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Questions and Answers
What is a key characteristic of Frequency Division Multiple Access (FDMA)?
What is a key characteristic of Frequency Division Multiple Access (FDMA)?
Which of the following signals represents cross-channel interference in a multi-carrier system?
Which of the following signals represents cross-channel interference in a multi-carrier system?
In Time Division Multiple Access (TDMA), what role do synchronization bits play?
In Time Division Multiple Access (TDMA), what role do synchronization bits play?
What does dynamic TDMA allow that is different from static TDMA?
What does dynamic TDMA allow that is different from static TDMA?
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What is the primary function of guard bands in FDMA?
What is the primary function of guard bands in FDMA?
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What is the spectrum spreading factor in a DSSS system?
What is the spectrum spreading factor in a DSSS system?
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What does the receiver multiplier do to the synchronized component of the received signal?
What does the receiver multiplier do to the synchronized component of the received signal?
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How is the fraction of power of the unwanted signal determined?
How is the fraction of power of the unwanted signal determined?
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In a DSSS system, what must be true for communication to be disrupted?
In a DSSS system, what must be true for communication to be disrupted?
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What is the effect of the DSSS system on the S/(N+I) ratio after the receiver multiplier?
What is the effect of the DSSS system on the S/(N+I) ratio after the receiver multiplier?
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What is the primary feature of pure ALOHA access?
What is the primary feature of pure ALOHA access?
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What inefficiency is commonly associated with TDMA in packet transmission?
What inefficiency is commonly associated with TDMA in packet transmission?
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Which access method allocates different frequency carriers to active users?
Which access method allocates different frequency carriers to active users?
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How does slotted ALOHA improve upon pure ALOHA?
How does slotted ALOHA improve upon pure ALOHA?
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Which of the following applications utilizes Code Division Multiple Access (CDMA)?
Which of the following applications utilizes Code Division Multiple Access (CDMA)?
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What is a significant feature of Code Division Multiple Access?
What is a significant feature of Code Division Multiple Access?
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In Time Division Multiple Access (TDMA), what do active users utilize?
In Time Division Multiple Access (TDMA), what do active users utilize?
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According to Shannon's formula, what does 'C' represent in the context of a Gaussian channel?
According to Shannon's formula, what does 'C' represent in the context of a Gaussian channel?
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Which method is characteristic of Code Division Multiple Access (CDMA)?
Which method is characteristic of Code Division Multiple Access (CDMA)?
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Which of the following conditions must be true for maximum transmission in a CDMA system?
Which of the following conditions must be true for maximum transmission in a CDMA system?
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What is a key aspect of the mixed access method mentioned in the content?
What is a key aspect of the mixed access method mentioned in the content?
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In the context of Shannon's capacity formula, what does 'W' signify?
In the context of Shannon's capacity formula, what does 'W' signify?
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Which scenario best describes carrier-sense multiple access protocols?
Which scenario best describes carrier-sense multiple access protocols?
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What is a major limitation of Frequency Division Multiple Access (FDMA)?
What is a major limitation of Frequency Division Multiple Access (FDMA)?
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What mathematical condition must be met in the context of parallel channel access using orthogonal signals?
What mathematical condition must be met in the context of parallel channel access using orthogonal signals?
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What does the term 'interference' refer to in a CDMA context?
What does the term 'interference' refer to in a CDMA context?
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What is the equation used to calculate the capacity when the properties of interference are close to Gaussian noise?
What is the equation used to calculate the capacity when the properties of interference are close to Gaussian noise?
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If K = 100 transmitters, how much spectrum is needed for each transmitter to maintain a data rate of 9.6 kbps?
If K = 100 transmitters, how much spectrum is needed for each transmitter to maintain a data rate of 9.6 kbps?
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What characterizes Direct Sequence Spread Spectrum (DSSS) systems?
What characterizes Direct Sequence Spread Spectrum (DSSS) systems?
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How does Frequency Hopping Spread Spectrum (FHSS) determine its frequencies?
How does Frequency Hopping Spread Spectrum (FHSS) determine its frequencies?
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What is the primary purpose of the code sequences in DSSS systems?
What is the primary purpose of the code sequences in DSSS systems?
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Which of these principles correctly describes the modulation process in DSSS?
Which of these principles correctly describes the modulation process in DSSS?
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What feature distinguishes chips from bits in DSSS systems?
What feature distinguishes chips from bits in DSSS systems?
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What does the formula $B = \frac{C}{1.44(K - 1)}$ represent?
What does the formula $B = \frac{C}{1.44(K - 1)}$ represent?
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Study Notes
Multiple Access in Mobile Networks
- Multiple communications need to be ensured without significant interference.
- Packet Access: The entire radio interface is allocated to a single user at a time. An example is ALOHA random access, where many terminals are connected for short periods. Repeated retransmission of corrupted data ensures successful communication sessions.
- Parallel Channel Access: Separate communications use non-overlapping or orthogonal signals.
Multiuser Packet Access
- Pure Aloha: Uncoordinated transmissions can lead to overlaps, creating collisions. This results in a low throughput.
- Slotted Aloha: Transmissions are synchronized to time slots, reducing collisions and increasing throughput compared to pure Aloha.
- Carrier-sense Multiple Access (CSMA): Transmitting stations sense the channel before transmitting to avoid collisions.
Pure ALOHA Access
- A graphical representation showing how transmissions can overlap between two nodes, creating collisions.
Slotted ALOHA Access
- A graphical representation showing how transmissions organized in time slots reduce collisions.
Throughput of ALOHA Access Schemes
- A graph comparing the throughput of pure Aloha to slotted Aloha under varying traffic loads. Slotted Aloha generally demonstrates higher efficiency at higher traffic loads.
Parallel Channel Multiple Access
- Frequency Division Multiple Access (FDMA): Active users are assigned different frequency carriers.
- Time Division Multiple Access (TDMA): Users sharing the same frequency bandwidth are allocated different time slots. Often mixed with FDMA for user groups.
- Code Division Multiple Access (CDMA): Users share the same frequency band but are assigned different codes rather than time slots or frequencies, providing flexibility.
FDMA
- Shows a graphic representation of the frequency allocation for uplink and downlink frequency bands. Guard bands prevent signal interference between adjacent channels.
Cross-Channel Interference
- The formula
c(t) = a₁ cos(2nf₁t) + a2 cos(2nf2t) + a3 cos(2nf3t)represents the interference of multiple signals. - Signals carrying information can interfere with each other when processed by non-linear amplifiers, generating new frequencies (e.g.
f' = 2f₁ - f2).
TDMA
- A diagram showing how time slots are allocated to users for communication. Each user has a unique time slot, maintaining non-interference.
TDMA Frame Structure
- A graphical representation of a TDMA frame, detailing timing slots, synchronization bits, information, and guard intervals.
Dynamic TDMA and FDMA Access
- Dynamic versions of these technologies adapt to variable transmission needs more efficiently than fixed ones.
Code Division Multiple Access (CDMA)
- Describes information in a channel is encoded with a specific code sequence.
- A key concept in spread spectrum communications, where code sequences distinguish between information sources.
- Shows an important diagram of the CDMA signal characteristics.
CDMA Channel Capacity
- The formula
C = W * log₂(1+S/N)relates channel capacity (maximum information transmission rate) to the bandwidth (W) occupied by the signal and the signal-to-noise ratio (S/N).
Exercise: Hiding the Signal in Noise
- A problem examining how much signal bandwidth is required to transmit data at 9 kbps given a specific signal-to-noise ratio (SNR) requirement using the channel capacity equation.
CDMA Channel Capacity with Multiple Transmitters
- How the capacity of a CDMA channel changes when multiple transmitters are sharing the same channel. The capacity increases with more transmitters with limited power while not severely reducing useful signal power.
- Describes a more complex model of multiple-transmitter CDMA, considering signal from K transmitting sources, finding an effective capacity when all sources are interfered, and assuming interference is close to Gaussian noise.
- Example on calculating the effective CDMA bandwidth requirement given a specific transmission rate and number of transmitters.
Spread Spectrum Technologies
- Direct Sequence Spread Spectrum (DSSS): The carrier is modulated with a high-rate code sequence, expanding the signal's bandwidth for better immunity to interferences.
- Frequency Hopping Spread Spectrum (FHSS): The carrier jumps among different frequencies, making it hard for interference to affect all hops.
- Time Hopping or Time-Frequency Hopping: Time slots or frequencies are determined by a code sequence for spread spectrum.
Direct Sequence Spread Spectrum (DSSS) Principles
- The principles involved in expanding the bandwidth of a signal through the modulation with a code sequence.
Example of DSSS System
- Shows the diagram of a transmitter and receiver components with the signal bandwidth expansion for interference immunity.
DSSS System Processing Gain
- The receiver multiplier's ability in compressing the signal spectrum (with synchronization) for its synchronized part while expanding the asynchronous part (i.e., noises) to achieve a better signal-to-noise ratio. An important measure of this capability is called the processing gain, denoted by variable k.
- How the receiver is able to selectively receive and effectively isolate the desired data signal amid interference and noise, illustrating the principles and importance of spread spectrum processing.
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Description
Explore the concept of multiple access techniques in mobile networks, focusing on packet access methods like ALOHA and CSMA. Understand how various strategies improve communication efficiency while minimizing interference. Test your knowledge on the nuances of pure and slotted ALOHA as well as parallel channel access.
