interact with each other. In order to meet today’s wireless communication standards, transceivers such as the Nutaq Radio420X must have flawless performance for each parameter.
 International Telecommunication Union. Isolation between antennas of IMT base stations in the land mobile service.http://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-M.2244-2011-PDF-E.pdf