Mobile App Development: Wireless Communication

 A phone connects to a wireless infrastructure network. Both WiFi and Cellular networks counts as types of wireless infrastructure networks. We call them infrastructure networks because they require deploying infrastructure in the form of wireless access points or wireless base stations.

Wireless networks happen because of signals, which are transmitted as electromagnetic waves.

There's a bunch of binary information, put on a wireless signal. Subsequently, the output is read. The actual information is encoded in the shape of the signal.

A pulse corresponds to 1, no pulse corresponds to 0. Each "batch of waves counts as one data packet. 

Radio waves propogate through a wireless channel. Radio waves are recovered through wireless communication and signal processing. Waves are used by MAC, or wireless medium access control.

Each network standards, provides a standardized set for each of these two problems. The solution runs in certain Wi-Fi network cards and drivers. New protocols have higher protocols. 

MAC is implemented as firmware and drivers. 

What people get in case of wireless channel distortion is NOT what is sent. There's a lot of issues, such as path loss, multi-path effects, noise, and other communications such as temporal variation. 

When a signal is recieved, it can be a lot quieter and less intense. The signal can also lose energy. 

Multi-path effect is also a possibility. 

The radiation is sending out in a sphere, so the total spherical surface area is 4 π d^2. However, the antenna doesn't get all of the energy. The power density is independent of wavelength so the power density at range is p = Pt/( 4 π d^2) Watts per square meter. Received power = power / surface area. 

To receive a lot of power, you need a bit antenna. 

Every one antenna has a different gain

Gt is transmit gain, Gr is receiving gain. Pr =  Pt * Gr * Gt *  Λ^2/( 4 π d^2) 

The only effect is not loss of energy. There is reflection at large obstacles, refraction, scattering at small obstacles, and a lot of other attributes. 

Refraction, Reflection, and Scattering. 

These signals can also easily detect movement, due to the distortions that happen to the electromagnetic waves

A major problem(s) is/are congestion and contention. There is one single station and multiple users at the same time. A base station cannot provision all users simultaneously. Some areas need more base stations (cities) and less stations for rural station. 

Next is demodulation. There are bits that have to be put on signals. The shaping of bits is called modulation, and to receive the data you have to go through a reverse process which is called demodulation 

The purpose of modulation is to shape a wave; that is, to improve robustness with channel distortion. The simplest approach is repetition. Signals can be represented in 2 domains, the time domain and frequency domain. 

AM stands for Amplitude Modulation. The wireless waves sit on a center frequency and are band limited, Wi-Fi is a good example of this. It has a center frequency of 2.4 GHz or 5 GHz, or a bandwidth of 20/40/80 MHz.

In order to modulate a signal, the signal has to be modulated by a particular cosine wave. When a signal is modulated in the time domain, convolution happens in the frequency domain. 

The equation is AM = s(t) x cos(2π Fc * t) x cos(2π Fc * t) which is basically a carrier code, taking a signal and modulating it over this specific signal. For this example, the period of the wave affects the specific frequency of a wave. I can then filter a high frequency using a low-pass filter. 

Wavelength =  Λc/Fc

This is spectrum allocation in the USA. Everything is at certain MHz.

Wi-Fi and LTE has similar modulation principles with different parameter setups. However, the LTE waveform needs to accommodate high mobility. Wi-Fi and LTE is allocated by different spectrum widths. 

Why do we need scheduline?

The radio channel is a limited resource and at any time, only one transmitter can access it. We need to ensure everyone can access the wireless channel in an efficient and fair manner. 

Cellular networks are very well orchestrated at a base station. Wi-Fi is disorganized, and use time sharing, whereas cellular networks have much higher standards. 

We need scheduling for energy efficiency, to allow multiple people to access the channel at the same time. 

If too many people access a 4G network at the same time, then unfortunately people cannot have quick access to a channel, and the phones will be going a very long amount of time in between when they are able to send packets. This is why the internet is so slow when there are big gatherings. 

Radios are able to send a wireless medium by broadcasting at different frequencies.

Medium access control allows multiple programs to share network at the same time.

Cellular has synchronous time sharing because each user follows a pre-schedule.