Millimeter-wave (mm-wave) wireless local area networks (WLANs) are expected to provide multi-Gbps connectivity by exploiting a large amount of unoccupied spectrum in e.g. the unlicensed 60 GHz band. However, to overcome the high path loss inherent at these high frequencies, mm-wave networks must employ highly directional beamforming antennas, which make link establishment and maintenance much more challenging than in traditional omnidirectional networks. In particular, maintaining connectivity under node mobility necessitates frequent re-steering of the transmit and receive antenna beams to re-establish a directional mm-wave link. A simple exhaustive sequential scanning to search for new feasible antenna sector pairs may introduce excessive delay, potentially disrupting communication and lowering the QoS. In this paper, we propose a smart beam steering algorithm for fast 60 GHz link re-establishment under node mobility, which uses knowledge of previously feasible sector pairs to narrow the sector search space, thereby reducing the associated latency overhead. We evaluate the performance of our algorithm in several representative indoor scenarios, based on detailed simulations of signal propagation in a 60 GHz WLAN in WinProp with realistic building materials. We study the effect of indoor layout, antenna sector beamwidth, node mobility pattern, and device orientation awareness. Our results show that the smart beam steering algorithm achieves a 7-fold reduction of the sector search space on average, which directly translates into lower 60 GHz link re-establishment latency. Our results also show that our fast search algorithm selects the near-optimal antenna sector pair for link re-establishment.

1. Smart mm-Wave Beam Steering Algorithm for
Fast Link Re-Establishment under Node Mobility in
60 GHz Indoor WLANs
Avishek Patra, Ljiljana Simić and Petri Mähönen
Institute for Networked Systems, RWTH Aachen University, Aachen, Germany

2. OUTLINE
INTRODUCTION TO MM-WAVE NETWORKS
BEAM STEERING PROBLEM
MOTIVATION FOR SMARTER SOLUTION
60 GHz CONNECTIVITY PRE-STUDY
SMART MM-WAVE BEAM STEERING ALGORITHM
SIMULATION SCENARIOS
RESULTS
CONCLUSIONS & FUTURE WORKS

3. INTRODUCTION TO MM-WAVE NETWORKS
• large unlicensed spectrum in mm-wave bands, e.g. 60 GHz
• exploiting mm-wave bands for multi-Gbps wireless
connectivity in WLAN, e.g. IEEE 802.11 ad
FILLER
• Challenges:
• high signal attenuation inherent at mm-wave
frequencies!
FILLER
• Solution:
• highly directional beamforming antennas
⇒ increase transmission range
1

4. • But in mm-Wave WLAN…
1) directional link formation = only when Tx and Rx
antenna sectors are both steered in correct directions
2) sector misalignments OR signal interruption ⇒ link
breakage
3) node mobility ⇒ more link breakages
FILLER
FILLER
⇒ link establishment and maintenance much more
challenging using directional antennas cf. vs. traditional
omnidirectional antennas
2
CONTD.INTRODUCTION TO MM-WAVE NETWORKS

5. • low latency, fast beam steering to re-establish links essential
for seamless connectivity and maintaining QoS
FILLER
• State of the Art:
Simple exhaustive sequential scanning of Tx and Rx
antenna sectors, e.g. in IEEE 802.11 ad
FILLER
• Our work:
Smart beam steering algorithm with reduced Tx-Rx sector
pair search space
3
CONTD.INTRODUCTION TO MM-WAVE NETWORKS

6. if RSS > Threshold
for given AP-UE
sector pair:
⇒ link established
⇒ “feasible sector
pairs”
e.g. S and S
4
...
user equipment (UE)
S4
S3
S2S1
S I
S i
...
S1
SJ
S j
S3 S2
S4
...
i
j
APΘ = 360°
I
UEΘ = 360°
J
APΘ
UEΘ
...
Pair = {S , S }
3 4
access point (AP)
AP, f UE, f
Feasible sector
BEAM STEERING PROBLEM
3 4

7. • links may be LOS or
NLOS
FILLER
depends on the
material properties
of the surrounding
indoor environment
FILLER
• a given AP-UE pair
may have multiple
feasible sector pairs
BEAM STEERING PROBLEM
5
8
UE1
UE3
UE2
5
6
4
AP
LOS = if no blockage and close enough
NLOS = reflected signals, penetrations
CONTD.
⇒

8. BEAM STEERING PROBLEM
• (re-) establishing link = searching until a feasible sector pair is
found
FILLER
• link (re-) establishment latency ∝ # sector pairs searched
before formation of link
FILLER
• Existing proposals: Exhaustive sequential scanning (# sector
pairs searched = total # sector pairs), e.g. IEEE 802.11 ad
• Finds optimal feasible sector pair
• But… high latency (∝ total # sector pairs)
6
CONTD.

9. MOTIVATION FOR SMARTER SOLUTION
• link re-establishment latency increases with increase of
antenna directionality
• especially under node mobility conditions
FILLER
⇒ highly detrimental to QoS
FILLER
Our aim: maintaining seamless connectivity and QoS in 60 GHz
WLAN requires frequent faster beam re-steering methods
FILLER
Our work: develop faster beam steering algorithm by smartly
restricting feasible sector pair search space
7

10. SMART MM-WAVE BEAM STEERING ALGORITHM
• Algorithm idea…
“Smart beam steering algorithm for link re-establishment that
searches for a new feasible sector pair over a reduced search
space in the vicinity of the previously known valid sector
orientation (previous feasible sector pair).”
FILLER
⇒ use of historical information, i.e. previous feasible sector
pair
FILLER
⇒ reduced search in vicinity of previous feasible sector pair
FILLER
8

11. SMART MM-WAVE BEAM STEERING ALGORITHM
• idea based on a look at the 60 GHz connectivity…
9
(a) Exhaustive sequential scan (b) Our Work
Previous feasible
sector pairs

12. • study AP-UE link formation in indoor scenarios
• determining all feasible sector pairs between AP and UE
FILLER
o indoor layouts with realistic material properties (for 60 GHz),
area of 10 x 10 m2
o AP – centrally located, UE – different locations at every 1 m
through indoor layouts
o ray-tracing signal propagation simulation using WinProp
o simulations done for every AP-UE sector pair and every UE
location in the indoor layouts
10
60 GHz CONNECTIVITY PRE-STUDY

13. FILLER
60 GHz CONNECTIVITY PRE-STUDY CONTD.
Indoor layouts (1) free space, (2) home,
(3) office, and (4) conference hall
Transmission power 0 dBm
Antenna gain (AP + UE) 25 dBi
Receiver sensitivity threshold – 78 dBm
AP Beamwidth Case 1: 30⁰ ; Case 2: 10⁰
UE Beamwidth Case 1: 30⁰ ; Case 2: 90⁰
11

14. 60 GHz CONNECTIVITY PRE-STUDY
-20
-30
-40
-50
-60
-70
-80
Received
Power
[dBm]
12
CONTD.
-20
-30
-40
-50
-60
-70
-80
Received
Power
[dBm]
-20
-30
-40
-50
-60
-70
-80
Received
Power
[dBm]
-20
-30
-40
-50
-60
-70
-80
Received
Power
[dBm]
1. free space
3. office
2. home
4. conference
hall

15. 0 2 4 6 8 10
0
1
2
3
4
5
6
7
8
9
10
0 2 4 6 8 10
0
1
2
3
4
5
6
7
8
9
10
60 GHz CONNECTIVITY PRE-STUDY CONTD.
13
Case 1: AP – 30⁰; UE – 30⁰ Case 2: AP – 10⁰; UE – 90⁰
• RSS at UE > Receiver sensitivity threshold ⇒ link established
• Complete set of feasible sector pairs for home layout:

16. * 1 hop = 1 m
• studying beam steering requirements (from previous to new
feasible sector pair) for 1–3 hop* UE movements
FILLER
0 5 10 15 20
0
0.2
0.4
0.6
0.8
1
CDF
 home
AP+UE
1 – Hop
2 – Hop
3 – Hop
1 – Hop
2 – Hop
3 – Hop
(θ 30 ;θ 30 )AP UE   
(θ 10 ;θ 90 )AP UE   
(θ 10 ;θ 90 )AP UE   
(θ 10 ;θ 90 )AP UE   
(θ 30 ;θ 30 )AP UE   
(θ 30 ;θ 30 )AP UE   
 home
AP+UE98%-ile case
60 GHz CONNECTIVITY PRE-STUDY CONTD.
14
• avg. total (AP+UE) beam
steering requirement (1-
hop movements) for 98%
cases ≈ 6
FILLER
• ‘insight’ for smart beam
steering algorithm based
on reduced search
around previous feasible
sector pair
Average beam steering requirement

17. SMART MM-WAVE BEAM STEERING ALGORITHM
• Algorithm details…
• start new feasible sector pair search for around previous pair
using a reduced search width parameter
• reduced search width parameter = combined search width for
AP and UE
• individual search width for AP & UE 1/∝ UE & AP beamwidth
respectively
• reduced search sector pairs arranged ∋ sector pairs requiring
least movement searched first ⇒ ensures coordination
• if feasible sector pair not found within reduced space, retort
to exhaustive sequential scan for unchecked sector pairs
15
CONTD.

18. SMART MM-WAVE BEAM STEERING ALGORITHM
16
CONTD.
= 6
for
this
work
Initialize reduced search width parameter
Compute AP & UE search widths
Obtain & sort reduced search sector pairs
Select first reduced search sector pair
For selected
sector pair,
RSS > threshold?
Select next
reduced
search
sector pair
All reduced
search sector pairs
checked?
NO
NO
Obtain unchecked sector pairs
(all sector pairs – reduced search sector pairs)
Select first unchecked sector pair
For selected
sector pair,
RSS > threshold?
Select next
unchecked
sector
pair
YES
All unchecked
sector pairs
checked?
NO
NO
No link for given AP & UE locations
YES
New feasible sector pair found
YES YES

19. SIMULATION SCENARIOS
• different indoor layouts – home, office, and conference hall
• mobility simulation through ‘walks’ (AP static, UE moved by
1-hop at a time)
• straight walks and random walks
• for random walks, orientation-unaware UEs and orientation-
aware UEs
• orientation-unaware UEs – previous feasible sector pair info.
corrupted at turnings
• orientation-unaware UEs – no ambiguity about previous
feasible sector pair
17

20. 0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
SIMULATION SCENARIOS
18
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10home
walks
office
walks
conference hall walks
CONTD.

21. RESULTS
• Performance metrics:
1. Search space and latency reduction – comparing #
sector pairs searched vs. total # sector pairs
2. Link optimality – comparing RSS for optimal feasible
sector pair and selected feasible sector pair
FILLER
• Results:
A. Straight walk in home layout
B. Random walk in home layout
C. Overall (all straight and random walks in all layouts)
19

22. A. Straight walk
Home layout:
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
Case 1:
AP – 30⁰
UE – 30⁰
RESULTS
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
CONTD.
20
Received power using
optimal sector pair,
RSS opt
Received power using
selected sector pair,
RSS SBS
Minimum received
power threshold, 
Selected sector pair
search space size,
|P|SBS
Complete sector pair
search space size,
|M| ( |M|= |P| )EX

23. RESULTS
21
CONTD.
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
A. Straight walk
Home layout:
Case 2:
AP – 10⁰
UE – 90⁰
Received power using
optimal sector pair,
RSS opt
Received power using
selected sector pair,
RSS SBS
Minimum received
power threshold, 
Selected sector pair
search space size,
|P|SBS
Complete sector pair
search space size,
|M| ( |M|= |P| )EX

24. RESULTS
A. Straight walk in home layout:
22
Avg. search reduction* Avg. RSS difference **
Case 1 90% ~ 0.03 dB
Case 2 75% ~ 0.00 dB
* Total # of sector pairs = 144 (both cases)
** RSS (optimal sector pair) – RSS (selected sector pair)
CONTD.

25. RESULTS
23
CONTD.
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
B. Random walk
Home layout:
Selected sector pair
search space size,
|P| , for direction–
unaware UE
SBS
Received power using
optimal sector pair,
RSS opt
Received power using
selected sector pair,
RSS , for direction–
unaware UE
SBS
Received power using
selected sector pair,
RSS , for direction–
aware UE
SBS
Minimum received
power threshold, 
Selected sector pair
search space size,
|P| , for direction–
aware UE
SBS
Complete sector pair
search space size,
|M| ( |M|= |P| )EX
Case 1:
AP – 30⁰
UE – 30⁰

26. RESULTS
24
CONTD.
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5 6 7 8 9 10
0
1
2
3
4
5
6
7
8
9
10
Selected sector pair
search space size,
|P| , for direction–
unaware UE
SBS
Received power using
optimal sector pair,
RSS opt
Received power using
selected sector pair,
RSS , for direction–
unaware UE
SBS
Received power using
selected sector pair,
RSS , for direction–
aware UE
SBS
Minimum received
power threshold, 
Selected sector pair
search space size,
|P| , for direction–
aware UE
SBS
Complete sector pair
search space size,
|M| ( |M|= |P| )EX
B. Random walk
Home layout:
Case 2:
AP – 10⁰
UE – 90⁰

27. RESULTS
B. Random walk in home layout:
max. RSS diff. between orientation unaware and aware UE = 0.17 dB
25
Avg. search reduction Avg. RSS difference
Orientation unaware UEs
Case 1 75% ~ 1.44 dB
Case 2 83% ~ 0.00 dB
Orientation aware UEs
Case 1 86% ~ 1.44 dB
Case 2 86% ~ 0.00 dB
CONTD.

28. C. Overall result (search space and latency reduction)
1 2 3 4 5 6
0
30
60
90
120
150
Conference
Hall
Home Office Home Office Conference
Hall
θ 30AP   θ 30UE   θ 10AP   θ 90UE  
Walk A
Walk B
Walk C
Walk D
Overall Average |P|
Average |P| θ 30AP   θ 30UE  ( ; )
Average |P| θ 10AP   θ 90UE  ( ; )
|P|
SBS
SBS
|P|EX
SBS
RESULTS
26
CONTD.
Avg. reduction
= 86% (7-fold)
Avg. reduction (Case 1)
= 89% (7-fold)
Avg. reduction (Case 2)
= 83% (7-fold)
Worst case reduction
= 66% (3-fold)

29. C. Overall result (link optimality)
RESULTS
27
CONTD.
1 2 3 4 5 6
0
0.03
0.06
0.09
0.12
0.15
0.18
Conference
Hall
Home Office Home Office Conference
Hall
θ 30AP   θ 30UE   θ 10AP   θ 90UE  
Walk A
Walk B
Walk C
Walk D
Overall Average (RSS - RSS )
(RSS-RSS)[dB]
opt SBS
optSBS
0.03
0.06
1.20
1.50
1.80
Avg. RSS diff.
= 0.02 dB
Avg. RSS diff. (Case 1)
= 0.02 dB
Avg. RSS diff. (Case 2)
= 0.02 dB
Worst case RSS diff.
= 1.44 dB

30. CONCLUSIONS & FUTURE WORKS
• low latency, fast beam steering algorithm that smartly
reduces feasible sector pair search space
• search limited based on (static) reduced search width
parameter
• 7-fold (avg.) / 3-fold (worst case) reduction in search space
and link re-establishment latency
• re-established links nearly optimal (avg. RSS diff. < 0.03 dB)
• incorporation of adaptive reduced search width parameter
• performance in outdoor scenarios
28