Stray and Overlapping STAs

1. Stray and Overlapping STAs Authors: Date: 2009-08-17 Graham Smith, DSP Group

2. Abstract The objective is to first estimate the practical problem of STAs that overlap with another network when the APs do not overlap. Then to look at the effects – what happens Then look to see if any solutions are required, and if so, what are they. Make proposals, if any Graham Smith, DSP Group

3. The Perceived Problem Looks obvious in these diagrams, but what about in practice? Graham Smith, DSP Group

4. Propagation Formula • Indoor propagation loss formula (11n) *, F in MHz, d in feet • For d<16.5ft • Lp = – 38 + 20 log F + 20 log d + Wall/Floor loss (Free Space formula) • For d>16.5ft • Lp = – 38 + 20 log F + 20 log 16.5 + 35 log (d/16.5) + Wall/Floor Loss • Std. Dev 3-4dB (Shadow Loss) • *Erceg et al (2004) as per 11n, Channel Model B – Residential • 10dB Outer Wall loss has been used in calculations • No internal wall or floor losses used in calculations • AP Antenna Gain 2dB • STA Antenna Gain 0dB Graham Smith, DSP Group

5. Terraced/Town Houses Terraced Houses @20ft -45dBm -52dBm -72dBm -88dBm -102dBm 3 Neighbors X X 150ft Worse Case position of STAs 60 ft, 4 walls = -100dBm HIDDEN X 150 ft, 2 walls = -96dBm HIDDEN -92dBm Neighbors 3 houses down, and opposite houses within 150 feet have potential to overlap Note: No internal wall losses, external wall loss only. STAs within range of each other, or another STA, when APs are not? – NO Graham Smith, DSP Group

6. Detached Houses 15ft Worse Case position of STAs 50 ft, 4 walls = -100dBm (no internal walls) HIDDEN X X -53dBm -70dBm -100dBm -125dBm 20ft 5ft X X Worse Case position of STAs 30 ft, 4 walls = -92dBm Add loss with one internal wall in middle house = -95dBm HIDDEN -53dBm -65dBm -95dBm -121dBm 20ft House opposite scenario as per previous slide – STAs are HIDDEN STAs within range of each other, or another STA when APs are not? – NO NOTE: 09/0474r0 by Alex Ashley (NDS) also predicts similar results(Simulation with 5 STAs per house) Graham Smith, DSP Group

7. Apartment Block 6 -45dBm -52dBm -72dBm -88dBm -102dBm -44dBm -63dBm -84dBm -99dBm Each Apartment 20 x 35 feet about 700 square feet -61dBm -77dBm -97dBm -77dBm -91dBm -105dBm -92dBm -103dBm • Compared to the AP signal strengths, no internal walls, STA signal strength is: • 4dB less due to antennas (-4) • 20ft less due to position in x axis (+2) • 10ft less due to position in z axis (+4dB) • i.e. either same as for AP or 2dB less RESULT, -2dB cf AP in x-axis 0dB cf AP in z-axis STAs within range of each other, when APs are not? – NO Graham Smith, DSP Group

8. Summary for domestic scenarios • In practice for residential scenarios, the overlapping STA is very rare if at all • The outer wall attenuation dominates • Now let us consider when the STA is outside of the building: • What is the effect on the two (non-overlapping) networks? • What is the effect on the STA? • Will this be a permanent situation? • How common? Graham Smith, DSP Group

9. Case 1 – Stray STA A and B are on same channel Stray STA (X) can see and be seen by B STA to B ~-81dBm (see below) STA to A ~-64dBm Channel selection ensures that B is the only other AP on same channel as A Graham Smith, DSP Group

10. What Happens – Case 1: Stray STA Case 1 – STA within range of both APs, (in the garden) • Possibly both ‘hidden’ and ‘seen’ nodes in network B • ‘Hidden’ nodes in network B have no effect • Worse case is STA ‘sees’ all other STAs and AP in B • TX STA sees communication (maybe just the preambles) in networks B plus its own network A (“Captive Effect”), resulting in reduced ability to gain the air (depends upon traffic), so STA can only use the “left over bandwidth” • Result is that only the STA is affected, not network B or the rest of A • RXAt best “Step-up Re-start” will result in reception for the STA from its own AP not being affected. (Network A signal strength is ~17dB higher than any transmission from network B)At worse, reception is noisy, resulting in dropped packets • SBA limits retries • Bottom line – STA is affected, mostly for TX, but not the networks Graham Smith, DSP Group

11. Extreme Example of Stray STA • Apartment Block around swimming pool • Outside STA ‘sees’ multiple networks • What Happens? • STA is effectively silenced or reduced TX due to ‘captive’ effect • RX also impeded due to numerous ‘hidden’ nodes • Overall effect is poor performance for an outside STA. No worse, no better due to OBSS . • Only “solution” is STA’s network needs to be on own, unique channelor else every other Network reduces it traffic – I don’t think so. Graham Smith, DSP Group

12. Case 2 – Two Overlapping STAs Networks A and B are hidden from each other STAs see each other at ~-78dBm (up to -92dBm) STAs see own QAPs at ~-64dBm Note: Distance from STA to QAP B is >200ft for B to be hidden from STA • What Happens? • TX - STAs slightly reduced in ability to gain air, only by the traffic on the other STA • RX – Step-up re-Start should enable good reception from AP Reception, at worse, only prohibited if two STAs transmitting at same time and stepping on each other. Effect depends on the relative traffic. • Basically Networks not affected , just the STAs (mostly TX). Not as much as ‘problem’ as Stray STAs. Graham Smith, DSP Group

13. Stray STAs – Solutions Notes: • Presence of Stray STAs is a dynamic, relatively short-lived problem • Stray STA could use CTS to self to improve its TX performance – this impinges on Network B • AP decision to re-scan (as below) should also depend upon the QLoad of the Stray STA Possible “solutions” • 08/1260r1 outlined method for a QAP to determine if there was an overlapping STA - Uses Beacon Report and continues channel search is Stray STA present • Requires the STA to be 11k compliant • AP B recognizes a ‘stray STA’, initiates a new Channel Search • Switch over after ~200ms, switch <5ms (assuming network is 11h) • Not a great incentive for B to do this, but the stray does present a possible hindrance. • Stray STA informs own AP that it is a stray • Use an “unsolicited” 11h Measurement Report to inform AP. AP can then initiate new channel search. Three spare bits are available. • This also works for the overlapping STA situation Note: These could result in several tries for a new channel… but so what? If no better channel found, OK, not made worse. If changing several times, no real new problem, channel change is not that a big deal. Graham Smith, DSP Group

14. Stray and Overlapping STAs Do we need to adjust OBSS solution to cater for them? • OBSS does not make situation worse, in fact correct Channel Selection does lessen chance of Stray STAs • Stray STAs occurrence is pretty unlikely • Enlarging QLoad to include QAPs at Distance 1 does not add any information that can be used for a Stray STA. • Only in case of a 1:2:1 Overlap is there a way to know about a hidden QAP, and then is the chance of a Stray STA enough to warrant Sharing based upon Distance 2 Networks? Basically, only the STA is affected, not the network • Solution #2 does not require any additions to the proposal, but could be added in informative text • Solution #3 does require addition to the proposal and has merit – Shall we add this? Graham Smith, DSP Group

15. Conclusions • Stray and Overlapping STAs will not occur if the STAs are within the confines of the house/apartment • Stray Overlapping STAs are corner cases • Temporary, dynamic • Fences etc. Make the occurrence pretty unlikely for houses • Stray STA does not adversely affect the performances of the two hidden networks, but its own performance is impaired • RX performance can be mitigated through “Step-up Re-Start” • APs can carry out new Channel Search at any time • If it sees a Stray STA for example • OBSS Channel Search lessens chance of Stray STAs • Could consider adding “unsolicited 11h Measurement Report”, sent by STA to inform AP of situation. Graham Smith, DSP Group

16. Proposal • Do not amend QLoad Element beyond reporting on self QLoad only • Add informative text covering possible actions when a Stray STA is present – AP can search for new channel • Should we consider adding a ‘Report’ for a STA to report that it is in a Stray STA situation? • If so, use 11h Measurement Report basic format? • Channel Number, Measurement Start time, Stop time, and Map (use bit 5 of Map for example) • Could do it for completeness, but is the ‘problem’ big enough to warrant it? – I personally do not think so Graham Smith, DSP Group