WiFi

Question 1

What are two categories WiFi standards fall under?

Answer 1

• Spread spectrum based: 802.11b

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

Question 2

What is multi path?

Answer 2

Signal from transmitter arrives at receiver through many paths

Question 3

Why OFDM?

Answer 3

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

Question 4

What is OFDM based wifi?

Answer 4

• 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

Question 5

What is Inter Carrier Interference (ICI)?

Answer 5

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

Question 6

Why does WiFi consume so much power?

Answer 6

The chip constantly keeps checking for on-air signal

Question 7

What are the channel selection?

Answer 7

• 20 MHz bandwidth per channel

- 3 non-overlapping channels

Question 8

What is UNII?

Answer 8

Unlicensed National Information Infrastructure

5 GHz channels

Question 9

What is Medium Access Control (MAC)?

Answer 9

• 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

Question 10

What are MAC requirements?

Answer 10

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

Question 11

What is Hidden Terminal Problem?

Answer 11

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

Question 12

What is the Exposed Terminal Problem?

Answer 12

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

Question 13

What is CSMA/CA?

Answer 13

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

Question 14

What is RTS/CTS?

Answer 14

• Any node hearing RTS and CTS will defer medium access

- RTS sent from sender, CTS sent from receiver

Question 15

Who sends Data and who sends ACK?

Answer 15

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

Question 16

What is NAV?

Answer 16

Network Allocation Vector

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

Question 17

What is the WiFi Direct feature?

Answer 17

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

Question 18

What is Dual-Band feature?

Answer 18

Simultaneously active in both 2.4 GHz and 5 GHz

Question 19

What is Tethering Feature?

Answer 19

• Connecting one device to another (Hotspot)

- Allows sharing internet connection of the phone with other devices

Question 20

How does WiFi based location tracking work?

Answer 20

• 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

Question 21

What is Location Fingerprinting?

Answer 21

A technique for location sensing based on WiFi signal.

Also known as scene analysis or pattern matching technique

Question 22

What is the two-phase process for Location Fingerprinting?

Answer 22

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

Question 23

What is RSSI?

Answer 23

Received Signal Strength Indicator

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

Question 24

What is RSSI accuracy in real WiFi chip?

Answer 24

Stand alone module

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

Question 25

What are the shortcomings of current WiFi systems?

Answer 25

- 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

Question 26

What are the 3 Ps?

Answer 26

- Price - Power - Performance

Question 27

Next-generation WLAN: Price

Answer 27

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

Question 28

Next-generation WLAN: Power

Answer 28

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

Question 29

Next-generation WLAN: Performance

Answer 29

Increasing demand for higher and robust wireless data-rates - Spatial dimensions - Higher signal bandwidth - Advanced coding and communication techniques

Question 30

What is WiFi?

Answer 30

Wireless Fidelity

Question 31

Why was 802.11b Spread Spectrum Technology developed?

Answer 31

- Avoid intentional jamming | - Spreads narrow band signal into broad band signal using special code

Question 32

What does direct sequence spread spectrum do?

Answer 32

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

Question 33

How does DSSS Transmitter works?

Answer 33

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

Question 34

How does DSSS Receiver works?

Answer 34

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

Question 35

What are the channel selection?

Answer 35

- 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