The networks around us are becoming increasingly dense. For instance, in the past a WiFi Access Point may have been expected to maintain only a single connection, but today it must be capable of supporting several. Similarly, the networks around us are becoming increasingly rich as well. Our personal gadgets increasingly play host to a rich and diverse set of concurrent wireless applications. A single device may wish to simultaneously access the Internet via WiFi, stream video and audio directly via WiFi-Direct from peer devices, and even triple up as a game controller to a gaming console via WiFi-Direct.
Meanwhile, a conflicting course is trending in mobile device design: the real-estate allocated for antennas and other RF components is becoming more and more limited. Equipment manufacturers are increasingly favoring slimmer profiles and larger battery sizes, leaving precious little room to support the radio on each device. However, portable consumer devices such as smartphones must accommodate a growing list of separate protocols (For instance in the ISM band alone – WiFi, WiFi-Direct, GPS, NFC, and Bluetooth need to be supported). Current practice is to use a separate radio and antenna for each protocol, shown in Fig. 2, but as the number of radios increases it becomes difficult to find enough space to separately place all the antennas these radios would need (e.g., the iPhone 4 “antenna-gate” was caused by antennas placed too closely). Is it possible to design a system capable of supporting multiple concurrent protocols whilst using only a single radio and antenna?
The key obstacle is that current radios cannot simultaneously transmit and receive on different arbitrary spectrum fragments with a single, shared Radio Frequency (RF) front end and antenna. The reason for this is that the transmitted signal causes high-powered self-interference, which saturates the RX chain and Analog-to-Digital Converter (ADC), consequently nulling the received signal. While the standard solution is to utilize static, analog RF filters to eliminate the self-interference, such an option is infeasible because spectrum fragmentation is dynamic – available spectrum in the ISM band varies in space and time, depending on the presence of other wireless networks. Consequently, if a radio wants to leverage all the available spectrum and be able to simultaneously transmit and receive on different fragments, the shared analog front end would need programmable analog filters that can be dynamically configured to let only the received signals through and filter out the self-interference. Analog filters however are typically statically configured and programmable analog filters that can be changed dynamically are expensive, lower performing, and impractical to deploy in current radios.
At SIGCOMM’12, we will be presenting the design and implementation of Picasso, a novel full duplex circuit design that sufficiently cancels (instead of filters) the self-interference in analog and prevents RX front end and ADC saturation, enabling the radio to cleanly recover the received signal. Our key contribution here is a circuit design that (1) isolates TX and RX signals at a single antenna by incorporating a circulator , and (2) exploits the fact that the self-interference signal travels through the fixed, known circulator channel to design a passive self-interference cancellation circuit. This allows a radio to simultaneously transmit and receive on arbitrary spectrum fragments even while using a single RF front end and antenna. Our design improves on all prior related work on full duplex wireless since they require at least two antennas (one for TX, one for RX), and these need to be separated by 15 to 20cm, which is untenable for small personal gadgets such as smartphones or tablets.
Picasso leverages this full duplex capability to build an abstraction that allows one to flexibly slice a single radio and antenna into separate independent slices operating on different spectrum fragments. Each slice is associated with a specific spectrum fragment in the ISM band (whose width/position can be programmatically specified). The key property is that the operation of each slice is decoupled from the other slices, i.e., the slice is free to run whatever narrowband PHY and MAC protocols it chooses, and the protocol behavior is not impacted by any other slice that may be present on the shared radio and antenna. Thus, in the above scenarios, the AP would have two slices corresponding to the two spectrum fragments and run two independent WiFi OFDM/CSMA protocols on the two slices in parallel. Similarly, a radio could be shared amongst multiple protocols (e.g. WiFi, Bluetooth, and NFC) by assigning independent slices to the appropriate spectrum fragment and running the corresponding protocol.
Further, to ensure that each slice can use existing, well-engineered narrowband PHY and MAC protocols on each slice, Picasso includes a reconfigurable filter engine that transparently takes signals spread over different spectrum fragments, and efficiently filters and resamples them so that the higher layers just see a simple sample stream consisting of narrowband digital samples. The higher layers are then free to process these samples with any narrowband PHY technique they choose, and schedule access to the slice with a MAC protocol of their choice. The slice, for all intents and purposes, appears as their own piece of spectrum centered at zero, operating on their own radio. Picasso thus completely abstracts out the complexity of spectrum fragmentation.
Steven Hong (Ph.D. Candidate)
Jeff Mehlman (Ph.D. Candidate)
Sachin Katti (Faculty)
Steven Hong, Jeff Mehlman, Sachin Katti, “Picasso: Full Duplex Signal Shaping”, In ACM HotNets 2011, Cambridge, Massachusetts, USA
Steven Hong, Jeff Mehlman, Sachin Katti, “Picasso: Flexible RF and Spectrum Slicing”, In ACM SIGCOMM 2012, Helsinki, Finland.