Podcast
Questions and Answers
The technique that permits more users to occupy a given band than other methods is known as ______.
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.
Orthogonal Frequency Division Multiplexing (OFDM) combines ______ and multiplexing.
modulation
One of the main benefits of spread spectrum is its resistance to ______ and interference.
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.
In OFDM, the single serial data stream is divided into multiple slower but ______ data paths.
The ______ is a key factor in the expression for processing gain G = BW/fb.
The ______ is a key factor in the expression for processing gain G = BW/fb.
Direct-sequence spread spectrum (DSSS) is also referred to as ______ multiple access.
Direct-sequence spread spectrum (DSSS) is also referred to as ______ multiple access.
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.
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.
One example of a frequency range is ______ MHz for bandwidth in processing gain calculations.
One example of a frequency range is ______ MHz for bandwidth in processing gain calculations.
Spread spectrum is a modulation technique that distributes a signal over a very wide ______.
Spread spectrum is a modulation technique that distributes a signal over a very wide ______.
Orthogonal Frequency Division Multiplexing is a method of wideband modulation that allows for the transmission of data over ______ subcarriers.
Orthogonal Frequency Division Multiplexing is a method of wideband modulation that allows for the transmission of data over ______ subcarriers.
The two primary types of spread spectrum techniques are Frequency-Hopping Spread Spectrum and ______.
The two primary types of spread spectrum techniques are Frequency-Hopping Spread Spectrum and ______.
In Frequency-Hopping Spread Spectrum, the frequency of the carrier is changed according to a predetermined ______.
In Frequency-Hopping Spread Spectrum, the frequency of the carrier is changed according to a predetermined ______.
In Frequency-Hopping Spread Spectrum, the synthesizer frequency changes at a rate higher than the ______.
In Frequency-Hopping Spread Spectrum, the synthesizer frequency changes at a rate higher than the ______.
The Direct-Sequence Spread Spectrum technique mixes the serial binary data with a higher-frequency ______ binary code.
The Direct-Sequence Spread Spectrum technique mixes the serial binary data with a higher-frequency ______ binary code.
The time that the synthesizer remains on a single frequency is known as ______.
The time that the synthesizer remains on a single frequency is known as ______.
The goal of wideband modulation methods is to transmit as many bits per ______ as possible.
The goal of wideband modulation methods is to transmit as many bits per ______ as possible.
The pseudorandom code generator can either be a special digital circuit or the output of a ______.
The pseudorandom code generator can either be a special digital circuit or the output of a ______.
One of the key advantages of spread spectrum is that it is essentially immune to ______.
One of the key advantages of spread spectrum is that it is essentially immune to ______.
The most widespread use of spread spectrum in cellular communications is referred to as ______ division multiple access.
The most widespread use of spread spectrum in cellular communications is referred to as ______ division multiple access.
In Direct-Sequence Spread Spectrum, a chip is the unit time for the ______ code.
In Direct-Sequence Spread Spectrum, a chip is the unit time for the ______ code.
After World War II, spread spectrum techniques were primarily developed by the ______ for secure communications.
After World War II, spread spectrum techniques were primarily developed by the ______ for secure communications.
The effect of spreading the signal provides a type of gain known as ______ gain.
The effect of spreading the signal provides a type of gain known as ______ gain.
Frequency-Hopping Spread Spectrum randomizes the signal so that it does not remain on any single ______ for a long time.
Frequency-Hopping Spread Spectrum randomizes the signal so that it does not remain on any single ______ for a long time.
In the context of spread spectrum, spectral efficiency refers to the effective use of ______ bandwidth.
In the context of spread spectrum, spectral efficiency refers to the effective use of ______ bandwidth.
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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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