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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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.
Binary Phase Shift Keying (BPSK) ModulationQuadrature Phase Shift Keying (QPSK) is similar, but uses four phases, 0o, 90o, 180o, 270o.
Spread Spectrum Modulated Unmodulated Multiple channels over the same bandwidth Single Channel Carrier
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.
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
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 (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
Orthogonal Frequency Division Mulitplexing (OFDM)
One channel detail, 20 MHz
Each carrier 312.5 kHz wide
Higher-Level Modulation Schemes for Higher Data Rates
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.