Wireless specifics by gjmpzlaezgx


									Wireless specifics
A Wireless Communication System


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.,
  GSM—Global Service for Mobile
  communications and IS-54, both use TDMA and
 CDMA (Code Division Multiple Access, e.g., IS-
  95, WCDMA, CDMA2000)
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

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
  policy—users form a queue waiting for their
  turns to use the connection. It’s 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.

Techniques used by all

Spread Spectrum Transmission
 FHSS (Frequency Hopping Spread
 DSSS (Direct Sequence Spread
OFDM (Orthogonal Frequency Division

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.

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.
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.

 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
 If the center frequency moves among 100 different
  frequencies, the required transmission bandwidth is at
  least 100 times as large as the original transmission

Example of FHSS

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.

FHSS in 802.11 or WiFi (p. 115, Example
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

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.

 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.
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 it’s not used for
  security. It’s used to spread the bandwidth.

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

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 don’s 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.

OFDM (Orthogonal Frequency
       Division Multiplexing)
What is OFDM
 Assume we need to send data at a speed of R
 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.

Advantages of OFDM

Robust against multipath interference
  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 don’t interfere with
   each other).
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)
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.
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.
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.
Assigning sub-channels


Time diversity
Frequency diversity
Space diversity

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)

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.

Frequency diversity

Frequency selective fading (p. 128, Fig.
Frequency hopping and IEEE 802.11
Frequency hopping and GSM

Space diversity

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

Coding techniques

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

Voice-oriented and data-oriented
 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
 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.
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.


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