802.11ac Wi-Fi Fundamentals Eric Johnson March 2014

Transcription

1 802.11ac Wi-Fi Fundamentals Eric Johnson March 2014

2 Agenda 11ac Standards Physical Layer Overview 11ac Data Rates Radio Realities Transmitters Receivers Antennas 11ac Beamforming 11ac Products 2

3 802.11ac Technology Overview Think of 11ac as an extension of 11n 11n specification introduced/leveraged: 2.4 and 5 GHz supported Wider channels (40 MHz) Better modulation (64- QAM) Additional streams (up to 4 streams) Beam forming (explicit and implicit) Backwards compatibility with 11a/b/g 11ac introduces 5 GHz supported Even wider channels (80 MHz and 160 MHz) Be?er modulaaon (256- QAM) AddiAonal streams (up to 8) Beam forming (explicit) Backwards compaability with 11a/b/g/n Refer to h?p://www ac.net for in- depth informaaon 3

4 Wider Channels 80 MHz channel widths supported in first generation 80 MHz is 4.5x faster than 20 MHz 80 MHz is contiguous Per packet dynamic channel width decisions Future releases will allow for 160 MHz channel widths 160 MHz can be either contiguous or in two noncontiguous 80 MHz slices 4

5 802.11ac Channels (FCC) UNII I and UNII II 2x 80 MHz 4x 40 MHz 8x 20 MHz Channel Freq (MHz) Band Edge Band Edge Weather Radar UNII II extended 3x 80 MHz 6x 40 MHz 12x 20 MHz Channel Freq (MHz) Band Edge Band Edge Band Band Channel Edge Edge Freq (MHz) US UNII III 1x 80 MHz 2x 40 MHz 5x 20 MHz 5

6 802.11ac Channels (ETSI) UNII I and UNII II 2x 80 MHz 4x 40 MHz 8x 20 MHz Channel Freq (MHz) Band Edge Band Edge UNII II extended 2x 80 MHz 5x 40 MHz 11x 20 MHz Channel Freq (MHz) Band Edge Band Edge

7 Understanding 11ac Data Rates 7

8 Terminology Symbol: basic element containing 1 to 8 bits of information Tone/Sub-Carriers: OFDM is made up of many tones. Each symbol is mapped to a tone. Cyclic Extension: technique used in OFDM to protect against multipath interference You need cyclic extension but it is dead air and consumes transmit time Guard Band: Space between channels. In these regions tones have a constant value of zero amplitude Pilot Tones: Used to train the receiver and estimate the channel Radio Channel: For Wi-Fi 20, 40, 80, or 160 MHz of spectrum Propagation Channel: everything that happens between the transmitter and receiver FEC: Forward Error Correction. Redundant information that is sent to assist the receiver in decoding the bits. 8

9 Sub-carriers Guard Tones 26 carriers 26 carriers 28 carriers 28 carriers -10MHz f c +10MHz 52 subcarriers (48 usable) for a 20 MHz non-ht mode (legacy a/g) channel -10MHz f c 56 subcarriers (52 usable) for a 20 MHz HT mode (802.11n) channel +10MHz 57 carriers 57 carriers -20MHz -10MHz f c +10MHz 114 subcarriers (108 usable) for a 40 MHz HT mode (802.11n) channel +20MHz 121 carriers 121 carriers -40MHz -30MHz -20MHz -10MHz f c +10MHz +20MHz +30MHz +40MHz 242 subcarriers (234 usable) for a 80 MHz VHT mode (802.11ac) channel An 80+80MHz or 16MHz channel is exactly two 80MHz channels, for 484 subcarriers (468 usable) OFDM subcarriers used in a, n and ac 9

10 QAM constellations Amplitude +1 Amplitude +1 Amplitude +1 Quadrature -1 Quadrature +1 Quadrature -1 Quadrature +1 Quadrature -1 Quadrature +1 Amplitude -1 Amplitude -1 Amplitude QAM constellation 64-QAM constellation 256-QAM constellation Constellation diagrams for 16-, 64-, 256-QAM 10

11 How do I get to the data rate for a given MCS? Basic Symbol Rate KHz 3.2 µs Cyclic Extension t/4 0.8 µs t/8 0.4 µs Bits Per Tone BPSK 1 QPSK 2 16 QAM 4 64 QAM QAM

12 Raw Data Rates #Tones * Bits per Tone * Symbol Rate 16 QAM, 20 MHz 52 * 4 * = 65 Mbps 12 12

13 Correct for Cyclic Extension 13 13

14 Apply FEC Coding 14 14

15 Transmitters 15

16 Transmitter Line Up Symbol Generation DAC Up Convert PA 16 16

17 Transmitter Terms Conducted Power This is the power that leaves the connectors EIRP: Effective Isotropic Radiated Power This is the conducted power (dbm) + antenna gain (dbi) in the direction of interest cable losses (db) Peak EIRP This is what is regulated It is the conducted power + peak gain cable losses dbm: log power ratio to milliwatt dbi: antenna gain relative to isotropic dbr: relative power eg:used with describing transmit mask 17 17

18 Symbol Stream Linear Amplitude Time (symbols) 18 18

19 802.11n Signal Frequency Domain Digital Domain 0 10 a After DAC Amplitude (db) PA Non Linearity Frequency (MHz)

20 Transmitter Non-Idealities DAC Quantization: this is due to the limited number of bits in a practical Digital to Analog Converter This noise source is not affected when the power is reduced PA Non Linearity: OFDM has a high Peak to Average Ratio. The peaks in the OFDM signal cause distortions which manifest as noise like shoulders Known as spectral regrowth For every one 1 db drop in tx power the regrowth drops by 3 db 2 db net The in channel noise is referred to as EVM Error Vector Magnitude The out of channel noise interferes with other Wi-Fi channels and determines how close we can space antennas 20 20

21 EVM As the depth of modulation increase the number of bits per symbol increases The in-band noise introduces uncertainty wrt to the actual symbol position Higher order modulations decrease the space between code points To make higher order modulations work the tx power needs to be reduced The EVM noise will add with interference and background noise 16 QAM 21 21

22 EVM Specfication and 22x tx table Modulation Coding Rate n EVM (db) ac EVM (db) BPSK 1/ QPSK 1/ QPSK 3/ QAM 1/ QAM 3/ QAM 2/ QAM 3/ QAM 5/ QAM 3/4 N/A QAM 5/6 N/A

23 Receivers 23

24 Receiver Line Up Symbol Decode ADC Down Convert LNA 24 24

25 Receiver Impairments Analog Compression Modern LNAs have very effective input power tolerance Digital Compression This is where a high power signal hits the Automatic Gain Control (AGC) Circuit. Gain drops and receiver sensitivity degrades The radio can be totally blocked if the power hits the Analog to Digital Converter (ADC) and consumes all the bits Intermodulation Again, the effective linearity of modern LNAs reduces the impact of this 25 25

26 DAS Interference: Example Without filtering any signal that hits the receiver above -45 dbm will cause a reduction of sensitivity The degradation continues until about -15 dbm at which point the signal is totally blocked With a 100 mw (20 dbm) DAS system at 2100 MHz Tx 20 dbm Effective rx antenna gain 3 dbi 1 st meter at 2100 MHz -39 db Power at 1m -19 dbm No impact distance 40 meters 26 26

27 Advanced Cellular Coexistence Proliferation of DAS and new LTE bands at 2.6 GHz are creating issue for Wi-Fi solution All new APs introduced by Aruba in the last 12 months and going forward have implemented significant filtering into the 2.4 GHz radio portion to combat this Design solution Use high-linear LNA followed with a high-rejection filter to achieve rejection target and little sensitivity degradation; Design target: Minimal Sensitivity degradation with -10dBm interference from 3G/4G networks (theoretical analysis). 27

28 Coverage Example 1. Sample coverage for 3x3 11n AP (or 3x3 11ac AP with 11n clients) in HT40 mode Coverage area sustains MCS5 and up 28

29 Coverage Example 2. Upgrade to 3x3 11ac AP with 11ac clients, still using 40Mhz channels (VHT40) Radius for 600Mbps (MCS9) area is 1/4 of that for 450Mbps (MCS7) 29

30 Coverage Example 3. Equivalent range for clients using 80MHz channels (VHT80) Rates roughly double, relative range for each of the MCS rates does not change, but 80MHz range is ~70% of equivalent (same MCS) 40MHz range 30

31 Relative Range ac Rates Signal level and range - db r MCS MCS MCS MCS MCS MCS MCS MCS MCS MCS Datarate 40MHz 80MHz MCS MCS MCS MCS MCS MCS MCS MCS MCS ,170 MCS ,300 31

32 Pros and Cons of ac Pros 1. APs can accommodate more users/devices Increased capacity 2. Standards based Explicit Beam-forming increases SNR Higher data rates over longer distances QAM Increased throughput at high SNRs Improved modulation and coding techniques 4. Multi-User MIMO (future generations) Improved utilization of RF capacity 5. Use of 5 GHz spectrum More non-overlapping channels Quieter RF environment 32

33 Pros and Cons of ac Caveats 1. Hardware update required to support ac Some features will not be available on legacy devices 2. Increased product cost Small premium for 3x performance Prices will come down 3. Supporting ac will result in increased load on the infrastructure 4. AP-225 requires 802.3at (PoE+) for full functionality & performance However, no restrictions on 11ac radio with 802.3af POE USB disabled, second Ethernet port disabled, 2.4GHz radio in 1x3:1SS mode 33

34 Wave 2 of 11ac What will wave ac deliver? MU-MIMO Use AP MIMO resources more effectively Transmit data to multiple devices simultaneously: for example 4SS AP streaming data to four 1SS clients simultaneously 4x4:4SS Benefit of additional stream mostly for MU-MIMO Not anticipating any 4x4:4SS client devices Adds 33% to max datarate VHT160 Doubles max datarate Practical problem: only 2 VHT160 channels available in entire 5GHz band Max 5GHz radio throughput triples again! 450 (11n 3x3 HT40), 1,300 (11ac 3x3 VHT80), 3,467 (11ac 4x4 VHT160) When will it be available? Radio chipsets available late 2014 Products in

35 Reasons not to wait for Wave 2 Unlikely to see any 4x4:4SS client devices Use of VHT160 not practical for typical enterprise deployment MU-MIMO is a nice-to-have optimization. How well it will work and what the real benefits are is still not entirely clear Requires new client devices (Wave 1 clients also not FW upgradeable) Wave 1 is here now (technology, products, market momentum), offering huge advantages over 11n. Wave 2 is the expected next step in the evolution of the technology. In general: the next wave in technology is always around the corner, something better is always coming Once Wave 2 is available, we ll for sure be talking about Wave 3. No different from when 11n 2x2 products were introduced and it was clear that 3x3 products would be available within 18 months. 35

36 11ad and what it means 60GHz band, three channels in most countries (each 2.16GHz wide), each providing up to 6.8Gbps PHY datarate No MIMO Challenges: Non-Line of Sight (NLOS) connections, range, penetrating obstacles (and people) Targeted to clean up a cluttered desk or TV cabinet Likely not appropriate for traditional AP use. But can be interesting for related applications like wireless docking, high-capacity WLAN hotspots, AP backhaul/aggregation, etc. It is being investigated (but no product plans as of yet) Standard is available, certification program in place Wi-Fi Alliance WiGig Alliance 36

37 Antennas 37

38 Antenna Basic Physics When the charges oscillate the waves go up and down with the charges and radiate away With a single element the energy leaves uniformly. Also known as omni-directionally 38 38

39 Building Arrays: 2 Elements By introducing additional antenna elements we can control the way that the energy 90 radiates 2 elements excited in phase Linear Plot λ / db Plot

40 Building Arrays: 4 Elements By introducing additional antenna elements we can control the way that the energy 90 radiates 4 elements excited in phase Equal amplitude Linear Plot db Plot

41 Building Arrays: 4 Elements By shaping the amplitude we can control sidelobes 4 elements excited in phase Amplitude 1, 3, 3, Linear Plot db Plot

42 Building Arrays: 4 Elements Phase By altering phase we can alter the direction that the energy travels 4 elements excited with phase slope Even amplitude Linear Plot db Plot

43 Reading Antenna Pattern Plots - Omni Azimuth Elevation -3 db Sidelobes 43 Omnidirectional Antenna (Linear View) 43

44 Reading Antenna Pattern Plots - Sector Front -3 db -3 db Back Side Azimuth Backlobe Sidelobes Elevation 44 Sector Antenna (Logarithmic View) 44

45 802.11ac Beamforming 45

46 Beamforming: Notes AP 22x series has 11ac beamforming support in 2.4 and 5 GHz bands Works with clients that support 11ac beamforming function This is at a minimum all 11ac client devices using Broadcom chipsets Support will have to come to all devices to compete with Broadcom offering 11ac beamforming is standards based first standard that is doing this the right way 11ac beamforming represents the consensus view of the 1000 s of contributors to the standards process 11ac beamforming is implemented in baseband. It works with all antenna subsystems The total number of beamforming combinations is effectively infinite 11ac actively tracks users so has a recent channel estimate between the AP and client that is updated frequently 46 46

47 Channel state information, implicit and explicit beamforming estimation Beamformer sounding frames Beamformee Actual CSI feedback from sounding Beamformed frames Explicit feedback for beamforming (802.11n and ac) 1 (Beamformer) Here s a sounding frame 2 (Beamformee) Here s how I heard the sounding frame 3 Now I will pre-code to match how you heard me Explicit feedback for beamforming 47 47

48 0.01 Client Antennas Antenna 1 Antenna 2 Antenna 3 E Field Amplitude h11 h21 h Wavelengths

49 Line of Sight 3 stream AP Smartphone 1 Antenna/1 Stream Client AP

50 Simple Reflection Let s introduce two reflection surfaces and look at the impact of one bounce on each side Client Virtual Antenna Pattern AP

51 Multi Stream Client The reflections allow beamforming to send different streams with different antenna pattern through the system Client Stream Stream 1 AP Stream 3 51

52 11ac Beamforming across an 80 MHz channel The standards based algorithm actually works out patterns for each sub carrier Below is the pattern for stream 1 at 5460, 5500, 5540 MHz

53 Aruba 11ac Solutions 53

54 AP-224/ ac 3x3 AP Enterprise class 3x ac Aggregate TCP platform throughput performance >1Gbps Two platform models: AP-224: external antennas (3x, dual band) AP-225: integrated antennas Advanced Cellular Coexistence support Dual radio: n 3x3:3 HT40 2.4GHz (450Mbps), support for TurboQAM ac 3x3:3 HT80 5GHz (1.3Gbps) 11ac beamforming supported in both bands Wired interfaces Network: 2x 10/100/1000Base-T Ethernet, with MACSec support USB 2.0 host interface, console port, DC power Will require 802.3at PoE (or DC power) for full functional operation Functional, but capabilities reduced when powered from 802.3af POE Enterprise temperature range, plenum rated, TPM $1,295 U.S. List 54

55 Thank You 55