Wireless Communcations

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# 181U Spring 2020
## Wireless Communications (Background)

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# Agenda

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* A little radio physics
* decibels and power
* link budget
* introduction to radio standards for IoT
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# Electromagnetic Waves

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* Characterized by: wavelength, frequency, and amplitude
* Examples: Light, x-rays, and radio waves

$$\lambda = c/f,   \ c = speed\ of\ light$$

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# Electromagnetic Spectrum

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Note: antenna length depends upon wavelength (typical: 1/4 wavelength)

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# Wavelength

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# Free Space Loss (FSL)

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As the wave propagate from the source it spreads over an
ever increasing area, so an antenna of a given size would be
able to capture a fraction of the wavefront that decreases
with the square of the distance.

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# Introduction to DB

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The **decibel (dB)** is 10 times the decimal logarithm of the ratio between two values.  Admits easy comparison across large scales
$$ L = 10log_{10}(\frac{P_2}{P_1})$$
$$\frac{P_2}{P_1} = 10^L$$

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# Review of logarithms

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$$if\ x = 10^y, then\ y = log_{10}(x)$$

Logarithms reduce multiplication to addition:

$$log(a \times b) = log(a) + log(b)$$

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# Using dB

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* Commonly used dB values

$$+10\ dB =  \text{10 times the power}$$
$$-10\ dB =  \text{one tenth the power}$$
$$+3\ dB =  \text{twice the power}$$
$$-3\ dB =  \text{half the power}$$

* For Example

$$\text{some power} + 10\ dB = \text{10 times the power}$$
$$\text{some power}-10\ dB =  \text{one tenth the power}$$
$$\text{some power}+3\ dB =  \text{twice the power}$$
$$\text{some power}-3\ dB =  \text{half the power}$$

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# Free Space Loss versus Distance at Different Frequencies

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# dBm and mW

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* What if we want to measure an absolute power with dB ?
  - We *have to define a reference*
* The reference point that relates logarithmic dB scale to the linear watt scale is:

$$ 1 mW = 0 dBm$$
* The new **m** in dBm refers that the reference is one **m**W and therefore a **dBm** measurement is a measurement of absolute power with reference to 1 mW.

* We can measure transission/received power in **dBm** and model loss/gain in in **dB**.  This allows us to relate energy (in transmission) with signal strength.

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# Using **dB** and **dBm** together

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Remember:
$$\text{some power} + 10\ dB = \text{10 times the power}$$
$$\text{some power}-10\ dB =  \text{one tenth the power}$$
$$\text{some power}+3\ dB =  \text{twice the power}$$
$$\text{some power}-3\ dB =  \text{half the power}$$
So:
$$10 mW + 10 dB \text{(of gain)} = 100 mW = 20dBm$$
$$100 mW - 10 dB \text{(of loss)} = 10 mW = 10dBm$$
$$50 mW + 3 dB \text{(of gain)} = 100 mW = 20dBm$$

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# milliwatts and dBm

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# Behavior of Radio Waves

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* The *longer* (lower frequency) the wavelength, the **further** it goes
* The *longer* the wavelength, the **better** it travels through and around things
* The *shorter* the wavelength, the more data it can potentially transport

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# Traveling Radio Waves

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Radio waves do not move in a strictly straight line  On their way from "point A" to "point B", waves may be subject to:

* Absorption
* Reflection
* Refraction
* Defraction
* Scattering

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# Wireless Link

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# Power in a wireless system

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# Link Budget

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**Link budget** is a way of quantifying performance of a radio link

* The received power in a wireless link is determined by
   - (+) transmitter power
   - (+) transmitter antenna gain
   - (-) tranmission path loss
   + (+) receiver antenna gain
* *If* the received power is greater than the **sensitivity** of the receiving radio, then communication is **feasible**.
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# Example Link Budget

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# Link budget

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* The **transmitter power** is limited by regulation
* In the 2.4 GHz unlicensed band (wifi, bluetooth) maximum power is 20 dBm in Europe and 30 dBm in the US
* Typical Receiver sensitivity
  -- nrf52840 BLE -- -96dBm sensitivity
  -- WF200 WiFi ==  -96.7dBm sensitivity, 17dBm transmitter
* Link budget also affected by coding of data -- LoRa/SigFox use spread spectrum codes to acheive greater effective sensitivity at cost of data rate.

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# Link Budget isn't the whole story

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*  The maximum range is determined by the *energy per bit received*  
  - Transmitted power
  - Sensitivity
  - **and** data rate.

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# Comparing Existing Technologies

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# Some Solutions

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* RFID
* WiFi
* Bluetooth and BLE
* Personal area networks
   - ZigBee,Thread
* Cellular

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# RFID

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* RFID very widely used
* Short distance (0.1-10 meters)
* Object (RFID tag) is applied to *thing* to be tracked
* RFID tag is read (written) by a device such as a cell phone 
* Technology behind contactless payment, library book tracking, inventory systems

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# RFID Frequencies of operation

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# IEEE 802.11 (WIFI) standards

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# Bluetooth

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* IEEE 802.15.1 standard
* Smart Mesh
* 79 channels 1Mhz wide + frequency hopping
* 2.4 GHz band
* Used mainly for speakers, health monitors, other short range applications

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# Bluetooth Architecture

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* **Master** node controls up to 7 active **slave** nodes and up to 255 inactive nodes.
* Slaves must communicate with the master
* Relatively restrictive configurations of slaves based upon standard *profiles*

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# Bluetooth Low Energy (BLE)

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* Based on IEEE 802.15.1
* Subset of Bluetooth 4.0
* Smart Mesh
* Wide OS support
* Used in smartphones, tablets, smart watches, ...

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# IEEE 802.15.4

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Standard for low-rate wireless personal area networks

* Minimal infrastructure, low poer
* Defines the physical (PHY) and medium access control (MAC) sublayer
* Targets small, power-efficient, inexpensive solutions
* Used by many upper layer protocols

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# IEEE 802.15.4 Topology

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# Mesh Networks based upon IEEE 802.15.4

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* 2.4 Ghz or 915 (868 europe) Mhz
* Mesh topology
* Zigbee
* Thread (emerging)

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# Emerging Standards

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# Summary

* Acknowledgements -- Slide Material drawn from:
   - "Wireless standards for IoT" by Ermanno Pietrosemoli: http://wireless.ictp.it/school_2019/slides/WirelessstandardsforIoT.pdf
   - "Radio Physics for IoT" by Ermanno Pietrosemoli:  http://wireless.ictp.it/school_2019/slides/Radio_Physics_for_IoT.pdf