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A basic STAR or Full Duplex (FD) dual antenna architecture is shown in Figure 1. It consists of three levels of cancellation to reduce the self-interference:
- The antenna which is the first level to reduce self-interference between the transmit and receive signals.
- The analogue (RF) cancellation provides a second level to prevent overloading the LNA and receiver circuitry. It often consists of a coupler to tap off part of the transmitted signal, manipulate its gain and phase with an attenuator and phase shifter respectively before injecting that into the receive path to cancel the self-interference.
- The digital cancellation is the third and final level. It also injects a version of the transmitted signal into the received path to further cancel self-interference.
Figure 1: A Dual Antenna FD Architecture
Editors note... the color scheme above is an excellent choice for presentations occuring on May 5 (Cinco de Mayo)
Most communications systems require a very high level of self-interference cancellation (SiC). For example, the 2.45 GHz Industrial and Medical Band (ISM) Wi-Fi has a maximum transmit power of 100 mW (20 dBm) and a receiver sensitivity in the order of -100 dBm. Assuming a 10 dB demodulator signal-to-noise ratio (SNR), 130 dB of SiC is required for reliable communication. In dual-antenna systems, a degree of isolation already exists as there are two antennas. This can be provided in three ways:
- A large physical separation between the antennas, which can be difficult where physical size is critical.
- Directional antennas whose beam patterns are such that they sit in each other’s nulls.
- Dual polarised antennas where one polarisation is for transmission and another for reception.
Antenna isolation is highly dependent on the environment, any nearby structure can reflect the transmitted signal directly into the receiver antenna. This is particularly crucial in mobile devices where the environment is changing. To counteract this the analogue and digital cancellation must continually update during operation by feedback from the baseband. Some applications like mmWave backhaul as produced by companies like MIMOTech and Dragon Wave X are less susceptible as they operated over line-of-sight (LoS) links with highly directional antenna. However, there are many applications where the two antennas are not possible either due to physical limitations or omnidirectional antenna are required. For these, single antenna options are preferred.
Single Antenna
A single antenna FD architecture is shown in Figure 2. It is similar to the dual antenna architecture, except that there is only one antenna and a combining or coupling network is required to interface both transmit and receive paths to that antenna. This combining network is generally one of four types:
- A circulator which has minimum insertion loss but is physically large and expensive.
- A Wilkinson power splitter which can be broadband but has 3 dB insertion loss.
- A hybrid coupler which has narrower bandwidth than a Wilkinson, but still with the 3 dB insertion loss.
- A rat-race coupler which is again has narrower bandwidth than a Wilkinson and also has 3 dB insertion loss.
Figure 2: A Single Antenna FD Architecture
All of these combining options provide only limited isolation between transmit and receive paths of around -30 dB. Any insertion loss in the combining network will reduce POUT – and hence efficiency – and increase the receiver noise figure (NF) – reducing sensitivity. However, the hybrid and rat-race do lend themselves to an additional architecture whereby the antenna combiner and analogue cancellation can be incorporated into a single unit.
Incorporated Combining Network and Analogue Cancellation
Combining the two elements in this way reduces the physical size of the FD system, but is only possible with the four-port hybrid and rat-race couplers. Their fourth port – called the isolation port – is usually terminated with a 50 Ω load. To increase SiC the 50 Ω load can be replaced with a variable impedance (ZV) as shown Figure 3, which reflects part of its incident power back into the combiner. If the gain and phase of this reflected signal is correct, it will cancel with any leakage through the combining network due to its limited isolation, thereby increasing SiC. This can additionally cancel any self-interference due to antenna mismatch, a problem unique to single antenna architectures.
Figure 3: A Single Antenna FD Architecture
Antenna Mismatch
Single antenna architectures are susceptible to antenna mismatch where a portion of the transmitted signal is reflected by the antenna back into the combining network and hence the receive path. This can be quite large as shown in Figure 4 where three Commercial-Off-The-Shelf (COTS) Wi-Fi dipole antennas were measured on a VNA. In the case of Antenna C the magnitude of the reflection at 2.45 GHz is -30 dB relative to the transmitted, of comparable size to the isolation of the combining network. This degrades to -20 dB at the edges of the ISM band. Antenna A is even worse. However, the analogue cancellation block or four-port combiner with ZV can compensate for antenna mismatch. For this to be the case though, the signal reflected from ZV must have identical amplitude as that from the antenna and be exactly in anti-phase.
Figure 4: Measured S11 of Three Dipole Antennas
Gain and Phase Imbalance
Assuming the antenna provides 20 dB of the 130 dB required SiC discussed above, the analogue and digital cancellation must provide an additional 110 dB. Achieving 110 dB by purely either analogue or digital is virtually impossible as the signals to be cancelled must be very closely matched. This is shown in Figure 5 where, for 60 dB cancellation the maximum tolerable gain and phase imbalance must be better than 0.1 dB and the phase 0.01°. The precision required suggests that analogue control elements are needed based on varactor and PIN diodes driven by high resolution digital-to-analogue converters (DAC) as oppose to digital control elements which tend to have very coarse resolution.
Figure 5: Gain and Phase Imbalance
The antenna and analogue cancellation sections reduce the self-interference presented to the LNA to below a level where it will not generate distortion which would interfere with the received signal. It is also important that the wanted signal and self-interference fit within the resolution of the analogue-to-digital converter (ADC). ADCs have a trade-off between resolution, sample rate and power consumption. The ADC sample rate is not only related to the signal bandwidth, but also the chosen receiver architecture, i.e. direct conversion or IF sampling. The remaining SiC will be then be handled with the digital cancellation.
Digital Cancellation
Where the antenna and analogue provide maybe 60 dB SiC, the remaining 70 dB must be provided digitally. Significantly more complex cancellation is possible in the digital domain if implemented on a digital signal processor (DSP) or field programmable gate array (FPGA). Whereas the analogue cancellation usually consists of just one path with single gain and phase control elements, in the digital domain, multi tap Finite Impulse Response (FIR) filters can be implemented to define complex gain and phase responses. This is necessary to compensate for the frequency dependent response of analogue components, particularly over large operational bandwidths.
Operational Bandwidth
The antenna response shown in Figure 4 indicates that the impedance of the sample antennas is likely to vary significantly over frequency, therefore the gain and magnitude of the antenna reflection will also vary over frequency. Single path analogue cancellation is only capable of providing a good gain and phase match over a very narrow bandwidth, resulting in a SiC profile with a sharp null. One solution is to have multiple analogue cancellation paths to match the gain and phase at multiple points over the bandwidth of interest. If the combined combining network and analogue canceller is used, ZV can be designed to mimic the impedance of the antenna, and therefore better track its impedance response. In the digital domain, the FIR filter can be increased in size, but there could be a limit here dependent on the application that FD is to be used with.
Use Cases
FD has recently been touted as a future Wi-Fi technology and was to be included in 802.11ax. However, the overheads proved too much, particularly on the digital side where a rapid update rate was required due to the dynamic operating environments. More likely applications are for LoS backhaul as mentioned above. FD has also previously been used in defence applications. In the late 70s Plessey Avionics sold the PTR3411 "Groundsat" VHF Transceiver.
Recently a renewed interest in the technology seems to be for half-duplex systems operating on two different, but closely spaced frequency channels. Adopting an FD type technology here can reduce or eliminate the high performance bandpass filters in the receiver frontends.