Achaleshwar Sahai

Passive Self-Interference Suppression for Full-Duplex Infrastructure Nodes

Recent research results have demonstrated the feasibility of full-duplex wireless communication for short-range links. Although the focus of the previous works has been active cancellation of the self-interference signal, a majority of the overall self-interference suppression is often due to passive suppression, i.e., isolation of the transmit and receive antennas. We present a measurement-based study of the capabilities and limitations of three key mechanisms for passive self-interference suppression: directional isolation, absorptive shielding, and cross-polarization. The study demonstrates that more than 70 dB of passive suppression can be achieved in certain environments, but also establishes two results on the limitations of passive suppression: (1) environmental reflections limit the amount of passive suppression that can be achieved, and (2) passive suppression, in general, increases the frequency selectivity of the residual self-interference signal. These results suggest two design implications: (1) deployments of full-duplex infrastructure nodes should minimize near-antenna reflectors, and (2) active cancellation in concatenation with passive suppression should employ higher-order filters or per-subcarrier cancellation.

On the Impact of Phase Noise on Active Cancelation in Wireless Full-Duplex

Recent experimental results have shown that full-duplex communication is possible for short-range communications. However, extending full-duplex to long-range communication remains a challenge, primarily due to residual self-interference, even with a combination of passive suppression and active cancelation methods. In this paper, we investigate the root cause of performance bottlenecks in current full-duplex systems. We first classify all known full-duplex architectures based on how they compute their canceling signal and where the canceling signal is injected to cancel self-interference. Based on the classification, we analytically explain several published experimental results. The key bottleneck in current systems turns out to be the phase noise in the local oscillators in the transmit-and-receive chain of the full-duplex node. As a key by-product of our analysis, we propose signal models for wideband and multiple-input-multiple-output (MIMO) full-duplex systems, capturing all the salient design parameters, thus allowing future analytical development of advanced coding and signal design for full-duplex systems.

Understanding the impact of phase noise on active cancellation in wireless full-duplex

Practical designs of wireless full-duplex are made feasible by reducing self-interference via active and passive methods. However, extending the range to long-range communication remains a challenge, primarily due to residual self-interference even after a combination of active cancellation and passive suppression methods is employed. In this paper, we study the factor that limits the amount of active cancellation in current designs of full-duplex. Through an experiment, we show that phase noise in the local oscillator limits the amount of active cancellation of the self-interference signal. Analysing the design proposed by [1, 2] in detail, we show that modifying the quality of the local oscillator can significantly increase the amount of active cancellation in full-duplex systems.

Asynchronous full-duplex wireless

The feasibility of a full-duplex physical layer, where a node can transmit and receive at the same time in the same frequency, has been established in recent work. Implicitly, most schemes to-date have assumed synchronous operation which allows clean training of all relevant channels. In this paper, we establish the feasibility of asynchronous full-duplex communication. We show that both modes of asynchronous full-duplex can be enabled: (i) where the start of transmission of a packet at a node precedes start of the reception from the same node, and (ii) where the start of reception precedes the start of transmission. We show that the former mode has a better performance, which is in fact comparable to the performance of synchronous full-duplex. Finally, we also show that the active suppression can be complemented by passive suppression with the use of optimal antenna placement on actual devices. An overall suppression of 80dB is shown possible in actual experiments making full-duplex feasible for realistic deployments.

On degrees-of-freedom of full-duplex uplink/downlink channel

Feasibility of full-duplex opens up the possibility of applying it to cellular networks to operate uplink and downlink simultaneously for multiple users. However, simultaneous operation of uplink and downlink poses a new challenge of intra-cell inter-node interference. In this paper, we identify scenarios where inter-node interference can be managed to provide significant gain in degrees of freedom over the conventional half-duplex cellular design.

On channel output feedback in deterministic interference channels

In this paper, we study the effect of channel output feedback on the sum capacity in a two-user symmetric deterministic interference channel. We find that having a single feedback link from one of the receivers to its own transmitter results in the same sum capacity as having a total of 4 feedback links from both the receivers to both the transmitters. Hence, from the sum capacity point of view, the three additional feedback links are not helpful. We also consider a half-duplex feedback model, where the forward and the feedback resources are symmetric and time-shared. Surprisingly, we find that there is no gain in sum-capacity with feedback in a half-duplex feedback model, when interference links have more capacity than direct links.

Capacity of All Nine Models of Channel Output Feedback for the Two-User Interference Channel

In this paper, we study the impact of different channel output feedback architectures on the capacity of the two-user interference channel. For a two-user interference channel, a feedback link can exist between receivers and transmitters in nine canonical architectures (see Fig. 3 ), ranging from only one feedback link to four feedback links. We derive the exact capacity region for the symmetric deterministic interference channel and the constant-gap capacity region for the symmetric Gaussian interference channel for all of the nine architectures. We show that for a linear deterministic symmetric interference channel, in the weak interference regime, all models of feedback, except the one, which has only one of the receivers feeding back to its own transmitter, have the identical capacity region. When only one of the receivers feeds back to its own transmitter, the capacity region is a strict subset of the capacity region of the rest of the feedback models in the weak interference regime. However, the sum-capacity of all feedback models is identical in the weak interference regime. Moreover, in the strong interference regime, all models of feedback with at least one of the receivers feeding back to its own transmitter have the identical sum-capacity. For the Gaussian interference channel, the results of the linear deterministic model follow, where capacity is replaced with approximate capacity.

On uplink/downlink full-duplex networks

Recent results in wireless full-duplex promise rate gains over the half-duplex counterpart when two nodes exchange messages with each other. However, when multiple full-duplex nodes operate simultaneously, the resulting network has increased internode interference compared to the half-duplex counterpart. The increased internode interference can potentially limit the rate gain achievable due to introduction of full-duplex capability. In this paper, we present new interference management strategies tha handle internode interference for full-duplex enabled network and achieve rate gains over its half-duplex counterpart.

Sum capacity of general deterministic interference channel with channel output feedback

In a two-user interference channel, there are four possible feedback paths - two from each receiver to the transmitters. This leads to 16 possible models of feedback. In this paper, we derive the sum capacity of two user deterministic interference channel for all sixteen cases. We find that whenever any of the direct link feedback from a receiver to its own transmitter is present, the sum-capacity is the same as when all four feedback links are present. Further when no direct link feedback is present, the sum capacity with one cross-link feedback and two cross-links of feedback is the same. This sum-capacity is the same as the sum-capacity when there is no feedback except in the regime of interference in which both interfering links are weaker than both the direct-links in which case the sum-capacity is the same as sum-capacity of the feedback model with all four feedback links.

Learning beyond local view: Value and information in the bits

Given certain amount of resources available for acquiring network-state information, what should be learned? In this paper, we study this fundamental question for a Z channel where each user has certain local view and beyond that it is allowed to learn k-bits of global network state information. We show that if the interference is unknown to both the transmitters, the best learning strategy is to quantize the signal to interference ratio and reveal it to both transmitters. However, if the interference is known to at least one of the transmitters, then a two-dimensional quantization of the global channel state is the optimal utilization of the k-bits.