Transmission Failure due to Inter-Stream/Inter-User Interference
Downlink Multi-User Beamforming (MUBF) is a key technique to scale throughput in dense wireless local area networks (WLANs) as it enables an Access Point (AP) to simultaneously transmit multiple independent data streams to different users in the same frequency resource block. Such multi-user transmission has been demonstrated in WLAN systems, massive MIMO systems, and is now standardized in IEEE 802.11ac and commercialized.
Unfortunately, client mobility, environmental mobility, and any source of precoding error (e.g., due to CSI feedback compression/quantization) can vastly degrade performance. In particular, imperfect beam steering does not merely result in a poorer quality signal at the receiver due to energy being directed away from the receiver: in a multi-user system, imperfect beam steering also increases inter-stream and interuser interference, i.e., influencing both the signal S and interference I components of the signal-to-interference-plus-noise ratio (SINR).
Beamformed Training in Downlink MU-MIMO
We propose M^3CS (Multi-user Multi-stream MCS), a technique for “just-in-time” multi-user bit-rate selection. In contrast to single-stream systems, multi-stream modulation and coding scheme (MCS) selection introduces the challenge of selecting multiple and potentially unequal MCS instead of just a single one: as mobility and other uncontrollable factors degrade SINR, the AP can maintain successful frame reception at the users by sufficiently reducing the MCS.
We design a multi-user inter-stream interference probing mechanism that proactively evaluates the MCS selection resulting from predicting per-user SINR based on the acquired CSIT. The AP triggers a multi-user probe by transmitting a minimum-length downlink multi-user frame at the rates determined using CSIT based MCS selection. The multi-user probing frame enables each user to infer channel variations since sounding occurred, as well as the inter-stream interference affecting the transmission. Thus, upon reception of this probing frame, each user measures its effective SINR (SINReff) and maps it to the corresponding preferred MCS.
Furthermore, since MCS is identified with an index (0 to 9 in 802.11ac), we can represent each MCS selection with only a few bits. CHRoME maps each MCS index to a predefined pseudo noise binary codeword (i.e., a correlatable symbol sequence or CSS). The transmission length and processing required to identify these sequences is significantly lower than what is required for decoding a packet, thus making them ideal for this application. The additional overhead introduced by the MUBF probing and feedback is far outweighed by avoiding rate under-selection (unnecessarily low MCS that wastes airtime) or over-selection (excessively high MCS that yields frame loss).
Spatially Multiplexed Fast Retransmissions
Despite the aforementioned resilience mechanisms, frames will occasionally be non-decodable due to excessive co-stream interference or mobility. Unfortunately, current retransmission strategies, inherited from the original CSMA design, require re-contention after a doubled backoff window. Consequently, physical layer parameters such as beamsteering weights are likely stale by the time retransmission is feasible, and therefore the time and resource penalty of channel sounding must be incurred again. In contrast, we design a soundless fast retransmission strategy in which the AP triggers a one-time immediate retransmission using the same CSIT as in the original transmission. Yet, because the original transmission failed, it is clear that the re-transmission strategy must be changed. Thus, because only a subset of the users’ transmissions will have failed, the retransmission will exploit the “liberated” degrees of freedom (DoF), e.g., an 8 antenna AP transmitting to 8 users with 2 failed frames will lead to an additional 6 DoFs for the retransmission to 2 users.
O. Bejarano, R. Hoefel, and E. Knightly, “Resilient Multi-User Beamforming WLANs: Mobility, Interference, and Imperfect CSI,” in Proceedings of IEEE INFOCOM 2016, San Francisco, CA, April 2016.