The challenges and benefits of achieving two-way wireless
With many wireless systems in use today, at any given time you are either talking, or you are listening…much like the walkie-talkies of years ago. This inability to simultaneously talk & listen is quite different from other two-way communication systems we are used to like the telephone and DSL…and it has a significant impact on elements such as bandwidth usage.
So why has two-way wireless been difficult to achieve up to now? Picture a situation where you are simultaneously trying to listen to someone who is whispering, while you yourself are shouting loudly. It’s not easy to do. From a wireless perspective, it is this idea of overcoming self-interference arising from a strong transmitted signal and a weaker received signal which has been quite challenging.
The driver to overcome this two-way wireless hurdle is driven by some significant expected benefits including:
- Improved quality of service and efficiency
- Increasing the available bandwidth for voice and data transmission by at least a factor of two
- Improved security
University of Waterloo makes two-way wireless a reality
The Coding and Signal Transmission Laboratory at the University of Waterloo, under the leadership of Professor Amir Khandani, has used Nutaq’s Software Defined Radio solutions to realize simultaneous two-way wireless transmission.
Consider the situation where you have one access point serving several different clients, where OFDM tones are divided among the clients (OFDMA) with two-way communications over each tone.
- Supported MIMO for each tone
- Allowed wireless clients to be asynchronous
- Used simple, small antennas combined with simple signal processing
- Did not require clients to have full duplex capabilities
The self-interference cancellation process
The key to achieving this two-way wireless breakthrough was implementing a self-interference cancellation mechanism. Referring back to the shouting & whispering analogy…they needed to cancel all the shouting so they could hear the whispered signal coming at them.
The main elements contributing to the self-interference cancellation were:
- A first stage of RF cancellation achieved through antenna design
- A second stage of RF cancellation achieved through active cancellation
- A cancellation and signal recovery achieved at baseband
In the diagram below (courtesy of Professor Amir Khandani and the CST Laboratory…original presentation can be found here) you can see the several complementary stages of interference removal & signal reconstruction. Nutaq’s Model-Based Design Kit (MBDK) was used to help the CST team accelerate the development of their signal processing algorithms, which contributed to the overall self-interference cancellation.
Hardware implementation & experimental results
The radio was based on 802.11, with a 20MHz bandwidth at 2.4GHz ISM band. A 64 tone OFDM signal was used with a transmission power of 30dBm. You can watch a video which captures the CST lab results in real-time here.
The measured results were impressive as can be seen from the data below. The residual self-interference to noise ratio after each of the interference cancelling stages was:
- After the antenna structure alone: 40dB
- After the analog cancellation stage: ~ 2dB
- After the digital cancellation stage (implemented on the Nutaq SDR platform): ~ .4dB
Potential applications of two-way wireless
Beyond more than doubling the bandwidth of voice & data services, two-way wireless can also open up other new applications including:
- Security enhancement
- Media-based wireless connectivity
- New methods for interference management
Professor Khandani continues his work in two-way wireless by working with industry to get two-way wireless to become part of wireless standards, accelerating its widespread implementation.