Dear all,
I've compiled Q/As (with edits) on the enhanced single radio/link MLD operation that I had with some of the members who were waiting in the queue during the presentation. I thought sharing the Q/As on the reflector will help other members
to have better understanding on the proposal and answer some of your questions. Please read through the Q/As and let me know if you have any question that is not covered.
[Q1] The enhanced single-radio MLD does not seem to be a "single-radio" to me.
[A] We called it single-radio since the MLD cannot operated on two links simultaneously as today’s single link operation for 11ax. For example, the MLD cannot receive two
data frames simultaneously and cannot transmit simultaneously. We agree that the naming might cause confusion so changing the name to
single-link MLD could better describe such operation and we are open to other suggestions.
[Q2] Can STA receive and transmit at any link any time?
[A] No. We assume the enhanced single-radio/link MLD listening to two links simultaneously but cannot Tx/Rx on any link at any time (e.g. no Tx/Tx, no Rx/Rx of any frames,
no STR). The listening includes CCA and receiving only specific frames (e.g. control frame).
[Q3] Based on slide 4, it seems to be dual-radio for control frames (e.g., RTS) (to be received on two links) and single-radio for data frames. However, the definition means
receiving and transmitting (any) frames on a single link at a time. You mean the single-link MLD is only for data transmission?
[A] We define a single radio MLD as an MLD that can transmit/receive frames on one link at a time. In our proposal, we are adding an extra capability to the single radio MLD
that can perform CCA and receive limited type of packets (e.g. RTS in non-HT PPDU and low MCS) on two links simultaneously. This doesn’t mean the AP MLD can transmit two RTS frames destined for the non-AP MLD on two links simultaneously. The AP MLD will transmit
one RTS on one link that is idle since the AP MLD knows that the non-AP MLD is a single-radio MLD that can process a frame exchange sequence that will follow after the RTS. Processing the RTS frame that is transmitted with limitations such as non-HT PPDU type
and low MCS just needs a fraction of the full receiver capability for processing any frame types transmitted in non-HT/HT/VHT/HE/EHT PPDU types, higher MCSs, and multiple spatial streams. Therefore we expect the enhanced single-radio MLD will have similar
complexity as today’s 11ax STA implementation but still benefit from multi-link operation.
[Q4] Your proposal is assuming an enhanced "single" radio with enhanced capability. My understanding is that it's more like two full front-end radios but an additional simplified
digital backend (PHY and MAC) as minimum requirement.
[A] We are assuming many single radio MLDs will be equipped with at least 2x2 MIMO capability and the idea is to reuse most of hardware that already exist in the single radio
MLD for the proposed method but add extra capability to listen to two channels simultaneously.
[Q5] You were assuming this minimum requirement only support some basic rate (like legacy OFDM rate, up to 24Mbps), and simple MAC format like RTS/CTS. My concern on this aspect
is: how about trigger frame? they can be fairly complicated in terms of frame structure, and also processing delay for parsing etc. How about a few MCS rates since trigger frame and in particular MU-RTS as replacement of RTS for protection purpose for MU,
can be transmitted by VHT or HE (supposed EHT as well) rate?
[A] The first frame of a frame exchange sequence is limited to a control frame such as RTS/MU-RTS to address such complications or long processing delay. The first frame of
a frame exchange sequence is also limited to use non-HT PPDU data rate up to 24 Mbps to manage the complexity. Other data rates/MCSs used in different PPDU types should be avoided to manage implementation complexity.
[Q6] Is there any radio or digital RX chain switch/tune time that needs to be considered to be practical? And if so, how much it would cost bandwidth/latency?
[A] The time budget after the reception of the RTS frame until reception of the data frame following the CTS is around 100 microseconds so we don’t see any issue in terms
of switching/tune time. In case there is a long link switch time due to a certain vendors implementation, link switch time could be added as a limitation of an MLD and indicate to the AP MLD and adjust its operation.
[Q7] As this hopping of channel among multi-link is initiated by the AP en-powered by its assumed capability, AP is choosing channel according to its local CCA. But we know it's
possible and non-trivial scenario where one channel/link is busy per the non-AP STA but not AP. The proposed mechanism would fail to handle this and the benefit diminishes (yet paid the cost of dual radio front-end.) Have you weighed in the cons or pros?
[A] A data frame is transmitted on a link where RTS/CTS exchange between an AP MLD and a non-AP MLD was successful, which is same as how today’s frame delivery is handled.
If the RTS/CTS exchange is not successful due to channel busy on the non-AP STA side, the data frame is not transmitted. We are assuming at least 2x2 MIMO configuration for a single radio MLD and reusing the front-end hardware so there is no extra cost on
the front-end side.
[Q8] If one channel is clean, what do we really gain from MLO/single radio (or even multiple radios)?
[A] If one channel is clean and the other is busy then MLO in general doesn’t help much since most of throughput will come from the clean channel.
[Q9] Does this limit to 20MHz only packets?
[A] No, it is not limited to 20MHz. The supported bandwidth will depend on the non-AP MLD’s capability as same as today’s single link operation. For example, to reserve medium
for 40MHz channel, RTS should be transmitted in 40MHz.
[Q10] The minimum required RX capability is 20MHz?
[A] Yes, since the frame exchange sequence should start with an RTS in non-HT PPDU (simplest form).
[Q11] This single radio actually is a single PHY, right? Assuming this single radio has multi-link RF chains, i.e. tx/rx and antenna. How does this single radio perform CCA when
receiving multiple signals from different RF chains as those signals are mixed up in the PHY?
[A] We define a single radio MLD as an MLD that can transmit/receive frame on one link at a time. In our proposal, we are adding an extra capability to the single radio MLD
that can perform CCA and receive limited type of packets on two links simultaneously. Each RF chain will be connected to a separate PHY for independent signal processing (depends on implementation) but can just process limited type of packets.
[Q12] Can the enhanced single radio MLD device receive a 1x1 data frame on one link without RTS/CTS? (assuming no collision)
[A] No. This is because we assume the additional capability that the single radio MLD has is limited. In this mode of operation, we assume the single radio MLD can listen
(CCA) and only process limited PPDU type/frame type on both links but cannot process data frame if received on the link that can just listen and process limited frames.
[Q13] Regarding the constraint of the enhanced single radio MLD that is not able to receive/transmit on both links. Are you assuming this constraint is dependent on the channel
separation, BW and related parameters? Or, more of a RF path limitation of the “enhanced single radio” which will exist independent of BSS operation parameters?
[A] The single radio constraint is independent of the BSS operation parameters. This can be viewed as the single-radio MLD similar to today’s 11ax STA except it can utilize
multiple links in a TDMA fashion. We are adding the extra capability on the single-radio MLD.