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