Wireless Communication Systems (Week 4) Flashcards
Radio Carrier Signals
It allows multiplexing. This means we can send many signals at once using different frequencies.
Amplitude Modulation (AM) System
Way of sending information by changing the amplitude of the high frequency carrier wave while keeping the frequency the same
Frequency Modulation (FM) System
Frequency modulation varies frequency while keeping amplitude constant
- Requires more power
Optical Communication System
Sends information using light instead of electricity
the system includes a
-light source (transmitter
-Transmission medium (path the light travels through)
-Light receiver (detector)
Optical Information Transmission (and the two modes)
The amount of light that reaches the detector tells us something about:
-The light source (how its behaving)
-The path or medium the light is travelling through
Two Modes
1. Medium Modulation
-The light stays on (constant)
-but the path is block or changed to send the signal
Ex: Someone is waiving their hand in front of a flashlight to blink a message
- Source modulation
-The path stays the same
-The light source itsled will change (blink/pulse) to send information
Free Space Links (Advantages and disadvantages)
Info is sent using modulated electromagnetic waves (like light or radio) through the air to one or more receivers.
Advantages:
-Long distance of communication
-Works over land air and space
disadvantages:
-Can be disrupted by weather
-hard to align perfectly
Infrared (IR) Remote control
They emit a near-infrared beam that is pointed towards the sensor of the device.
Single channel remote control (one button, single function), sends a signal to trigger a sensor or switch.
multichannel remote (multiple button, multiple functions).The signal is changed by using different frequencies for each function. Then the receiver must demodulate (decode) the signal and use filters to separate and understand what command was sent out.
There is IR light everywhere so the remote needs a way to stand out - it does this by sending blinking IR light (called modulation). The receiver is looking for that blinking pattern and will ignore all other IR sources
IR transmitter
You can use a basic transitor circuit to send IR signals but using a microcontroller is better since you have more control and flexibility. The microcontroller sends pulses through its output pins to generate signals at specific frequencies.
IR Receiver Module
It recives IR signals from something like a remote control and turns them into electrical signals that your device can understand
Pulse Code Modulation (PCM)
used to ensure the transmitted IR signal gets to its correct destination (or the target device receives only the signal its meant to)
Two states:
1. Mark (Burst)
-The LED flashes light at a high frequency
This means the transmitter is ON
- Space
-The LED is off
The light flashes in a pattern that represents 1s and 0s so the receiver understands what its saying
Pulse Distance Encoding
The length of the pulse burst (mark) is always the same.
what changes is the time between the burst (space)
if its a short gap its =0
if its a long gap its =1
Pulse Length Encoding
Length of the burt is diffrent for a logical 0 and 1. The gap stays the same
Bi-Phase (or Manchester) Encoding
Way to send bits using fixed size and time solts. So half the time is pulse =ON, half the time is a space= OFF. The order of the ON/OFF tells the receiver if its a 0 or a 1.
for example: first half is on(light pulse), second half is Off (no light) then its a 0.
NEC Transmission Code
Burts at a carrier frequency of 38 kHz.
How it works:
1. sends a long burt of light to signal that its about to begin
2. Then pauses for 4.5 ms (Break before the message)
3.The actual command is encoded in a series of bits using pulse distance encoding.
Has a fixed word length.
The address and the data are always sent twice. First the normal version, then the inverted version is sent.
LEader code (9ms burst -LED blink, 4.5 ms space -LED off, Adress of fixed word size, inverted address, data of fixed word size, inverted data)
Bluetooth
Low power short range wireless communication system.
ISM Band (Industrial Scientififc Medical)
Bluetooth uses ISM band to communicate wireless.
Its a band (Not one fixed frequency)
Its open everyone (because of this there are many sources of interference).
A high frequency wave (2.4GHz) can only transmit short distance (about 10 m) unless power is increased
Piconets and Devices
Piconet- Small bluetooth network
Central device- Like a phone
Peripheral device- Keyboard, headset, sensor
The central device controls all communication. It can send or request data from any of the peripherals. Peripherals can only communicate through the central device.
1 central device can connect up to 7 peripherals at once.
Interface Avoidance (Hint there are two ways)
Because bluetooth uses ISM band to communicate it needs to avoid interference with other devices.
It does this by finding unused parts of the ISM band. If a channel is noisy or crowded it will just jump to another channel.
OR
it uses Spectrum Spreading (FHSS =frequency Hopping spread spectrum). Bluetooth rapidly hops between frequencies making it less likely to clash with other devices.
-Bluetooth uses 79 random frequencies, where both devices know the hopping pattern so they stay in sync
-It will divide the frequency band into several hop channels
This is Important because Bluetooth is low power (short range) so its more likely to face interferences.
Bluetooth connection Process
- Inquiry
Once device sends a signal seeing if anything is there. If a nearby device is there it will reply with its Bluetooth address and many its name and some details - Paging (Connecting)
Once devices knows each other’s address one will try to establish a connection. It will send a connection request to the known address - Connection
Devices are now paired and can communicate
Bluetooth Profiles
Profile defines the type of communication (e.g audio, file transfer, keyboard input)
Specifications = How Bluetooth works
Both devices must support the same profile to connect successfully
Serial port profile
This is an example of the bluetooth profiles
It lets Bluetooth trlplace wired serial communication. It makes two Bluetooth devices feel like they are connected by RX and TX wires.
This is good for short burst of data like commands or sensor readings
Wifi
Wireless networking technology that allows devices to talk to each other and the internet without cables.
ESP NOW (unicast, encryoted, broacast)
Lets ESP wifi device to talk to each other without a connection (no router needed).
Unicast- Up to 20 devices can be paired as peers and controlled (Senda a message to a specific device)
Encrypted- up to 6 devices and be paired and controlled (This is a more secure way of communication)
Broadcast - No limit, but need to consider interference (Sends to all nearby ESPs)
