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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!
QAM bits
code rate BW streams MAC eff.
PHY 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
11ac 3x3, 80 MHz, 64QAM, max 6 0.833 80 3 0.65 975 63411ac 3x3, 160 MHz, 64QAM, max 6 0.833 160 3 0.65 1950 126811ac 4x4, 160MHz, 256QAM, max 8 0.875 160 4 0.65 3640 2366
11ac maximum (8x8 etc.) 8 0.875 160 8 0.65 7280 4732
11ac Potential Data Rates vs 11n11ac Throughputs
11acWave 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?)
J
11ac: wider channels
• 80 MHz Channels
14
01
36
13
21
28
12
41
20
116
112
10
81
04
10
0
16
51
61
15
71
53
14
9
64
60
56
52
48
44
40
36IEEE channel #
20 MHz
40 MHz
80 MHz
5170MHz
5330MHz
5490MHz
5710MHz
5735MHz
5835MHz
80 MHz
U-NII Worldwide U-NII 3
WLANRadar
U-NII 2U-NII 1
WLANWLAN 80 MHzRadar
WLAN - 160 MHz
160 MHz (potentially noncontiguous) channels in ~2014?
More spectrum coming soon
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
• Meeting spectral mask is challenging• Operating mode change requires 10s of milliseconds
Secondary 40MHz
Primary20MHz
Secondary20MHz
36+40 44 48
Mode 1
Mode 2
Mode 3
11ac: 256QAM
6
Modulation SymbolsBits/Sym.
Relative speedup
BPSK 2 1QPSK 4 2 2.0016QAM 16 4 2.0064QAM 64 6 1.50256QAM 256 8 1.33
QPSK
256QAM
11ac MAC Changes
• Frame size and MAC efficiency• Dynamic bandwidth operation• CCA thresholds differently handled• New RTS/CTS rules
MAC efficiency
PSDU size (Bytes)
450Mbps 1.3Gbps 2.6GbpsUDP Goodput (Mbps)
MAC efficiency
UDP Goodput (Mbps)
MAC efficiency
UDP Goodput (Mbps)
MAC efficiency
1500*32 347 77% 734 56.5% 1019 39.2%
64K (max 11n) 362 80.5% 810 62% 1180 45.5%
172K 1038 80%
295K 2000 77.5%
Preamble/PHY header
MAC header Payload
AIFSRandom backoff
tEffectiveOverhead
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 channel20MHz 40MHz 80MHz
WiFi signal -72 -72 -69
Non-WiFi signal -62 -59 -56
• 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
Secondary20time
time
BA
Secondary40time
timeSIFS
BA
BA
BA
RTS
RTS
RTS
RTS
CTS
CTS
CTS
CTS
Interference at responder
RTS/CTS in 11ac
Wave 2 – Multi-User (MU-MIMO)
11ac : DL-MU-MIMO
• 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
AP
PCPhone
PhonePC
2 streams
1 stream
1 stream
2 streams
DL-MU: basic parameters
• Max. # of users per packet : 4• Max. # of space-time streams per user : 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
• 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 silicon14
Stream 1Stream 2Stream 3Stream 4
Single-User MIMO (11n)
Multi-User MIMO (11ac)
Stream 1
Stream 2
Stream 3
Stream 4
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
15
STA 1
STA 2
STA 3
AP
STA 1
STA 2
STA 3
BA
BAR
BA
BAR
BA
SIFS SIFS SIFS SIFSSIFS
Error Recovery (same for sounding)
PIFS
• 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
DL-MU: TXOP sharing
Voice (primary AC) Data
Video
Thank You