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Full-Duplex Radio Integrated Circuit Could Double Radio Frequency Data Capacity
Zothecula writes Full-duplex radio communication usually involves transmitters and receivers operating at different frequencies. Simultaneous transmission and reception on the same frequency is the Holy Grail for researchers, but has proved difficult to achieve. Those that have been built have proven complex and bulky, but to be commercially useful in the ever-shrinking world of communications technology, miniaturization is key. To this end, engineers at Columbia University (CU) claim to have created a world-first, full-duplex radio transceiver, all on one miniature integrated circuit.
The (copper) Gigabit Ethernet PHY transmits and receives simultaneously on four wire pairs. It accomplishes this with a hybrid that subtracts the transmitted signal from the one being received. Last year some newer WiFi access points debuted that could do the same thing with RF. (Gigabit Ethernet is technically RF too because each of the four wire pairs operate at around 125MHz. WiFi access points operate in the 2.4GHz and 5.4GHz bands.)
The issue is that a strong transmission in the same band as a receiver can desense the receiver. This can also be done with a cavity duplexer if you need input and output in the same band on adjacent frequencies, but you pay for it with geometric space (since cavity duplexer dimensions are a fraction of the wavelength in free space multiplied by the materials velocity factor). This can be problematic on HF and VHF bands, but UHF and microwave can get away with duplexers the size of a brick. Unfortunately, that's still too much for mobile phones since it's too big to fit in someone's pocket.
This may be a solution, but it is not clear there is actually a problem it solves.
Does this enable more total data to be transmitted where there are multiple users in a band? When they are using spread spectrum and reception conditions are poor, and one or both ends are moving through buildings or spaces occupied by reflective surfaces?
I am sure someone will buy the patent, but much less sure it will turn out to be value for money.
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Hi all, I was perusing through all the comments, and as one of the authors of the work, I thought I would clarify some of the points that were raised to aid the discussion: 1. The chip targets same-channel full duplex, meaning the transmitter and the receiver work in the same frequency channel at the same time, and are not separated by polarization, modulation format etc. Therefore, since transmitted signals are around +20dBm and receiver sensitivity levels are around -90dBm, nearly 110dB of suppression through isolation (across a pair of antennas or a circulator) and echo (aka self-interference or SI) cancellation must be achieved (as one of the people above has correctly pointed out). Such a high degree of SI cancellation requires that SI cancellers be implemented in all domains (RF, analog and digital, each yielding a part of the total SI suppression). 2. As one of the people above has pointed out, even if the signals were separated in modulation format for instance, the transmitter SI would be so powerful that it would saturate the receiver front end before modulation-format-based separation can be achieved in the digital domain. So echo cancellation at the receiver front end is required. 3. As someone points out, circulators and echo cancellers have existed for quite a while and have been implemented in many ways. The innovation here is that we perform echo or SI cancellation at RF in a single chip, which has not been done before. 4. Moreover, the SI cancellation approach can tackle echos that experience significant delay (as high as 20ns) while still fitting with an IC form factor through the use of on-chip reconfigurable high-Q filters, enabling cancellation of wideband signals (>20MHz enabling use for WiFi). 5. Finally, indeed the varying environment is a challenge and the RF and digital SI cancellers need to be reconfigured periodically (milli-seconds). Hope this helps.
flatulus: Thanks for the comments. They are spot on.
- It is true that there are benefits beyond full duplex, namely in reducing duplexer filter requirements for FDD. We have received commercial interest for this application as well. LTE provides support for 24 FDD bands, a lot more than 3G. Having 24 fixed-frequency duplexers in a handset is near impossible. So, there is interest in tunable duplexers that can cover multiple bands but inevitably have reduced isolation and greater insertion loss than conventional fixed-frequency duplexers. Self-interference cancellation can be used to enhance the isolation back to the 55dB levels seen in conventional fixed-frequency duplexers. In our ISSCC paper, which has not yet been uploaded to IEEExplore, we show measurements for the FDD use case as well.
- It is true that cellular base stations will require 20-30dB higher isolation. I think the higher-gain antennas do offer an isolation advantage but not as much as the increase in antenna gain because the antenna-to-antenna coupling is a near-field phenomenon. In base stations, where form factor is less of a concern, discrete-component based approaches can be used as a first line of defense, followed by IC-based fully-integrated cancellation. Also, WiFi base stations and cellular small cells have lower transmit power levels and so are more direct applications for this IC technology.
- I did not follow the MWC, but as far as I know, in the literature, SAW-less receivers for TDD have been reported and have made it into phones, but duplexer-less receivers for FDD have not yet been reported. The SAW-less receiver problem is easier because one has to deal with jammers picked up from the environment, which tend to be a lot weaker than transmitter self-interference.