“It is generally not possible for radios to receive and transmit on the same frequency band because of the interference that results. Thus, bidirectional systems must separate the uplink and downlink channels into orthogonal signaling dimensions typically using time or frequency dimensions,” Andrea Goldsmith, Wireless Communications
Much of wireless research takes half-duplex operation for granted, using time or frequency division for bi-directional communication. But some research studies    , European R&D projects , as well as some commercially deployable solutions , have proven that full duplex communication is possible. Full-duplex communication is considered a way to potentially double the speed of wireless communication and is considered a key technique for 5G systems.
Achieving full-duplex communication involves solving many challenges. Cancelling the self-transmitted signal in various places within the transceiver is of key importance. This means that cancelling must be done:
- At the level of the antennas: This involves using two transmit antennas and one receive antenna. Assuming a wavelength λ, the two transmit antennas are placed at distance d and d + λ/2 from the receive antenna. Offsetting the two transmitters by half a wavelength causes their signals to add destructively and cancel one another. This creates a null position where the receive antenna is able to receive the other, much weaker, signal.
- At the level of analog circuits (before the A/D converter): This implements self-interference cancellation in the analog domain using a noise cancellation circuit. The transmit signal is fed to the circuit as a noise reference, which subtracts it from the received signal, after adjusting for phase and amplitude.
- In the digital baseband: This uses the received digital samples after the analog-to-digital conversion in the receive path. The transmitted samples are stored in a local memory. The received samples are correlated with the transmitted samples to determine the beginning of the transmitted packet and its phase in the received samples. The transmitted samples are rotated samples that almost completely remove the transmitted signal from the received signal. There are a couple of existing baseband interference cancellation techniques, like ZigZag and self-interference cancelation (SIC).
Figure 1 shows the block diagram of a full-duplex system implemented using the three self-cancellation techniques. This full-duplex system assumes reception and transmission each use a single radio.
Figure 1. Existing full-duplex design with three cancellation techniques
In their video , Philip Levis, an assistant professor at Stanford University, addresses the issue of full-duplex wireless. Full duplex has the potential to revolutionize a large number of wireless systems. I highly recommend watching the video.
Are you are interested in live discussions, networking and meaningful input in the area of 5G? Don’t wait to join our LinkedIn group, Towards 5G.
References: A. Goldsmith, Wireless Communications, Cambridge University Press, NY, USA 2005.  Achieving Single Channel, Full Duplex Wireless Communication Jung Il Choi, Mayank Jain, Kannan Srinivasan, Philip Levis, Sachin Katti  Full Duplex Radios, Dinesh Bharadia, Emily McMilin, Sachin Katti  Practical, Real-time, Full Duplex Wireless Mayank Jain, Jung Il Choi, Tae Min Kim, Dinesh Bharadia, Siddharth Seth, Kannan Srinivasan, Philip Levis, Sachin Katti, Prasun Sinha  Beyond Full Duplex Wireless Kannan Srinivasan, Steven Hong, Mayank Jain, Jung Il Choi, Jeff Mehlman, Sachin Katti, and Philip Levis  DUPLO – Full-Duplex Radios for Local Access: http://www.fp7-duplo.eu/  Kumu Networks, http://kumunetworks.com/  Full Duplex Wireless, Philip Levis: https://www.youtube.com/watch?v=Xxr-GGoelKU  Towards 5G, LinkedIn Group: https://www.linkedin.com/groups?home=&gid=7440729.