Each wireless sensor standard defines its own modulation and channel access methods. These two methods are central to any sensor technology performance.
Define Multiple Access.
Techniques on how resources are allocated to users or how users share the resources.
Define Modulation.
The process of translating the baseband message signal to a modulated carrier signal (band pass) at frequencies that are very high compared to the baseband frequencies.
Which two types of Multiple Access exist?
Coordinated channel access: allocation of radio resources (channel) handled by the system
Uncoordinated channel access: random distribution of radio resources
How can the performance of a modulation scheme be measured?
Power efficiency
Bandwith efficiency
Name some famous modulation schemes and their use cases.
ASK: short range sensor
FSK: short and long range sensor
PSK: Mobile Celluar, Wireless Broadband
What is meant by the spread spectrum?
Applying different codes (e. g. direct sequence) to a digital signal to get a spreaded signal.
The signal needs to be despreaded to be readable.
Illustrate the differences of the following Multiple Access techniques:
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)
Explaing the following Multiple Access techniques:
FDMA: Users are assigned a channel each
TDMA: Users are assigned timeslots
CDMA: Users share bandwidth by using code sequences that are orthogonal to each other (spread and shift)
FDMA:
TDMA:
The Aloha Protocol is a popular Multiple Access technology. Explain its basic operation.
Devices can transmit data whenever they have a message to send on a channel
If two or more devices transmit simultaneously on the same channel, their messages will collide and be corrupted
Devices that detect a collision will wait for a random amount of time before trying to transmit again
Illustrate a two-way handshake.
What is the hidden & exposed node problem?
And how can it be solved?
When sensing range > transmission range:
Solution: Perform a 4-way-handshake (RTS/CTS/DATA/ACK-operation)
Define the terms Frequency Band, Bandwith, and Throughput.
Frequency Band is the range of frequencies in the spectrum, that has a defined minimum and maximum frequency intervals.
Bandwidth is the difference between the upper and the lower frequencies in the band.
(Network) Throughput is the rate of successful message delivery over a communication channel. [bits/s]
Which Modulation and Multiple Access do LoRa, Bluetooth and RFID use?
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