Wireless specifics

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Wireless specifics
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A Wireless Communication System
Antenna
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Technologies for Cell phones to Handle Multiple
Users

• Unique feature for voice communications
• FDMA (Frequency Division Multiple Access) in 1G
  cellular phone technology
• TDMA (Time Division Multiple Access, e.g.,
  GSMGlobal Service for Mobile communications and
  IS-54, both use TDMA and FDMA)
• CDMA (Code Division Multiple Access, e.g., IS-95,
  WCDMA, CDMA2000)

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Technologies for Cell Phones to Handle Up and
Down Links

• TDD (Time Division Duplex) forward (down link)
  and reverse (up link) channels use the same
  frequency band but alternating time slots
• FDD (Frequency Division Duplex) forward and
  reverse channels use different carrier frequencies

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Technology used by WiFi, etc. to handle multiple
users

• It is a time division method.
• Unlike the TDMA in cell phones, in WiFi no
  specific time slots are assigned to users.
  Instead, it is basically a first-come-first-served
  policyusers form a queue waiting for their
  turns to use the connection. Its very much like
  the rule used in a bank or a computer network.
• It is good for data transmission, but not that
  ideal for voice transmission.

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Techniques used by all

• Spread Spectrum Transmission
• FHSS (Frequency Hopping Spread Spectrum)
• DSSS (Direct Sequence Spread Spectrum)
• OFDM (Orthogonal Frequency Division Multiplexing)

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What is Spread Spectrum Transmission

• The traditional transmitted signal has a
  bandwidth of the same order as the information
  signal at the baseband. For example, the
  bandwidth of a voice signal is about 4 kHz (the
  baseband). After modulation, as it being
  transmitted it still occupies several thousand Hz
  but at a much higher frequency.
• The spread spectrum signal occupies a much larger
  bandwidth.

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Why spread spectrum

• It is robust against frequency selective fading
  in urban and indoor environments.
• It is robust against interference emitted by
  machines, microwave ovens, etc.
• It can provide additional security.
• It can provide greater operational flexibility
  and system capacity, as in CDMA.
• It is required by regulation to use spread
  spectrum in unlicensed ISM (Industrial,
  Scientific and Medical) bands.

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Spread spectrum methods

• Frequency hopping spread spectrum (FHSS)
• The transmitter constantly, often randomly,
  shifts the center frequency of the transmitted
  signal.
• Only the machine that knows the hopping pattern
  can receive the signal.
• Direct sequence spread spectrum (DSSS)
• Each transmitted bit is spread into N smaller
  pulses (chips) before transmission.
• Only the machine that knows the pattern of the
  spread can retrieve the signal.

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FHSS

• Invented by Austrian-born movie star Hedy Lamarr
  to protect guided torpedoes from jamming
• The shifts in frequency (hops) occur according to
  a random pattern that is known only to the
  transmitter and the receiver. (Actually it is
  pseudo random It is generated by an algorithm,
  so it is not really random but to a person who
  does not know the pattern, it looks random.)
• If the center frequency moves among 100 different
  frequencies, the required transmission bandwidth
  is at least 100 times as large as the original
  transmission bandwidth.

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Example of FHSS
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FHSS and GSM (p.115, Example 3.14)

• If the channel coincides with a deep frequency
  selective fading or when the cochannel interfence
  from another cell using the same frequency is
  excessive, the distortion in the received voice
  signal will be large. A slow frequency hopping
  of 217.6 hops per second can be used in GSM to
  tackle these problems.

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FHSS in 802.11 or WiFi (p. 115, Example 3.15)

• Uses 78 hopping channels each separated by 1 MHz.
  These frequencies are divided into three
  patterns of 26 hops each corresponding to channel
  numbers (0, 3, 6, 9, , 75), (1, 4, 7, 10, ,
  76), (2, 5, 8, 11, , 77). These choices are
  available for three different systems to coexist
  without any hop collision.
• 2.5 hops per second

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FHSS in Bluetooth (p.129, Example 3.24)

• Uses a fast frequency hopping (1,600 hops per
  second) over 79 MHz of bandwidth. That is, it
  hops over 79 channels each separated by 1 MHz.

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DSSS

• The transmitter spreads one bit, say a one or a
  zero (you either have a one or a zero in the
  digital world), into many (N) smaller chips (they
  are a sequence of zeros and ones) according to a
  code known to the transmitter and the receiver.
• The receiver, using the code and a correlator,
  put the spread chips together to get the original
  bit.
• The bandwidth of the original signal will be N
  times wider after the spreading because the chip
  rate is N times faster than the bit rate.
  Therefore the signal will be more robust against
  interference and fading.
• The code used for spreading and de-spreading can
  be secret, if only the transmitter and the
  receiver know it, thus providing a security
  measure.

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DSSS in 802.11

• The code (called Baker code) used in 802.11 to
  spread the data bits is given by 1, 1, 1, -1,
  -1, -1, 1, -1, -1, 1, -1. So one bit of data is
  spread to become 11 chips.
• The Baker code is not a secret code, so its not
  used for security. Its used to spread the
  bandwidth.

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DSSS in 802.11 (p. 117, Fig. 3.23)
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More about DSSS

• The bandwidth of the transmitted DSSS signal is N
  times as large as that of the original signal.
• CDMA uses DSSS. Each user is given a unique code
  that other users dons know. Although a user can
  receive the signals sent to other users by the
  transmitter, in the de-spreading process only the
  signal sent to that user can be detected. The
  interference generated by other users is very
  small.

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OFDM (Orthogonal Frequency Division Multiplexing)
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What is OFDM

• Assume we need to send data at a speed of R
  symbols/sec.
• We break the data sequence into N (an integer,
  say, 48) sub-sequences. The data rate of each
  sub-sequence will be R/N, much slower than the
  original sequence.
• N carriers are used, each having a different
  frequency and each sending one sub-sequence.
• At the receiver end, the N sub-sequences are put
  together to get the original data sequence.

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Advantages of OFDM

• Robust against multipath interference because
• In each sub-sequence the symbols are N times
  longer than the original symbols.
• The longer the symbol, the weaker the multipath
  interference (the signals representing the same
  symbol but coming from multiple paths will be
  close enough compared with the width of the
  symbol so they dont interfere with each other).

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Advantages of OFDM

• Robust against frequency selective fading
• To battle the frequency selective fading (signals
  at certain frequencies might be much weaker than
  that of other frequencies), error-control coding
  can be used in each subchannel.
• If the signal for a particular subchannel(s) is
  weak, the transmitting power of that subchannel
  can be increased to compensate for the fading.
• High spectral efficiency (high bit rate to
  bandwidth ratio)

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Drawbacks of OFDM

• Complexity you need to put together N signals to
  rebuild the original one.
• Sensitive to Doppler Shift. When receiver is
  moving at a high speed, the received frequency
  will shift, too, and that can cause problem for
  OFDM.
• High peak-to-average-power ratio (PAPR) and thus
  lower efficiency.

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Example OFDM in 802.11a (p.109)

• 64 subchannels are used, among which 48 are used
  for data transmission, the remaining 16 are for
  other purposes.
• The symbol rate of each channel is 250 kilo
  symbols per second (ksps).
• The actual data rate for the user is 48X250 ksps
  12 Msps.
• The overall bandwidth is 20 MHz.

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SOFDMA (Scalable Orthogonal Frequency Division
Multiple Access)

• Used in 802.16e (WiMax for mobile users)
• The same OFDM technique will be used, but each
  user may only get a part of the spectrum,
  depending on the application the user is running.
  TDMA is also used.
• In 802.16-2004 (WiMax for fixed users) OFDM is
  used and a user get all the available spectrum.
  Users are separated by TDMA.

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Assigning sub-channels
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Diversity

• Time diversity
• Frequency diversity
• Space diversity

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Time diversity DSSS and RAKE receiver

• Using the signals from different paths to get one
  stronger signal. Signals from different paths
  arrive at the receiver at different time
  instances. Longer paths create longer delays.
• Due to multiple paths, each bit sent by the
  transmitter can create several peaks at the
  output of the correlator (Fig. 3.25, p. 122).
• The RAKE receiver is designed to put the peaks
  together. (p. 124, Fig. 3.26)

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Time diversity (contd.)

• Multipath reception in CDMA
• Chip rate 1.25 Mcps, symbol rate 4,800 Sps
• Can resolve multipath components 1/1.25 Mcps
  800 ms apart.
• A multipath spread of up to 1/4800 bps 2.08 ms
  cannot cause ISI.

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Frequency diversity

• Frequency selective fading (p. 128, Fig. 3.30)
• Frequency hopping and IEEE 802.11
• Frequency hopping and GSM

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Space diversity

• Four methods to take advantage of space diversity
  (p. 132, Fig. 3.32)
• Trisectored antennas for CDMA

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Coding techniques

• Error control coding
• Coding for spread spectrum (CDMA)
• Orthogonal codes

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Voice-oriented and data-oriented networks

• Voice-oriented networks use the so-called
  fixed-assignment methods. Each user is assigned
  a slot of time, a portion of frequency band, or a
  specific code for the entire length of the
  conversation.
• Data-oriented networks use random-access methods.
  Users share the same medium (air or wire). Since
  data arrive at random instances, the medium will
  be assigned to each user in a random fashion.

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Comparison of two methods

• Fixed assignment ensures constant connection,
  which is needed for voice communication, but can
  have low utilization rate.
• Random access method is more suitable for data
  communication, because data arrive in bursts.