11ac: 5G WiFi
• The trigger for 5GHz everywhere
• Led by Apple and other consumer specialists
– In-home device sync, video, backup, etc
– “Gigabit WiFi” on retail shelves
• 11ac’s best feature is that it is 5GHz only!
11ac Data
Throughputs
11ac Potential
Rates vs 11n
QAM code
bits rate
PHY
BW streams MAC eff. Mb/s
TCP
Mb/s
11n 1x1, 2.4 GHz max
6 0.833
20
1
0.65
75
49
11n 2x2, 5 GHz max
6 0.833
40
2
0.65
300
195
11n 3x3, 5 GHz max
6 0.833
40
3
0.65
450
293
11n best (4x4 5GHz) max
6 0.833
40
4
0.65
600
390
6 0.833
80
3
0.65
975
634
6 0.833
160
3
0.65
1950
1268
8 0.875
160
4
0.65
3640
2366
8 0.875
160
8
0.65
7280
4732
11ac 3x3, 80 MHz, 64QAM,
max
11ac 3x3, 160 MHz, 64QAM,
max
11ac 4x4, 160MHz,
256QAM, max
11ac maximum (8x8 etc.)
11ac
Wave 1
• 80MHz channels
• 256QAM modulation
– Potentially could be added to 2.4GHz
• MAC improvements
• Ubiquitous support on TX-BF (hopefully)
Wave 2
• MU-MIMO
• 160MHz channels
– non adjacent (future?)
11ac: wider channels
• 80 MHz Channels
5735
MHz
5835
MHz
149
153
157
161
165
5710
MHz
100
104
108
112
116
120
124
128
132
136
140
IEEE channel #
20 MHz
5330 5490
MHz MHz
36
40
44
48
52
56
60
64
5170
MHz
40 MHz
80 MHz
More spectrum coming soon
 160 MHz (potentially noncontiguous) channels in ~2014?
WLAN - 160 MHz
Radar
WLAN
U-NII 1
WLAN
U-NII 2
Radar
80 MHz
WLAN
U-NII Worldwide
80 MHz
U-NII 3
J
Dynamic bandwidth (BW) operation
• Currently channel must be free on whole 40MHz even if
using 20MHz only
– Wider channel increases likelihood of interfering signal in
extension
– Dynamic BW capability is desired to utilize BW more efficiently
– Not required, but most vendors support it
Secondary
40MHz
36+40
Primary
20MHz
Secondary
20MHz
44
48
Mode 1
Mode 2
Mode 3
• Meeting spectral mask is challenging
• Operating mode change requires 10s of milliseconds
11ac: 256QAM
QPSK
256QAM
Bits/ Relative
Modulation Symbols Sym. speedup
BPSK
QPSK
16QAM
64QAM
256QAM
6
2
4
16
64
256
1
2
4
6
8
2.00
2.00
1.50
1.33
11ac MAC Changes
•
•
•
•
Frame size and MAC efficiency
Dynamic bandwidth operation
CCA thresholds differently handled
New RTS/CTS rules
MAC efficiency
AIFS
Random
backoff
Preamble/
PHY header
MAC
header
Payload
Overhead
PSDU size
(Bytes)
1500*32
Effective
450Mbps
1.3Gbps
2.6Gbps
UDP
Goodput
(Mbps)
MAC
efficiency
UDP
Goodput
(Mbps)
MAC
efficiency
UDP
Goodput
(Mbps)
MAC
efficiency
347
77%
734
56.5%
1019
39.2%
80.5%
810
62%
1180
45.5%
1038
80%
2000
77.5%
64K (max 11n) 362
172K
t
295K
Note: BestEffort QoS is assumed
11ac strengthened Clear Channel Assessment
(CCA) requirement
• Must detect WiFi signals with energy level above
threshold on all channel widths
– Preamble detection not required
• Different back-off thresholds for WiFi vs non-WiFi
signals
– Device must be either smart enough to differentiate WiFi
signals vs other energy or use lowest threshold for all
Energy Detection threshold (in dBm) for signals not
occupying primary 20MHz channel
20MHz
40MHz
80MHz
WiFi signal
-72
-72
-69
Non-WiFi signal
-62
-59
-56
RTS/CTS in 11ac
• Under 11n RTS/CTS rules for wider channels, a
responder responds with CTS using full wide channel
• If a transmitter receives a CTS on 20MHz channel, it
responds with data using full width channel
(20/40/80MHz etc.)
• This rule does not account for possible interference
on the wider channels
Primary
RTS
CTS
BA
Secondary20
RTS
CTS
BA
RTS
CTS
BA
RTS
CTS
BA
Secondary40
SIFS
Interference at responder
time
time
time
time
Wave 2 – Multi-User (MU-MIMO)
11ac : DL-MU-MIMO
2 streams
1 stream
Phone
1 stream
Phone
PC
AP
2 streams
PC
• Down-link Multi-User MIMO (spatial division multiple
access)
• Makes use of multiple physical paths through the
environment to enable multiple simultaneous
transmissions to different receivers
• Impressive capacity improvements if you can fit 8-16
antennas per band on an AP.
12
DL-MU: basic parameters
• Max. # of users per packet
• Max. # of space-time streams per user
:4
:4
– Collision free maximum # of space-time streams
for SU packet = 8
• Max. total # of space-time streams over all users: 8
DL-MU – Spatial Multiplexing
Single-User MIMO (11n)
Stream 1
Stream 2
Stream 3
Stream 4
Multi-User MIMO (11ac)
Stream 3
• Historically
– Access points and clients were using same/similar baseband
• MU requires different designs
– AP complexity increased due to MU support in both HW and SW
– MU capability costs likely to push market to different AP and
client BB
14 silicon
DL-MU: Sequential Block ACK
• Sequential Ack is used because no UL-MU support
• One user send immediate BA
• Other users are polled using BAR
STA 1
AP
STA 2
BAR
BAR
STA 3
SIFS
STA 1
SIFS
BA
SIFS
PIFS
15
SIFS
BA
STA 2
STA 3
SIFS
Error
Recovery
(same for
sounding)
BA
DL-MU: TXOP sharing
• After random back off the winning AC will be
allowed to transmit
• Other ACs are allowed to piggy back in MU packet as
long as it doesn’t extend the TXOP
– If primary AC is voice, max TXOP is 1.5 seconds
– If primary AC is video, max TXOP is 3 seconds
Video
Voice
(primary AC)
Data
Thank You