Spread Spectrum Techniques Overview

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Questions and Answers

The technique that permits more users to occupy a given band than other methods is known as ______.

CDMA

Orthogonal Frequency Division Multiplexing (OFDM) combines ______ and multiplexing.

modulation

One of the main benefits of spread spectrum is its resistance to ______ and interference.

jamming

In OFDM, the single serial data stream is divided into multiple slower but ______ data paths.

parallel Signup and view all the answers

The ______ is a key factor in the expression for processing gain G = BW/fb.

channel bandwidth Signup and view all the answers

Direct-sequence spread spectrum (DSSS) is also referred to as ______ multiple access.

code-division Signup and view all the answers

A common format in OFDM is to space the subcarriers equally across the channel by a frequency that is the ______ of the subcarrier symbol rate.

reciprocal Signup and view all the answers

One example of a frequency range is ______ MHz for bandwidth in processing gain calculations.

1.25 Signup and view all the answers

Spread spectrum is a modulation technique that distributes a signal over a very wide ______.

bandwidth Signup and view all the answers

Orthogonal Frequency Division Multiplexing is a method of wideband modulation that allows for the transmission of data over ______ subcarriers.

multiple Signup and view all the answers

The two primary types of spread spectrum techniques are Frequency-Hopping Spread Spectrum and ______.

Direct-Sequence Spread Spectrum Signup and view all the answers

In Frequency-Hopping Spread Spectrum, the frequency of the carrier is changed according to a predetermined ______.

sequence Signup and view all the answers

In Frequency-Hopping Spread Spectrum, the synthesizer frequency changes at a rate higher than the ______.

data rate Signup and view all the answers

The Direct-Sequence Spread Spectrum technique mixes the serial binary data with a higher-frequency ______ binary code.

pseudorandom Signup and view all the answers

The time that the synthesizer remains on a single frequency is known as ______.

dwell time Signup and view all the answers

The goal of wideband modulation methods is to transmit as many bits per ______ as possible.

hertz Signup and view all the answers

The pseudorandom code generator can either be a special digital circuit or the output of a ______.

microprocessor Signup and view all the answers

One of the key advantages of spread spectrum is that it is essentially immune to ______.

jamming Signup and view all the answers

The most widespread use of spread spectrum in cellular communications is referred to as ______ division multiple access.

code Signup and view all the answers

In Direct-Sequence Spread Spectrum, a chip is the unit time for the ______ code.

pseudorandom Signup and view all the answers

After World War II, spread spectrum techniques were primarily developed by the ______ for secure communications.

military Signup and view all the answers

The effect of spreading the signal provides a type of gain known as ______ gain.

processing Signup and view all the answers

Frequency-Hopping Spread Spectrum randomizes the signal so that it does not remain on any single ______ for a long time.

frequency Signup and view all the answers

In the context of spread spectrum, spectral efficiency refers to the effective use of ______ bandwidth.

radio Signup and view all the answers

Study Notes

Spread Spectrum

It is a modulation and multiplexing technique that distributes a signal and its sidebands over a very wide bandwidth
It was first used in military communication systems due to its inherent security and jamming resistance.
It gained civilian use in the mid 1980s via FCC authorization.
It has been implemented in cellular telephones including CDMA.

Frequency Hopping Spread Spectrum (FH/SS)

Serial binary data modulates a conventional two-tone FSK, or higher MFSK.
The FSK output is then mixed with a frequency synthesizer output
The synthesizer is driven by a pseudorandom code generator, either a dedicated digital circuit or the output of a microprocessor.
The code generator produces a random sequence of 0s and 1s, giving its output the appearance of digital noise.
The pseudorandom code is actually predictable, as it repeats after a certain number of bit changes. This predictability is why the code is classified as “pseudo” random.
The frequency synthesizer changes frequencies at a rate higher than the data rate.
The FSK tone stays constant for one data interval but the frequency synthesizer switches rapidly through multiple frequencies within that interval.
The dwell time is defined as the duration the synthesizer remains on a single frequency.
FCC regulations mandate a minimum of 75 hopping frequencies and a maximum dwell time of 400 μs.

Direct Sequence Spread Spectrum (DS/SS)

Serial binary data is XORed with a serial pseudorandom code.
One bit time for the pseudorandom code is called a chip.
The rate of the code is called the chipping rate.
It is also known as Code-Division Multiple Access (CDMA), or SS multiple access.
CDMA is used in satellite systems for multiplexing signals on a single transponder.
It's also widely used in cellular systems, allowing more users to occupy a given band compared to other methods.

Processing Gain

Spreading the signal using DS/SS provides a gain called processing gain that improves the overall signal-to-noise ratio.
Higher processing gain signifies greater interference resistance.
Processing gain (G) is calculated as: G = BW / fb.
BW represents the channel bandwidth and fb stands for the data rate.
Example: If BW is 1.25 MHz and fb is 13 kbps, then G = 1.25 MHz / 13 kbps = 96.15.

Benefits of Spread Spectrum

Security: Spread spectrum is inherently secure due to its wide bandwidth and pseudorandom noise.
Resistance to Jamming and Interference: The signal is spread over a wide bandwidth, making it difficult to jam.
Band Sharing: It permits multiple users to share the same frequency band without interfering with one another.
Resistance to Fading and Multipath Propagation: The spreading effect helps mitigate fading and multipath effects common in wireless communication environments.
Precise Timing: Spread spectrum signals contain precise timing information, which is crucial for synchronization in communication systems.

Orthogonal Frequency Division Multiplexing (OFDM)

It is a combination of modulation and multiplexing.
It is also called multicarrier modulation (MCM).
Despite its initial proposal in the 1950s, its practical implementation was delayed until the late 1990s due to complexity and cost.
Modern advancements in fast DSP chips have made OFDM more practical.
It transmits data using multiple carriers spread across the channel bandwidth.
This technique spreads the signals over a wide range of frequencies, making them robust against noise, fading, reflections, and multipath effects common in microwave communication.

Orthogonal Frequency Division Multiplexing (OFDM) Continued

Single serial data stream splits into multiple slower parallel data paths, each modulating a separate subcarrier.
The parallel data paths are modulated using various modulation techniques like QPSK or 16-QAM.
This technique is efficient in reducing the peak power for the signal due to the parallel nature of transmission.
Each subcarrier is spaced by a frequency that's the reciprocal of the subcarrier symbol rate.
This equal spacing property makes the subcarriers orthogonal.
Despite its complexity, OFDM offers excellent performance in multipath environments.
This makes it popular in both wired and wireless applications like DSL, Wi-Fi, and 4G/5G mobile systems.
It is well-suited for high-speed data transmission, where the ability to achieve high data rates in the presence of multipath fading is crucial.
It is a core technology in many modern communication systems.

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Explore the fascinating world of spread spectrum techniques and their applications, first used in military communications and later adapted for civilian technologies. This quiz covers both the fundamentals of spread spectrum and delves into the specifics of Frequency Hopping Spread Spectrum (FH/SS). Test your knowledge on modulation methods and their historical significance.