Physical Layer (2)

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Physical Layer (2)
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Goal

• Physical layer design goal send out bits as fast
  as possible with acceptable low error ratio
• Goal of this lecture
• Review. Modulation, demodulation, maximum
  likelihood detection
• Introduction to wireless communications, CDMA,
  OFDM, MIMO
• Introduction to error control codes

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Review

• What is modulation/demodulation?
• What is the waveform of a BPSK signal carrying
  bits 00110101?
• What is the three general steps to demodulate a
  received signal?
• What is detection? What is maximum likelihood
  detection?
• With BPSK, -1 and 1. The noise could take 3
  values, -1, 0, 1, with probability 0.1, 0.8, and
  0.1, respectively. What is the error probability?
  

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Cellular Phone Networks

• User base station Telephone network
• FDMA Frequency division multiplexing
• How to make sure that you are using this band,
  not that band?
• TDMA Time division multiplexing
• CDMA Code division multiplexing

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GSM Global System for Mobile Communications

• Second generation cell phone system (digital,
  first generation analog).
• GSM-900 and GSM-1800 are most widely used
• GSM-900 uses 890 - 915 MHz to send information
  from the mobile station to the base station
  (uplink) and 935 - 960 MHz for the other
  direction (downlink).
• FDMA TDMA
• Each user transmitting on a frequency and
  receiving on another frequency.
• 124 pairs of 200 KHz channels. Each channel
  divided into time slots for 8 users.
• Each user is has a chance to transmit every 4.615
  ms. Each time he can send 114 data bits
  24.7kbps.

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CDMA

• Described in IS-95.
• A good analogy in the book You have a group of
  people in a room. TDMA means they talk in turn.
  FDMA means that those who wants to talk sit in
  different corners and cant hear other pairs.
  CDMA means each pair talks in a different
  language and other peoples voices are noise to
  them.

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CDMA

• The whole bandwidth is used by every user.
  Meaning that they can send out symbols really
  fast.
• The trick is to make what A sent appear as 0 to
  B.
• Because we have a fast symbol rate, for each data
  bit, we send out, say, 8 bits, call the small
  bits chips.
• Given a bit, if 1, send out, say,
  -1,-1,-1,1,1,-1,1,1, and if 0, 1,1,1,-1,-1,1,-1,-1
  
• This is called the chip sequence.
• The key is that each station has a unique chip
  sequence (language), and different languages are
  orthogonal. Fig. 2-45.

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Wireless LAN Physical Layer

• 802.11b,g in the 2.4G band and 802.11a in the 5G
  band. People now consider 802.11 as the notion of
  MAC layer protocol, while a, b, g, or n, are
  about physical layer.
• 802.11b. 1, 2, 5.5, 11Mbps.
• 1Mbps BPSK modulation. 1 bit into 11 chips with
  Barker sequence 1 1 1 -1 -1 -1 1 -1 -1 1 -1.
  Why spread spectrum? Required by FCC but was
  later removed
• 2Mbps QPSK.
• 5.5M and 11M use some bits to select chip
  sequence and use two bits for QPSK
• 802.11a. Up to 54Mbps. OFDM.
• 802.11g. Up to 54Mbps. OFDM.

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OFDM (Orthogonal Frequency Division Multiplexing)

• In wireless communications, in addition to the
  bandwidth limit and additive noise, you also have
  multipath fading!
• The faster your symbol rate is, the more badly
  you will be affected by multipath fading.
• In effect, OFDM is like DSL given a wideband
  channel, divide it into many sub channels. Each
  sub-channel can be modulated/demodulated
  independently. Because each sub-channel is of a
  much smaller bandwidth, multipath fading is much
  less severe.
• In implementation, use IFFT and FFT.