# [07/02/2021] Introduction to the Cellular Network ### Salman's Notes --- ### What is the Cellular Network? Different from Wi-Fi, **Cellular Networks have to transmit data to a large amount of dynamic users through a medium that is very succeptible to interference and noise**. Therefore, extra preparations must be conducted before any transmission of data. Functions such as Coding (adding bits to recover any bits lost due to interference) and Modulation (converting signals or data into other signals so that they can be transmitted with the right frequency). ![](https://i.imgur.com/0aS8Dph.png) *Figure 1. The role of coding and modulation in mobile communication.* Figure 1 shows the process of coding and modulation done to data so before it is elligible for transmission. ### How does the Cellular Network handle those Users? To cope with the gargantuan amount of users that need to connect to a Cellular Network, **these networks employ a function/device called the Scheduler**. 4G and 5G Cellular Networks use Orthogonal Frequency-Division Multiplexing (OFDM), which is a Scheduling Mechanism that divides or multiplexes data into different frequencies at the same time and making sure that no interference happens. ### Cellular Network Scheduling in 4G In 4G, there are a set of 12 orthogonal frequencies and each frequency is modulated differently (this approach is called the Orthogonal Frequency-Division Multiple Access, which means that one block of data may correspond to multiple users at the same time). Each of these orthogonalities correspond to a 15-kHz frequency band thus adding up to a total of 180-kHz of the available frequency spectrum. This frequency spectrum can be visualized as a 2D space in regards to the time and the frequency of each block of data. ![](https://i.imgur.com/fBbtPu9.png) *Figure 2. Spectrum abstractly represented by a 2-D grid of schedulable Resource Elements.* The Cellular Network Scheduler works with a set of Physical Resource Blocks (PRB) which consists 84 blocks as shown in Figure 2 (12 blocks in the Y axis multiplied with 7 blocks in the X axis). **The Scheduler is responsible for the coding and modulation that is used within each PRB, and may use any method or algorithm to get the job done.** It corresponds to feedback of the QoS Class Identifier that tells the scheduler how important the block is based on its Quality of Service. ### Cellular Network Scheduling in 5G Instead of using the same waveform, 5G uses different waveforms for multiplexing the data. For example, data that is used for IoT devices and for places with difficult connectivity will use the sub-1 GHz bands with each block possibly having 2 waveforms which are 15 kHz and 30 kHz. They also have different intervals for each waveform, (0.5 for the former and 0.25 ms the latter). Mobile broadband and devices which require High Availability will most likely be given the 1 GHz to 6 GHz bands with 15, 30, and 60 kHz waveforms. Lastly, devices which are near or have in-line coverage will be given frequencies of above 24 GHz, or the mmWaves. It is up to the Scheduler to figure out where to allocate the data given its use case and which waveform will be given for that data which will in turn decide how the data will be coded and modulated. ### References Title: 5G Mobile Networks: A Systems Approach Authors: Larry Peterson and Oguz Sunay Source: https://github.com/SystemsApproach/5G License: CC BY-NC-ND 4.0 Link: https://5g.systemsapproach.org/primer.html#id2 ---