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ECE150 (Mobile Embedded Systems) > WiFi > Flashcards

WiFi Flashcards

1
Q

What are two categories WiFi standards fall under?

A
  • Spread spectrum based: 802.11b

- OFDM based: 802.11 a/g/n/ac/ax/ad

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2
Q

What is multi path?

A

Signal from transmitter arrives at receiver through many paths

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3
Q

Why OFDM?

A
  • Inter Symbol Interference -> time-domain effect of multi path
  • Frequency selectivity -> frequency-domain effect of multi path
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4
Q

What is OFDM based wifi?

A
  • Parallel data transmission on several orthogonal subcarriers with lower rate
  • Maximum of one subcarrier frequency appears exactly at a frequency where all other subcarriers equal zero
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5
Q

What is Inter Carrier Interference (ICI)?

A

Zero ICI achieved if OFDM symbol is sampled exactly at its center frequency.

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6
Q

Why does WiFi consume so much power?

A

The chip constantly keeps checking for on-air signal

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7
Q

What are the channel selection?

A
  • 20 MHz bandwidth per channel

- 3 non-overlapping channels

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8
Q

What is UNII?

A

Unlicensed National Information Infrastructure

5 GHz channels

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9
Q

What is Medium Access Control (MAC)?

A
  • Infrastructure network contain wired network and access points
  • There are access points outside of the infrastructure network
  • Ad hoc networks are outside of that as well
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10
Q

What are MAC requirements?

A
  • Avoid interference among simultaneous transmissions but enable as many non-interfering transmission as possible and maintain fairness among transmission
  • Asynchronous - common clock not used
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11
Q

What is Hidden Terminal Problem?

A
  • Interference: If ‘X’ transmits while A and B are in session
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12
Q

What is the Exposed Terminal Problem?

A

Even though Y is outside the communication zone of A and B, X is, so Y cannot receive anything from X

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13
Q

What is CSMA/CA?

A
  • WiFi uses Carrier-sense multiple access with collision avoidance protocol which listens before talking
  • Solves Hidden and Exposed terminal problems
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14
Q

What is RTS/CTS?

A
  • Any node hearing RTS and CTS will defer medium access

- RTS sent from sender, CTS sent from receiver

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15
Q

Who sends Data and who sends ACK?

A
  • Sender sends Data and receiver sends ACK, happens after RTS and CTS
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16
Q

What is NAV?

A

Network Allocation Vector

  • full exchange with ‘virtual’ carrier sense
  • NAV (RTS) and NAV (CTS)
17
Q

What is the WiFi Direct feature?

A
  • International standard established by WiFi alliance
  • WiFi connection without a wireless access point
  • Using WiFi protected setup
  • No extra hardware required
  • Supported in android
18
Q

What is Dual-Band feature?

A

Simultaneously active in both 2.4 GHz and 5 GHz

19
Q

What is Tethering Feature?

A
  • Connecting one device to another (Hotspot)

- Allows sharing internet connection of the phone with other devices

20
Q

How does WiFi based location tracking work?

A
  • WiFi triangulation works similarly to GPS: requires 3+ points of reference to calculate a location
  • Each AP records RSSI (Received Signal Strength Indicator) readings from device and transmits RSSI information to WLAN controller which uses a locating algorithm to determine device position
21
Q

What is Location Fingerprinting?

A

A technique for location sensing based on WiFi signal.

Also known as scene analysis or pattern matching technique

22
Q

What is the two-phase process for Location Fingerprinting?

A
  1. Radio map of observed Signal Strength (SS) values from different locations are recorded during offline calibration phase
  2. SS values observed at a user mobile device are compared to radio map values using proximity matching algorithms to infer current user locations
23
Q

What is RSSI?

A

Received Signal Strength Indicator

  • Signal strength at antenna
  • Measured after ADC
  • Gain between antenna and ADC has to be compensated
24
Q

What is RSSI accuracy in real WiFi chip?

A

Stand alone module

  • Check antenna gain
  • Check for any other gain elements between antenna and chip
  • Check automatic gain control (AGC) algorithm
25
Q

What are the shortcomings of current WiFi systems?

A
  • Too many short data frames
  • Lowest Bandwidth 20 MHz not utilized most of the time
  • Significantly lower system efficiency
  • Overlapping Basic Service Set (BSS) in dense deployments block each other from transmitting
  • Performance in outdoor hotspots is poor compared to cellular service
26
Q

What are the 3 Ps?

A
  • Price
  • Power
  • Performance
27
Q

Next-generation WLAN: Price

A

Demand design innovation and process-geometry advancements to shrink die-size

  • 50% area reduction
  • Productizing internal power amplifier (IPA)
  • Programmable hardware/engine architecture to reuse/map multiple algorithms on same hardware
28
Q

Next-generation WLAN: Power

A

Long Battery Life is key

  • Efficient management of sleep modes during idle times
  • Clock gating, fast napping, fast scan and restore memory, and power islanding
  • Adjust bit width of each and every block
29
Q

Next-generation WLAN: Performance

A

Increasing demand for higher and robust wireless data-rates

  • Spatial dimensions
  • Higher signal bandwidth
  • Advanced coding and communication techniques
30
Q

What is WiFi?

A

Wireless Fidelity

31
Q

Why was 802.11b Spread Spectrum Technology developed?

A
  • Avoid intentional jamming

- Spreads narrow band signal into broad band signal using special code

32
Q

What does direct sequence spread spectrum do?

A

XOR of signal with pseudo-random number (chipping sequence)

33
Q

How does DSSS Transmitter works?

A

user data + chipping sequence -> X -> spread spectrum signal + radio carrier -> modulator -> transmit signal

34
Q

How does DSSS Receiver works?

A

radio carrier -> demodulator (outputs received signal) -> lowpass filtered signal + chipping sequence -> X -> products -> integrator -> sampled sums -> decision -> data

35
Q

What are the channel selection?

A
  • 22 MHz bandwidth/channel
  • 11 channels but 3 non-overlapping and supports rates from 1 to 11 Mbps, realistically 4-5 Mbps max
  • Limits number of access points in range of each other to 3

ECE150 (Mobile Embedded Systems) (12 decks)

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