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Baseband and Broadband. Baseband – wired only – the signal goes down the wire without any modulation Broadband – wired and wireless – the signal is modulated on a carrier. Channel Frequencies and Channel Spacing. Figure 4-25: Power vs Frequency of the Transmitter Output.

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baseband and broadband
Baseband and Broadband
  • Baseband – wired only – the signal goes down the wire without any modulation
  • Broadband – wired and wireless – the signal is modulated on a carrier.

Modulation Types and TerminologyNot all of these are important for this course. The ones you should know are Amplitude Modulation, On-Off Keying, Frequency Modulation, Frequency Shift Keying, Phase Shift Keying, Quadrature Phase Shift Keying, and Quadrature Amplitude Modulation. We will discuss these in the following slides.

  • Keying is the inclusion of digital data carried over an analog carrier.
  • It is a type of modulation, namely modulation of digital data.
  • There are numerous types of keying, all designed to get as much data as possible through a given bandwidth.

Binary Phase Shift Keying (BPSK) ModulationQuadrature Phase Shift Keying (QPSK) is similar, but uses four phases, 0o, 90o, 180o, 270o.

bits and symbols
Bits and Symbols
  • By combining different types of modulation, it is possible to transmit using more than just a on - off (1 – 0) signal.
  • If the transmission can be sent and received with four different values, you can transmit 2 bits at once.
  • If you can use 8 values, you can transmit 3 bits. 16 values gives you 4 bits, etc.
spread spectrum technologies
Spread Spectrum Technologies

Spread Spectrum Modulated Unmodulated Multiple channels over the same bandwidth Single Channel Carrier


Originally, 3 different channel options for 802.11:

  • 1. Infrared (similar to TV remote controls)
  • 2. Frequency Hopping Spread Spectrum (FHSS) 2.4 GHz
  • 3. Direct Sequence Spread Spectrum (DSSS) 2.4 GHz
  • (Additionally, Time Hopped Spread Spectrum (THSS) can
  • also be used to help save battery life on wireless receivers)
  • Radiated Power is limited
  • 1W in U.S.
  • Approx. 20mW in Europe, 1mW in Japan
frequency hopping spread spectrum
Frequency Hopping Spread Spectrum

Frequency-Hopping Spread Spectrum (FHSS) is a method where the user is rapidly shifted from channel to channel across a given spread spectrum frequency band. A code is used to provide a pattern for the frequency shifts. Codes can be selected so that multiple users all hopping across the same band at the same time will have minimal mutual interference. FHSS is used extensively in wireless LAN applications.


Frequency Hopping Spread Spectrum (FHSS)

Digital multiple access scheme in which carrier frequencies of individual

users are varied in pseudorandom fashion within a wideband channel.


User i

User j


Data of each user is broken into uniform sized bursts, which are transmitted

on different channels within the total spectrum band.

Instantaneous bandwidth any one transmission burst is much smaller than

total spread bandwidth.





Spectrally-inefficient if used by a single user

Complex frequency synthesizer is required

Error correction is required

Provides a level of security; interception difficult

without knowledge of pseudorandom sequence

of frequency hops.

FH signal is somewhat immune to fading with

error control coding and interleaving.

RF signal is dehopped at the receiver using a frequency synthesizer

controlled by a synchronized pseudorandom sequence generator.

Examples: Bluetooth and HomeRF


802.11 FHSS

  • 22 hop patterns to choose from
  • 79 hopping frequencies
  • 50 hops/sec.
  • 1 or 2 Mbps
  • Does not scale well to higher data rates.
code division multiple access
Code Division Multiple Access

Code Division Multiple Access (CDMA) allows all users to share the same radio channel at the same time. Users are separated by different codes. Although all stations operate on the same carrier frequency, the receiving station knows in advance the specific code assigned to the desired transmitting station. It can extract the desired transmitter information from the desired station and reject all other stations that are using other codes. The codes are digital sequences of binary ones and zeros. In a CDMA access system, the codes are selected so that they are different. In CDMA terminology, the codes are said to be orthogonal. A channel, in CDMA terms, is therefore a unique code assigned to a base station and a mobile station rather than a separate frequency. CDMA has become a common term used synonymously with Direct Sequence Spread Spectrum communications technology.

direct sequence spread spectrum
Direct Sequence Spread Spectrum

Direct Sequence Spread Spectrum (DSSS) uses a carrier wave that is modulated with a binary code whose bit rate is much faster than the information bit rate. This code is called the spreading code. The spreading code allows all users to simultaneously transmit across the entire spread spectrum bandwidth. The receiver knows the spreading code of the transmitter it wants to listen to. It uses the spreading code to pick out the signal of that transmitter and to ignore all the other transmitters.


Direct Sequence Spread Spectrum (DSSS)

A narrowband message signal is multiplied by a very large bandwidth

signal called the spreading signal.

Spreading signal is a pseudonoise sequence (PN) composed of

+1 and –1 (derived from a PN sequence of 1 and 0).

“Chips” of duration Tc



There are Tb/Tc chips in a spreading

signal. Each user’s bit is multiplied by

the spreading signal assigned to that




message bit, Tb



  • Every user uses the same set of frequencies at the same time. Their
  • signal is distinguishable because of the unique PN sequence assigned
  • to each system user.
  • Receiver extracts desired user’s signal by correlating received signal
  • with the user’s spreading sequence.
  • Since users have PN spreading sequence, they are near orthogonal,
  • only some residual interference seeps into the detecting of a desired
  • user’s bit. “Orthogonal” means that signals are similar in structure, but
  • mathematically independent of one another.
  • Strict power control is needed to ensure all users received power is
  • the same, otherwise it is hard to “pick out” the weaker signal.

802.11b Spreading

  • 802.11 transmitter combines the message bits with the
  • a spreading sequence bits using a binary adder.
  • For 1Mbps and 2Mbps operation, spreading code is the
  • 11-chip Barker sequence, which is 10110111000.
  • 5.5Mbps and 11Mbps operation of 802.11b uses complementary
  • code keying (CCK) to provide spreading sequences at these
  • higher data rates.
  • CCK derives a different spreading code based on fairly complex
  • functions depending on the pattern of bits being sent. Modulator
  • simply refers to a table for spreading sequence that corresponds
  • to the pattern of data bits being sent.
  • CCK has good performance in multipath environments.

Orthogonal Frequency Division Mulitplexing (OFDM)

  • Used with the 802.11g and European-based HiperLAN/2
  • wireless LAN standards.
  • Also used in 802.11a
  • In addition, OFDM has also been around for a while supporting
  • the global standard for asymmetric digital subscriber line (ADSL).

OFDM Modulation

  • 802.11a uses OFDM which breaks a single 20 MHz channel
  • of a high-speed data carrier into 52 lower-speed subcarriers.
  • Each subchannel is 312.5 kHz wide. These subchannels are
  • then transmitted in parallel. OFDM uses 48 of these subchannels
  • for data and the remaining four as pilot subcarriers.

52 subcarriers


8 Channels

One channel detail, 20 MHz

Each carrier 312.5 kHz wide


Higher-Level Modulation Schemes for Higher Data Rates

  • To achieve higher data rates, higher-level modulation schemes
  • are used on each subcarrier.
time hopped spread spectrum
Time-Hopped Spread Spectrum

Time-Hopped Spread Spectrum (THSS) is a third type of spread spectrum communications. With THSS, the transmitter is pulsed on and off. The period and the duty cycle are varied by a code similar to FHSS. The advantage of a pulsed THSS system is the very low duty cycles that are used, which means they have very low power consumption and battery drain. THSS is often used in hybrid systems with FHSS.