Wi-Fi

• A name for local area wireless computer networking technology

• Based on IEEE 802.11 standards
  on Wireless Local Area Networks (WLANs)

• Used as a synonym for WLAN (Wireless LANs)

• physical layer와 data link layer에서 정의된다

802.11 standards: The main stream

Legacy 802.11 (1997)

• 주파수 대역 : 900MHz, 전송속도 : 2Mbps
• Frequency hopping spread spectrum (FHSS)

802.11b (1999)

• 2.4GHz band, 11Mbps
• Direct sequence spread spectrum (DSSS)

802.11a (1999)

• 5GHz band, 54Mbps
• Orthogonal frequency division multiplexing (OFDM)

802.11g (2003)

• 2.4GHz band, but OFDM and 54Mbps
• compatible with 802.11b

802.11n (2009)

• 2.4GHz and 5GHz band
• OFDM
• up to 600Mbps with 40MHz channel, 4x4 MIMO

802.11ac (2014)

• Gigabit throughput
• Wider RF bandwidth, more MIMO streams, high-density modulation

and many others...

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Wireless Channel

• signal은 transmit (TX) antenna로부터 wireless channel로 전송된다
• signal은 공기 중을 날아서 receive (RX) antenna에게 도착한다

Signal Attenuation

• signal이 공기 중을 날아갈수록, power는 감쇄한다
  

Pathloss

• The receiver (Pr) is basically a function of transmit power (Pt) and the distance between transmitter and receiver
• Signal attenuation based on distance is called pathloss

Receiver Sensitivity

• Receive antenna는 signal power가 receiver sensitivity보다 강할 때만 signal을 받을 수 있다
  • Receiver sensetivity depends on the hardware
    

SNR

• signal이 RX sensitivity보다 강하게 받아졌다고 하더라도,
  receiver가 signal을 decode할 수 있을지는 SNR(Signal-to-Noise Ratio)에 달려있다
  $SNR = P_{signal}/P_{noise}$

• 때때로, SINR(Signal-to-Interference-plus-Noise Ratio)이 이용되기도 한다

• decoding하기 위한 minimum SNR은 mudulation과 coding에 의존한다

• Higher modulation and coding level requires higher SNR

Fading

• received signal power에 영향을 주는 요소는 Pathloss와 Fading이 있다

• Fading : Deviation of attenuation affecting a signal

  • Slow fading (large-scale fading, shadowing)

    • 단순히 장애물 때문에 발생하는 감쇄의 차이

  • Fast fading (small-scale fading)

    • 파장의 강화 or 상쇄로 인한 감쇄의 차이
      

    Slow (large-scale) fading

    • Large obstruction such as hill or large building obscures the main signal path between the transmitter and the receiver
    • Modeled using log-normal distribution

    Fast (small-scale) fading

    • Caused by multipath propagation
    • Amplitude and phase change imposed by the channel varies considerably over the period of use

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Wi-Fi : Physical Layer Aspects

Physical Layer에서,,,

• Signal generation from bits
• Recovery of bits from the received signal
• Tightly coupled with physical medium
• Modulation and coding scheme (MCS)

Modulation

• 정보를 담고 있는 modulating signal을 이용해서
  carrier signal의 하나 이상의 properties를 구분하는 과정

• carrier signal : a high frequency signal for carrying data

  • E.g. WLAN channels
    

Modulation

• ASK, FSK, PSK
• QAM (PSK + ASK)

Modulation used in Wi-Fi

• BPSK (Binary PSK) : encode 1 bit on a symbol
  • 2 different signals

• QPSK (Quadrature PSK) : encode 2 bits on a symbol
  • 4 different signals

• 16-QAM : encode 4 bits on a symbol
  • 16 different signals

• 64-QAM : encode 6 bits on a symbol
  • 64 different signals

Coding (Channel Coding)

• message가 에러에 tolerable할 수 있도록
  error-correcting codes (ECC)를 삽입한다
  • E.g. Hamming (7, 4) code
    : corrects 1-bit error in a 7-bit message

• Coding rate
  • message에서 data bit의 비율
  • E.g. 3/4 coding rage : 75% data, 25% ECC
    높아질수록 데이터 효율은 증가하지만,
    에러 발생했을 때 correction이 줄어든다

MCS (Modulation and Coding Scheme)

• modulation scheme과 coding rate를 정의한다
• transmitter는 receiver에게 어떤 MCS level을 사용할 지 알려줘야 한다
• MCS table
  

• MCS level의 선택은 link quality에 의존한다
• Higher MCS level requires higher SNR
• SNR이 동적으로 변할 수 있기 때문에,
  그에 맞춰서 MCS level도 변한다

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MIMO (Multiple Input Multiple Output)

• communication performance를 향상시키기 위해
  transmitter와 receiver 둘 다 여러 개의 안테나를 사용한다

advantage

• array gain : improves spectral effiency
  • transmit different data on multiple antennas
  • 안테나 간의 공간 분리를 이용해서
    동일한 전력 수준으로 더 멀리 있는 수신기까지 신호 전송이 가능해진다
  • M transmit antenna, N receive antenna가 있을 때,
    array gain = min(M, N)

• diversity gain : improves link reliablity
  • transmit the same data on multiple antennas using space-time coding
  • 여러 개의 안테나별로 fading이 다르기 때문에
    그 중 가장 좋은 걸 고르거나 합쳐서 SNR을 향상시킬 수 있다

Physical layer: OFDM

• A specialized FDM (frequency division multiplexing)
• 서로서로 orthogonal인 sub-carrier들이 선택된다
  • 서로서로 interfere하지 않는다.

• can easily adapt to channel conditions
• robust against narrow-band co-channel interference
• robust against inter-symbol interference (ISI)
  and fading casued by multipath propagation
• High spectrum efficiency compared to spread spectrum techniques
  

802.11n OFDM - 20MHz channel

• 20MHz channel, divided into 56 subcarriers
• 실제로는 52 subcarriers가 data에 사용되고,
  4 subcarriers는 pilot이다
• 각 subcarrier : 312.5kHz,
  signal bandwidth : 17.5MHz
• symbol duration : 3.6us (guard interval : 0.4us 포함)
  

802.11n OFDM - 40MHz channel

• 40MHz channel, divided into 114 subcarriers
• 108 subcarriers used for data,
  6 subcarriers for pilot
• Each subcarriers : 312.5kHz
  signal bandwidth : 35.625MHz
• symbol duration : 3.2us + guard interval

802.11n OFDM - Modulation & Coding

600Mbps를 만들기 위해,

• Modulation : 64QAM
  • subcarrier 하나에 6 bit가 code된다
• Coding : 5/6
  • 16.6% redundant bits for error correction
• Data bits per OFMD symbol (40MHz)
  • 6 (5/6) 108 subcarriers = 540 bits
• Number of OFDM symbols per second
  • 1s / 3.6us = 277,777
• Data rate
  • 540 * 277,777 = 150Mbps

• MIMO
  • Linear capacity growth with minimum number of antennas
  • Use 4x4 MIMO : 150Mbps -> 600Mbps