Architecture of a General Cellular Communication System

In this post, we will have a look at what cellular communication entails. We will take an in-depth look at the architecture of a general cellular communication network and its associated terminologies. We shall also be looking at the components of a cellular network that facilitates communication.

A cellular communication network is one that uses a large number of low-power wireless transmitters to create cells. Each cell is fixed only for its service area, and not for the service users, ensuring mobility. The addition of new cells into the network is also a fairly simple process, promoting modularity in expansion. The concept of cellular networks was developed by Bell Labs in 1971.

Cellular System Architecture

The increase in demand for cellular services combined with the limited availability of the frequency spectrum available for commercial cellular use has promoted engineers to look for techniques to use the available spectrum as efficiently as possible, without sacrificing coverage area. The solution to this was to deploy the entire network as cells.

Cells

• Cells are basic geographical units of the cellular network.
• Basically, they are a geographical region where a cellular network is deployed.
• The shape of the cells should ideally be somewhere in between a square (distance to corners are quite large) and a circle (high changes of overlapping with other cells).
• They are hence generally modeled as hexagons (they aren’t hexagons in reality though), as each vertex is equidistant from the center and also each cell can accommodate up to six neighbors without the threat of overlapping.
• These cells can take up any shape (or even be of no particular shape) in real life.
• The area covered by each cell is governed by factors such as subscriber density, demand, terrain, and population.
• The size of the cells is to be taken care of as well because smaller cells require more infrastructure for a smaller area, and frequent handovers are to be taken care of due to the smaller area, increasing unnecessary traffic in the home MSC.
• Each cell has its dedicated broadcasting facility and its own frequency at which it operates.

Based on the radius of the area of coverage, cells are further divided into the following types:

Cell Clustering

• A cluster is a group of cells.
• Generally, clustering is done in such a way that no two adjacent cells operate on the same frequency.
• This is done to ensure minimal interference between two adjacent cells, as well as to address the frequency spectrum availability problem.
• If we consider the cells to be hexagonal, the minimum number of cells to be present in a cluster is seven.
• This ensures that each cell sharing a border has a unique frequency at which it operates, as shown in the figure below.

Cell Splitting

• To cater to a large number of users in densely populous areas, such as cities, the cells are further divided into sub-cells, and this process is known as cell splitting.
• The sub-cells have antennas that have been reduced in height to sufficiently cover the whole sub-cell region.
• This, in turn, increases the capacity of the cell.
• But splitting the cell into smaller parts can lead to interference, as the band of frequencies in use within a cell is the same.
• To counter this, cell-sectoring is implemented where omnidirectional antennas are replaced by several directional antennas, as shown in the figure below.

Frequency Reuse

• One of the underlying principles behind cellular communication is to make the most of the limited frequency band available.
• This can be done by reusing the same frequencies for multiple cells, while still satisfying the criteria of no two adjacent cells operating on the same frequency.
• The coverage area of a cell is known as its footprint.
• The footprint is limited by a boundary.
• This ensures that the same group of channels can be used in different cells that are far away from each other so that their frequencies do not interfere.
• The first figure shows how this is done while splitting the available frequency band into seven sections.
• As the band is split into seven sections, the frequency reuse factor would be 1/7.

Handoff

• Maintaining cellular connectivity even while the subscriber moves from one location to another, across the cells, poses a problem as each cell operates at a different frequency.
• Here, the concept of Handoffs might come in handy (pun intended).
• Handoff is a process in cellular communication for automatically transferring the call from one radio frequency to the other, without interrupting the user’s conversation.

Components of a Cellular System

The following diagram gives a brief overview of all the components in the Cellular Network.

Cellular systems offer wireless and mobile telephone stations for the same service provided by fixed stations over conventional wired loops. The components of the cellular system, which we shall see now, facilitate the interconnectivity of different cells and cell networks.

Public Switched Telephone Network (PSTN)

• The Public Switched Telephone Network (PSTN) is a collection of all the circuit-switched telephone networks (which includes all the exchanges, hubs, switching centers along the lines) in the world.
• These can be operated either by a national, regional, or local body and are the central most component of any telecommunication system.
• Be it the optical fiber lines (for internet connectivity), telephone lines, satellite communication, and, of course, for cellular systems as well.
• The PTS Network interconnects all the above-mentioned modes of communication, facilitating calls and services across different systems.

The technical operation of the PSTN is standardized by a division of the International Telecommunications Union (ITU) called the ITU Telecommunication Standardization Sector (ITU-T), which standardizes telecommunication technologies across the world. Although PSTN is not limited or specific just to Cellular Communication Systems, its definition and functionality are added facts to know.

Now we move on to the components that are specific only to mobile communication.

Network Switching System (NSS)/Mobile Telephone Switching Office (MTSO)

• The Network Switching System (NSS), often replaced by the term Mobile Telephone Switching Office (MTSO), is the heart of the cellular communication system, which is connected to the PSTN, thereby providing connectivity.
• The NSS also performs switching of calls between different users with the help of the Mobile Switching Center (MSC).
• It also offers other functionalities such as billing of mobile services and authentication.
• The Switching System of Cellular Communication Networks includes the following components:

• Mobile Switching Center (MSC)
  • The Mobile Switching Center (MSC) performs the switching of calls from mobile users and fixed (landline) or mobile users and is the heart of the NSS.
  • The MSC is also responsible for the management of additional mobile services like billing, registration, authentication, location updating, among many others.
  • This is done with the help of other components of the NSS, such as the VLR, HLR, and AuC.
  • An MSC is responsible for a collection of cells or base stations, and there can be multiple MSCs within a circle, which is dependant on factors such as population and subscriber density.
  • Different MSCs are allocated to different service providers.

• • However, for every circle, there is a special MSC that further interconnects every circle. This is known as the Gateway Mobile Switching Center (GMSC) and is used to route long-distance calls.

• Home Location Register (HLR)
  • The Home Location Register (HLR) is one of the two databases for the storage and management of subscriptions.
  • It contains details of every subscriber whose SIM card was brought from somewhere under the jurisdiction of the MSC of that particular operator.
  • These include details such as the service profile, location information, and activity status of the user.
  • A major feature of the HLR is that its entries are permanent.

• Visiting Location Register (VLR)
  • The Visiting Location Register (VLR) is a database that is updated as soon as a user whose subscription details are not present in the HLR enters the vicinity of an MSC of the service provider.
  • This means that the VLR, as the name suggests, contains details of all users “visiting” the jurisdiction of the MSC, commonly known as “roaming“.
  • The VLR stores the same information as that of the HLR, differing only by the fact that the entries made into the VLR are temporary and get erased as soon as the user leaves the MSCs vicinity, preventing unnecessary data from clogging the memory.

• Authentication Center (AuC)
  • The Authentication Center (AuC) is responsible for making sure that the call is safe and secure by authenticating the mobile user making the call.
  • This is done by cross-verifying the user’s details such as the identity code, and the secret key stored in each user’s SIM cards and making sure that they match before the connection is made.
  • The details of the user are obtained from the HLR (VLR in case of roaming).
  • The AuC also ciphers the radio channel, preventing attacks on the network, keeping it secure.

• Equipment Identity Register (EIR)
  • The Equipment Identity Register (EIR) keeps a record of all authorized mobile equipment (that can access the cellular system) in the network, identifying them by the device’s Internation Mobile Equipment Identity (IMEI).
  • This elevates the security of the network by keeping spoofers off the system, as well as providing back up during thefts.
  • In case of theft, when the network operator is informed of this, it tracks the IMEI of the stolen device with the help of the EIR and obtains the location of the device in use with the help of the AuC.
  • A thing to keep in mind is that this works only if the culprit tries to access the network with the stolen device, either by trying to make a call or by sending an SMS message.

Some additional functionalities that are provided by the MSC are:

• Billing Unit – It works on the policies offered by the service provider. Once the call is accepted, the caller and the duration of the call is recorded, which is then added to the caller’s bill.
• Maintenance Control – It supervises and monitors the health of various circuits in the system, and tracks malfunctions of the electronics circuits with the help of suitable control algorithms.
• Power Control Unit – This Unit controls the power of the signal being transmitted from the MSC to the mobile units to prevent distortion of the signals.

Base Station System (BSS)

The Base Station System (BSS) is the gateway for the mobile user into the network and hence is also known as the Radio Access Network. It mainly has two components, which are listed below:

• Base Transceiver Station (BTS)
  • The Base Transceiver Station (BTS) houses the radio transceivers that interact with the mobile user and receiver and transmit signals to and from the user.
  • It mainly contains equipment for encrypting and decrypting communication signals, spectrum filtering tools like bandpass filters, and antennas (of course).
  • In most cases, a BTS generally covers a cell and is responsible for providing connectivity for the whole cell. Other major components of a BTS are:
    • Transceiver (TRX) – Provides for the transmission and reception of signals, to the mobile user as well as the higher entities like Base Station Controller (BSC).
    • Power Amplifier (PA) – Amplifies the received signal to be transmitted, to compensate for attenuation in the channel.
    • Combiner/Multiplexer – Combines feeds from different transceivers to be able to send it over one antenna.
  • The antennae of the BTS are generally directional antenna, which are responsible for their own sectors within the cell, to be able to cater to a more extensive customer base.
• Base Station Controller (BSC)
  • The Base Station Controller (BSC) is in charge of tens or hundreds of BTS that are assigned to it.
  • This means that the jurisdiction of a BSC covers a certain number of cells, providing an interface between the BTS and the MSC.
  • The BSC is the brains in the BSS and handles the allocation of radio channels, receives measurements from mobile phones, and takes care of handovers between two BTS.
  • A BSC can be seen as a BTS but with additional power and responsibilities, such as being able to act as a switching center.
  • A major component of the BSC that sets it apart from the BTS is the transcoder that converts the 13 Kbps voice channel used over the radio link amongst the BST, to the standard 64 Kbps channel used in the PSTN.

Mobile Stations (MS)

• Mobile Station (MS), also known as Mobile Equipment (ME), is the name given to all the user equipment, like cellular phones and landline phones, that are a part of the cellular network.
• People use the MS to access the cellular network for communication, which in turn connects to the BTS.
• An MS is responsible for converting the user input, be it audio, video, or text to radio waves before being sent.
• Every Mobile Station has a unique International Mobile Equipment Identity (IMEI) number associated with it, to keep track of the hardware.

Terminologies related to Cellular Systems

Having had an overview of the general cellular network, we have come across quite a lot of technical jargon or “terminologies” related to the Cellular System, which we shall have a look at in this section.

Subscriber Identity Module (SIM) card

The SIM card is the first step to getting an MS to be a part of a cellular network, which contains a unique Internation Mobile Subscriber Identity (IMSI) number and its associated key, which is used to identify and authenticate calls and other interactions from the user. Apart from the IMSI number, the SIM card also has other unique keys associated with it, which are listed below:

• Integrated Circuit Card IDentifier (ICCID) number that uniquely identifies the SIM card chip.
• A Personal Identification Number (PIN)
• A Personal Unblocking Code (PUC) used to unlock the PIN
• The most commonly known one is the “Mobile Station International Subscriber Directory Number” or MSISDN, which is colloquially known as one’s mobile phone number.

They come in different sizes, namely the Mini-SIM, Macro-SIM. and Nano-SIM.

Hand Offs/ Handovers

Handoffs or Handovers refer to the process of transferring the call from one BTS to another, usually when the signal strength goes down. This generally occurs when the MS is on the move across multiple cells. We shall have a detailed look at handovers pretty soon in one of the posts that come next.

Signal Strength

• The signal strength is an estimate of the level of connectivity of the MS
• Their values can be found out by different methods such as Received Signal Strength Indicator (RSSI), Reference Signals Received Power (RSRP), Reference Signal Received Quality (RSRQ), Signal-to-Interference-plus-Noise Ratio (SINR), and Received Signal Code Power (RSCP).
• They are estimations of the power levels of an RF client device.
• Generally, the RSSI values can be found in anyone’s mobile device, by going to About Device -> SIM Status ->Signal Strength. This is the RSSI value of the device, and the following table can be used to check your connectivity:

Dwell Time

Dwell time is defined as the time a call may be maintained within a cell without any handovers and is dependent on factors such as propagation, interference, and distance between MS and BTS.

Channels

Wireless communication, in general, happen over channels, which are just a collection of frequencies. Over the duration of the course, we shall come across various types of channels associated with cellular communication. Two channels that are quite fundamental are:

• Control Channel – frequencies reserved for sending and receiving of commands between two entities. In cellular systems, it is of two types, listed below.
• Forward Control Channel – refers to the frequencies used by a telephone tower to communicate with all nearby cellular phones, i.e. a BTS to MS communication.
• Reverse Control Channel – refers to the frequencies used by cellular telephones to communicate with a telephone tower, i.e. an MS to BTS communication.

How are Calls Placed?

After having learned all the terminologies and the components of the cellular network system, let’s have a look at how a call is placed.

• When a call is placed, the Mobile Switching Center (MSC) nearest to the caller gets a request to be connected to the callee’s MSC, which forwards the request to them.
• Once they accept the call, a channel is assigned to them through which they talk for the entirety of their conversation.
• When a user talks into their mobile equipment, the microphone in the device converts the voice into a digital signal, which gets sent by the antenna in the device as electromagnetic waves to the nearest Base Transceiver Station (BTS).
• From the BTS it is sent to the Base Station Controller (BSC), which in turn forwards it the corresponding MSC, and then towards the BSC and BTS of the callee.
• At the callee’s mobile equipment, the antenna receives the electromagnetic waves and converts it back to digital signals, which gets converted into voice at the speaker of the callee.
• In the case of either the caller or callee being in motion, the MSC suggests for a handover between two BTSs along the direction of motion, maintaining the quality of the call. We’ll discuss handoff and all the different types of handoff in a later post in this wireless and cellular communication course.

Now, one question that would probably be in everyone’s mind is how such a narrow spectrum gets distributed for over a billion subscribers, if not more. To satisfy this, certain techniques are employed, such as the Multiple Access techniques like CDMA, TDMA, and WCDMA, to complement the reuse of frequency, ensuring that everyone gets to enjoy their daily quota of mobile data. We will study more about these Multiple Access techniques later in this course.