Practical, real-time, full duplex wireless
Summary (4 min read)
1. INTRODUCTION
- Wireless radios today are generally half duplex.
- Antenna cancellation, when combined with the other mechanisms, allows full duplex operation: Choi et al. evaluate working 802.15.4 (1mW) full duplex prototypes which are close to (within 8%) an ideal full duplex system.
- Antenna cancellation-based designs have three major limitations.
- Furthermore, having two transmit antennas creates slight null regions of destructive interference in the far field.
- While manual tuning is sufficient for lab experiments demonstrating a proof of concept, it leaves open the question of whether it is possible to build a full duplex system that can automatically adapt to realistic environments.
2. RADIO DESIGN AND FULL DUPLEX
- This section provides background on the basics of radio design as well as full duplex.
- It explains the basic challenges in building a full duplex radio, the existing techniques for doing so, and the limitations of those techniques.
2.1 Radio Design
- Figure 1 shows the basic design of a modern radio receiver.
- Because digitally sampling a 2.4GHz signal would require very high speed sampling at the Nyquist frequency of 4.8GHz, radios downconvert a RF sig- 1These channel sounders are wideband (∼240MHz) radios used for RF profiling, programmed to generate a single wideband pilot pattern for measuring the channel.
- Nal to a baseband signal centered around 0Hz.
- To transmit a packet, a radio generates digital samples for the desired waveform, converts them to a baseband signal with a digital to analog converter (DAC) and upconverts the baseband to RF.
2.2 Cancellation
- The problem is that a node hears not only the signal it wants to receive, but also the signal it is transmitting.
- By canceling this self-interference from what it receives, a full duplex node can in theory decode a received signal.
- Digital cancellation, while helpful, is by itself insufficient: current systems in the research literature cancel up to 20-25 dB [8, 7].
- The limitation is that ADCs have a limited dynamic range: since self-interference is extremely strong, an ADC can quantize away the received signal, making it unrecoverable after digital sampling.
- One approach to analog cancellation uses a second transmit chain to create an analog cancellation signal from a digital estimate of the self-interference [5], canceling ≈33dB of self-interference over a 625kHz bandwidth signal.
2.3 Full Duplex
- Motivated by the above limitations, recent work has proposed antenna placement techniques [4, 5].
- The separation between the receive and transmit antennas attenuates the self-interference signal [5], but this antenna separation is not enough.
- The key idea behind antenna cancellation is to use a second transmit antenna and place it such that the two transmit signals interfere destructively at the receive antenna.
- Combined with the analog and digital cancellation described above, the design is able to cancel 50-60dB of self-interference, which is sufficient to operate a full duplex 802.15.4 radio.
- The third limitation relates to the details of Choi et al.’s design.
3.1 Eliminating the Bandwidth Constraint
- The design is based on a simple observation: any radio that inverts a signal through adjusting phase will always encounter a bandwidth constraint that bounds its maximum cancellation.
- The key insight of this paper is that one can use a balun in a completely new way, to obtain the inverse of a self-interference signal and use the inverted signal to cancel the interference.
- To cancel selfinterference, the radio combines the negative signal with its received signal after adjusting the delay and attenuation of the negative signal to match the self-interference.
- Nal before combining it, which may be hard to achieve in practice.
- The rest of this section examines these practical concerns and how to solve them.
3.2 Balun Benefits
- The first wire is an ideal selfinterference path and has a 20dB attenuator representing the antenna separation.
- An RF combiner adds the two signals on the received side to measure the canceled signal.
- In both cases, the passive delay line and attenuator provide fine-grained control to match phase and amplitude for the interference and cancellation paths to maximize cancellation.
- In comparison, the balun based circuit provides a good degree of cancellation over a much wider bandwidth.
- Based on Figure 5, the authors can obtain the best possible cancellation with balun and phase-offset cancellation for a given signal bandwidth.
3.3 Auto-tuning
- The results in Figure 6 show that, if the phase and amplitude of the inverted signal are set correctly, balun cancellation can have impressive results across a wide bandwidth.
- Since the QHx220 is an active component, the authors call this cancellation scheme Balun Active Cancellation.
- It emulates a variable delay by controlling the attenuation of the in-phase and quadrature signals (gi and gq), adding them to create the output.
- Hence the authors can use the same gradient descent algorithm for tuning the two attenuation factors in QHx220, gi and gq .
- It then reverses direction, reduces the step size and attempts to converge to the optimal point.
3.4 Digital Cancellation
- But this cancellation only handles the dominant self-interference component between the receive and transmit antennas.
- The full duplex radio design uses digital cancellation (DC) to cancel any residual interference that persists after balun cancellation.
- The estimation uses the least square algorithm [15] due to its low complexity.
- Let s[n] be the known transmitted digital sample at time n fed into the FIR filter.
- A node needs to re-estimate its channel state at a period below the channel coherence time.
4. FULL DUPLEX MAC
- The most interesting possible benefits of full duplex occur above the physical layer.
- Prior work, for example, argued that MAC layer techniques that exploit full duplex can mitigate hidden terminals and improve fairness [4, 13].
- The lack of a real-time full duplex MAC layer prevented experimentally evaluating these claims.
- The authors describe the design and implementation of a simple full duplex MAC.
- Later sections evaluate a real-time full duplex network using this MAC.
4.1 Design
- In CSMA/CA, the hidden terminal problem occurs because a half-duplex receiver cannot inform other nodes of an ongoing reception.
- Full duplexing naturally mitigates the hidden terminal problem because the receiver can immediately start a transmission that also suppresses nearby nodes.
- Furthermore, when a WLAN network is saturated, a bottleneck occurs at the AP because it gets the same transmit opportunity as each client while serving more flows.
- The authors MAC uses busytones as a way to mitigate this problem.
- Whenever a node finishes transmitting a packet before finishing receiving, it transmits a predefined signal until its reception ends.
4.2 Real-Time
- The previous section has described the design of a full duplex MAC protocol.
- The earlier the destination address is in the packet and the faster the hardware can initiate a secondary transmission, the lower the secondary response latency becomes.
- Typical software-defined radios such as USRP [2] cannot meet these requirements, due to latency between hardware and the host PC as well as their need to store packets as digital samples rather than bits.
- Each OFDM symbol is 8µs long, thus QPSK modulation achieves 12Mbps bitrate without any channel coding.
- Primary transmissions use the existing half duplex MAC implementation from the same reference design which mimics 802.11 behavior.
5. EVALUATION
- This section provides experimental results for the performance of the adaptive full duplex system described in Sections 3 and 4.
- The evaluation has two parts: physical layer performance of the cancellation design and effect of a full duplex MAC on hidden terminals and network fairness.
5.1 Cancellation
- The authors evaluate the cancellation design by measuring how much it can attenuate the self-interference signal.
- As mentioned in Section 3.3.1, active components like QHx220 introduce non-linearities which limit the efficacy of balun and digital cancellation.
- Figure 11 shows the cancellation achieved with the combination of balun active and digital cancellation as the transmit power of the self-interfering signal varies.
- Each data point takes 26µs, 10µs to change the QHx220 settings and 16µs to measure RSSI over two OFDM symbols.
- Figure 13 shows the cumulative distribution of how many iterations the algorithm takes to converge.
5.2 MAC performance
- The previous section proves full duplex feasibility with real-time self interference cancellation over wideband WiFi signals.
- Specifically the authors focus on the hidden terminal and fairness problems.
- The reason is that the data load of each flow is 3Mbps, which is lower than the link capacity of around 8Mbps.
- Full duplex should increase the overall throughput by a factor of two, while the results show only a 45% overall increase.
- Due to bursty traffic, sometimes the queue at the AP does not have packets for all clients.
6. COMPARISON WITH MIMO
- The performance evaluation in the previous section shows the clear benefits of the full duplex in the MAC performance.
- The authors compare the case when the nodes use the two antennas to implement a 2x2 MIMO system vs implement a full duplex system.
- Without channel state, the transmitter cannot change its rate in response to channel changes and should fix its rate in advance.
- This section shows that for different channel conditions the performance of full duplex can exceed or lag behind the performance of a similarly resourced 2x2 MIMO system.
- If so, this raises an interesting and new degree of freedom.
7. DISCUSSION
- A balun cancellation-based full duplex radio, while an important step forward, leaves several open questions and new possibilities and future research.
- One issue that comes up numerous times in the design is the sensitivity and precision of the components used in cancellation.
- These accuracies are clearly possible – a small group of researchers were able to achieve them with commodity components.
- Using in-packet techniques to update channel estimates on a per-packet basis can address this challenge.
- While the full duplex system presented does not preclude coexistence with existing half duplex systems, secondary transmissions need to know whether the primary is full duplex capable, otherwise there may be poor interactions with link layer retry counts.
8. CONCLUSION
- This paper presents design for a full duplex radio based on a novel cancellation technique called balun cancellation.
- Unlike prior designs, balun cancellation can work on wideband, high power signals, such that it is possible to build full duplex 802.11n devices.
- These devices can automatically tune their cancellation circuits and so operate in dynamic environments.
- The paper describes the implementation of a full duplex prototype that uses an 802.11-like OFDM link layer which can operate in real time.
- Using this prototype, it evaluates a full duplex MAC layer, validating prior hypotheses that full duplex can prevent many hidden terminals and improve wireless LAN fairness.
Did you find this useful? Give us your feedback
Citations
[...]
2,084 citations
Cites background or methods from "Practical, real-time, full duplex w..."
...Categories and Subject Descriptors C.2.1 [Computer Communication Networks]: Network Architecture and Design—Wireless communication General Terms: Algorithms, Design, Experimentation, Performance Keywords: Full Duplex, Interference Cancellation, Non-linear Cancellation...
[...]
...Designs which use an extra transmitter chain report an overall total of 80dB of self-interference cancellation (we have been able to reproduce their results experimentally)....
[...]
...…suggests that any in-band full duplex system has to be able to cancel all the above distortions in addition to the main signal component itself, since all of these are within the frequency band we are transmitting and receiving on and act as strong self-interference to the received signal itself....
[...]
...• A full duplex system has to reduce non-linear harmonic components that are 80dB above the noise floor, so any full duplex technique has to provide at least 80dB of non-linear self-interference cancellation....
[...]
1,752 citations
1,549 citations
Cites methods from "Practical, real-time, full duplex w..."
...5...
[...]
...In bidirectional IBFD, two modems simultaneously exchange messages over the same frequency band, as further described in Section III....
[...]
...14...
[...]
...10...
[...]
1,398 citations
1,041 citations
Cites background or methods from "Practical, real-time, full duplex w..."
...Miura and Bandai [127] proposed a full-duplex node architecture with directional transmission and omnidirectional reception based on the full-duplex architecture in [122]....
[...]
...received from a local transmitting antenna is usually much stronger than signal received from other nodes [122]....
[...]
...First, the current full-duplex systems have the bandwidth at the order of 40 MHz [122], and the practical implementation of the full-duplex systems for the mmWave communications with a bandwidth of several GHz should be investigated and demonstrated....
[...]
...[122] use the balanced/unbalanced (balun) transformer to support wideband and high power systems....
[...]
References
12,542 citations
1,623 citations
"Practical, real-time, full duplex w..." refers background or methods in this paper
...Motivated by the above limitations, recent work has proposed antenna placement techniques [4, 5]....
[...]
...Well known digital and analog techniques, even when combined together, are not sufficient to cancel self-interference sufficiently for full duplex [4]....
[...]
...propose an additional technique, called antenna cancellation [4]....
[...]
...Hence in our current implementation we have to make do with a component that provides an approximation, the QHx220 noise cancellation chip [11], which prior full duplex designs have used [4, 12]....
[...]
...The state of the art in full duplex operates on narrowband 5MHz signals with a transmit power of 0dBm (1mW) [4]....
[...]
977 citations
"Practical, real-time, full duplex w..." refers background in this paper
...Motivated by the above limitations, recent work has proposed antenna placement techniques [4, 5]....
[...]
...[5], which uses two separate TX chains....
[...]
...Antenna separation uses the fact that the distance between the transmit and receive antennas naturally reduces self-interference due to signal attenuation [5]....
[...]
...One approach to analog cancellation uses a second transmit chain to create an analog cancellation signal from a digital estimate of the self-interference [5], canceling ≈33dB of self-interference over a 625kHz bandwidth signal....
[...]
...The separation between the receive and transmit antennas attenuates the self-interference signal [5], but this antenna separation is not enough....
[...]
671 citations
"Practical, real-time, full duplex w..." refers background in this paper
...Implementing DC for a full duplex radio, however, is more challenging than other uses of digital cancellation, such as successive interference cancellation (SIC) [8] and ZigZag decoding [7]....
[...]
...Digital cancellation, while helpful, is by itself insufficient: current systems in the research literature cancel up to 20-25 dB [8, 7]....
[...]
439 citations
Related Papers (5)
Frequently Asked Questions (14)
Q2. What future works have the authors mentioned in the paper "Practical, real-time, full duplex wireless" ?
A balun cancellation-based full duplex radio, while an important step forward, leaves several open questions and new possibilities and future research. Exploring the effect on the TCP layer may be an interesting future work.
Q3. What is the main limitation of the balun cancellation technique?
Balun cancellation has no bandwidth constraint: it can in theory support arbitrary bandwidths and cancel arbitrarily high transmit powers.
Q4. What is the importance of having a preloaded packet?
Transmission overlap depends on solving two technical challenges, minimizing secondary response latency and having transmission flexibility in preloaded packets.
Q5. What are the limitations of the QHx220?
The QHx220 can cancel 20-25 dB for a 10MHz bandwidth signal but it introduces numerous complications, such as non-linearities and distortions, which complicate digital cancellation significantly.
Q6. How many packets can be recovered from a full duplex radio?
While an SIC implementation that recovers 80% of otherwise lost packets is a tremendous success, a full duplex radio that drops 20% of packets is barely usable.
Q7. What is the basic approach to cancelling a signal?
The basic approach is to estimate the attenuation and delay of the self-interference signal and match the inverse signal appropriately.
Q8. Why is the frequency response of the self interference channel estimated?
Since the training symbols are defined in the frequency domain – each OFDM subband is narrow enough to have a flat frequency response – the radio estimates the frequency response of the self interference channel as a complex scalar value at each subcarrier.
Q9. Why is balun cancellation not perfect across the entire band?
The key reason is that the balun circuit is not frequency flat, i.e., different parts of the band are inverted with different amplitudes.
Q10. How many dB of cancellation can a full duplex radio have?
Digital cancellation, while helpful, is by itself insufficient: current systems in the research literature cancel up to 20-25 dB [8, 7].
Q11. What is the cancellation algorithm for a wide bandwidth?
The results in Figure 6 show that, if the phase and amplitude of the inverted signal are set correctly, balun cancellation can have impressive results across a wide bandwidth.
Q12. What is the difference between balun cancellation and other cancellation techniques?
Unlike prior designs, balun cancellation can work on wideband, high power signals, such that it is possible to build full duplex 802.11n devices.
Q13. What is the difference between a full duplex radio and a SIC?
Unlike SIC or ZigZag, which use DC to recover packets which would have otherwise been lost, a full duplex radio uses DC to prevent the loss of packets which a half duplex radio could receive.
Q14. How does the hardware auto responder logic pick the correct transmission queue?
Each node maintains per-destination transmission queues, and the hardware auto responder logic automatically picks the correct transmission queue to send a secondary packet from based on the header of the primary reception.