Fundamentals of Cellular Communication and GSM Networks

Fundamental Concepts of Cellular Communication

Cellular communication is a form of wireless communication that allows mobility by transceiving signals through a network of fixed base stations. Historically, mobile devices were large and cumbersome, described metaphorically as "bricks," but they have evolved into ubiquitous tools used for navigation (e.g., Waze, Google Maps) and communication (e.g., SMS). The primary challenge in cellular communication is managing the limited frequency spectrum while supporting a large number of users. To solve this, the concept of frequency reuse is employed, where the geographic area is divided into "cells," and the same frequencies are reused in distant cells to avoid interference.

Several key processes define the user experience in these networks. "Handover" or "handoff" refers to the seamless transition of a call or data session from one base station to another as the user moves. This process ensures that the connection is not dropped when the user crosses cell boundaries. The network is organized into a hierarchy: the individual cells are the smallest units, which are grouped into clusters, and these are managed by higher-level controllers and switches.

Network Architecture and Components

A cellular network consists of several major components categorized into the mobile station and the fixed infrastructure. The Mobile Station (MS) is the user's device, which includes the Mobile Equipment (the physical phone) and the Subscriber Identity Module (SIM). The SIM card stores the user's identity and subscription data, allowing the user to access the network from different physical devices.

The Base Station (BS) serves as the primary contact point for the mobile station. It is responsible for transmitting and receiving radio signals within a specific cell. Each cell is typically represented as a hexagon in theoretical models, though real-world shapes vary. Base stations are connected to a Base Station Controller (BSC), which manages radio resources for one or more base stations, including frequency allocation and handovers.

The Mobile Switching Center (MSC) is the heart of the network. It handles the routing of calls and data between mobile users and other networks, such as the Public Switched Telephone Network (PSTN). The MSC also manages mobility tasks, tracking the location of users as they move between different coverage areas. When a user moves between cells managed by the same MSC, it is an "intra-MSC handover." If they move to an area managed by a different MSC, it is an "inter-MSC handover."

Link Types and Information Flow

Communication in a cellular network occurs over two primary links: the Uplink and the Downlink. The Uplink (also known as the reverse link) is the transmission path from the Mobile Station to the Base Station. The Downlink (or forward link) is the transmission path from the Base Station to the Mobile Station. These links use different frequency bands to allow simultaneous two-way communication (duplexing).

User Databases and Security Management

The network maintains several databases to track users and ensure security. The Home Location Register (HLR) is a central database containing permanent information about every subscriber authorized to use the network, including their current location (at the MSC level) and service profile. The Visitor Location Register (VLR) is a temporary database maintained by the MSC for users currently roaming in its service area. The VLR retrieves data from the user's HLR to provide services locally.

Security is managed by the Authentication Center (AuC), which stores encryption keys and validates the identity of the user during the login process. Additionally, the Equipment Identity Register (EIR) tracks physical devices using their International Mobile Equipment Identity (IMEI) number. The EIR can be used to block stolen or unauthorized devices from accessing the network using a "Black List," while authorized devices are on the "White List."

Multiple Access Techniques: FDMA and TDMA

To allow multiple users to share the same frequency spectrum, networks use multiple access techniques. Frequency Division Multiple Access (FDMA) divides the spectrum into several narrow frequency channels. Each user is assigned a specific frequency for the duration of their call. While simple, it is inefficient because the channel remains occupied even if the user is silent.

Time Division Multiple Access (TDMA) improves efficiency by dividing a single frequency channel into multiple time slots. Users take turns transmitting in their assigned time slots. This allows multiple users to share the same frequency simultaneously. Each TDMA frame consists of a set number of slots, and the cycle repeats rapidly to provide the illusion of a continuous connection. TDMA requires precise synchronization between the mobile station and the base station.

Capacity Calculations and TDMA Parameters

The capacity of a TDMA system is determined by several variables. Let $N$ be the total number of time slots per frame, $C$ be the number of control slots reserved for signaling, and $X$ be the number of frequency channels assigned to a cell. The number of traffic channels $S$ available for users in a single cell is calculated as:

$S = (N - C) \times X$

If we consider a cluster of cells where frequencies are reused, the total number of available channels in the cluster depends on the bandwidth. Let $B_{total}$ be the total bandwidth allocated to the cellular provider and $BW$ be the bandwidth of a single channel. The total count of frequency channels $K$ is:

$K = \frac{B_{total}}{BW}$

If the geographic area is covered by $M$ cells, the total system capacity $P$ (total number of simultaneous users) is:

$P = S \times M$

Traffic Theory and the Erlang B Model

Telecommunications engineers use traffic theory to determine how many channels are needed to support a specific population with a target Grade of Service (GoS). The GoS represents the probability that a call will be blocked because all channels are busy. Traffic intensity is measured in Erlangs ($E$).

For a single user, the offered traffic intensity $A_v$ is defined as:

$A_v = \lambda \times H$

Where $\lambda$ is the average number of call requests per unit time and $H$ is the average duration of a call (holding time). If there are $V$ users, the total offered traffic $A$ is:

$A = A_v \times V$

The probability of blocking in a system with $C$ channels is given by the Erlang B formula:

$GoS = P(\text{Blocking}) = \frac{\frac{A^C}{C!}}{\sum_{k=0}^C \frac{A^k}{k!}}$

As the number of channels increases, the efficiency of the trunking (sharing a pool of channels) improves, allowing the system to support significantly more users for the same GoS.

The GSM Standard: Architecture and Specifications

The Global System for Mobile Communication (GSM) is a digital cellular standard that utilizes a combination of FDMA and TDMA. It operates in several frequency bands, most notably GSM-900 and GSM-1800. In GSM-900, the Uplink uses $890\text{-}915\,MHz$ and the Downlink uses $935\text{-}960\,MHz$. Each channel has a bandwidth of $200\,kHz$.

GSM voice coding uses Regular Pulse Excitation - Long Term Prediction (RPE-LTP). Human speech is divided into $20\,ms$ segments, each compressed into $260$ bits. This results in a source bit rate of:

$\frac{260\,bits}{20\,ms} = 13\,kbps$

To protect against errors, these bits undergo channel coding (including convolutional coding and parity checks), increasing the bit count. The bits are then "interleaved" to spread them across multiple bursts, which protects the signal from bursty interference.

GSM Frame and Burst Structure

A GSM frame lasts for $4.615\,ms$ and is divided into $8$ time slots. Each time slot, known as a "burst," lasts for $0.577\,ms$. The "Normal Burst" used for traffic contains a total of $156.25$ bits, structured as follows:

1. Tail Bits ($3$ bits): Used to define the start of the burst.
2. Data Bits ($57$ bits): The first half of the payload.
3. Signaling/Stealing Bit ($1$ bit): Indicates if the burst carries traffic or urgent signaling.
4. Training Sequence ($26$ bits): Used by the receiver to synchronize and compensate for multi-path distortion.
5. Signaling/Stealing Bit ($1$ bit).
6. Data Bits ($57$ bits): The second half of the payload.
7. Tail Bits ($3$ bits): Used to define the end of the burst.
8. Guard Period ($8.25$ bits): A silent interval to prevent overlapping between bursts due to propagation delays.

The modulation technique used in GSM is Gaussian Minimum Shift Keying (GMSK), a form of digital frequency modulation that is spectrally efficient.

Subscriber and Equipment Identification

GSM units are identified by two primary codes. The International Mobile Subscriber Identity (IMSI) is stored on the SIM card and consists of:

• Mobile Country Code (MCC): $3$ digits.
• Mobile Network Code (MNC): $2$ digits.
• Mobile Subscriber Identification Number (MSIN): Typically $9\text{-}10$ digits.

The International Mobile Equipment Identity (IMEI) identifies the hardware itself and consists of:

• Type Allocation Code (TAC): Identifies the device model and manufacturer.
• Final Assembly Code (FAC): Identifies the factory.
• Serial Number (SNR).
• Spare (SP): A check digit.

Short Message Service (SMS)

SMS is a store-and-forward service for short text messages (up to $160$ characters). The SMS architecture includes a Short Message Service Center (SMSC). If a recipient is unavailable, the SMSC stores the message and attempts to deliver it once the user reconnects to the network. SMS can be Mobile Originated (MO) or Mobile Terminated (MT).

Introduction to CDMA (IS-95)

Code Division Multiple Access (CDMA), specifically the IS-95 standard, differs significantly from GSM. Instead of dividing by time or frequency, CDMA allows all users to share the same wide frequency band ($1.25\,MHz$) simultaneously. Each user's signal is encoded with a unique mathematical code. The receiver uses the same code to extract the specific user's signal from the background noise.

Key features of CDMA include:

• Soft Handover: The device connects to a new cell before breaking the connection with the old one, resulting in zero interruption.
• Power Control: The network constantly adjusts the mobile station's transmit power to ensure all signals arrive at the base station with similar strength, maximizing capacity.

Comparison: GSM vs. IS-95 CDMA