Simultaneous Beam Training

1. Simultaneous Beam Training Authors: Date: March 2010

2. Background • Beamforming is necessary to achieve multi-Gbps throughput in 60 GHz band. • Necessary to overcome severe path loss & directionality in 60 GHz. • For a robust 60 GHz network: • STAs needs to beam-train periodically to account for blockage, movement, change of orientation, etc. • STAs need to use dedicated service (time) periods to beam-train, to prevent traffic disruption to other 60 GHz devices. • The beam training overhead can be significant for 60 GHz network with multiple STAs.

3. Example Beam-Training Overhead Calculation • Consider a conference Room scenario with 4 pairs of STAs communicating: • Assume beam training uses 15.3c long preamble (4.7μs) • Beam training by 4 pairs of STAs using separate dedicated times can lead to significant overhead. • This significantly reduces perceived network throughput

4. Solution: Simultaneous Beam Training • Allow multiple pairs of STAs to perform beam training simultaneously • Each pair of STAs uses a different training sequence in order to mitigate interference • The different training sequences may be based on different Golay codes • It is easy to generate multiple Golay codes using the same hardware with good cross-correlation properties • Example: Two complementary Golay sequences of length 128 with good correlation properties • Simultaneous training of even 2 pairs of devices can reduce the beam training overhead by 50%

5. Conference Room Simulation Setup • We will demonstrate performance of simultaneous Tx beam training of 2 pairs & 3 pairs of STAs • Channels generated using the TGad Conference Room channel model, and as defined in the TGad evaluation methodology • Transmitter parameters: • Training sequence is transmitted across each of Tx beams in a random order • 19 Tx beams total of 60o HPBW, covering the half-space z>0 • Receiver parameters: • Training sequence is received in an omni-directional mode (covering z>0) and using a simple correlator detector • Receiver selects the best Tx beam by comparing the strength of the received training sequences across all beam directions. • A random delay of 0-20 chips has been added to model in-room propagation delays. • Performance results are averaged over 100 channel realizations, and 10 noise realizations and beam-ordering per channel realization.

6. Case #1: Simultaneous Training of 2 Pairs of STAs • Assume STA2STA1 training uses Golay code a128 from 802.15.3c • Assume STA7STA8 training uses Golay code b128 from 802.15.3c • To illustrate benefit of interference suppression using distinct Golay codes, performance is compared to the case where every STA uses a128 as the training sequence • 50% reduction of training overhead with no performance degradation

7. Case #2: Simultaneous Training of 3 Pairs of STAs • Assume STA2STA1 and STA7 STA8 trainings use Golay codes a128 and b128 respectively, from 802.15.3c. • The third pair STA6STA3 training uses concatenation of Golay codes a64 and b64 from 802.15.3c Note: Failure events can be readily flagged as they correspond to low measured SINR of the selected beam. • 67% reduction of training overhead with minimal performance degradation.

8. Enabling Simultaneous Beam Training in TGad • The Simultaneous Beam training concept can be overlayed on any beam training protocol: • Standard specifies a few Golay based training-sequences with good cross-correlation properties. • AP allocates the same service (time) period to more than one pair of STAs for beam training. • AP allocates distinct training sequences to these pairs of STAs • Alternately, STAs pseudo-randomly pick a distinct training sequence (e.g: based on STA-ID). • STAs can re-use existing HW to generate and detect these additional Golay based training-sequences. • Each STA needs to generate or detect only one Golay based training sequence, at any given time.

9. Conclusion • Enabling simultaneous beam training of multiple pairs of STAs using different Golay-based training sequences can significantly reduce network overhead • Presented examples from the Conference Room Scenario where 3 pairs of STAs can simultaneously perform beam training, resulting in 67% training overhead reduction. • The additional Golay-based training sequences can be generated and detected without change to existing hardware • We are open to comments/suggestions to incorporate this concept in TGad.

10. References [1] A. Maltsev et al., “Channel Models for 60 GHz WLAN Systems,” 802.11-09/0334r6, 01-20-2010. [2] E. Perahia, “TGad Evaluation Methodology,” 802.11-09/0296r16, 01-20-2010.

11. Straw Poll • Do you support inclusion of the technique,    - Simultaneous Beam Training as described in 10/0252r0 in the TGad draft amendment? • Y: • N: • A: