16.3 Cellular Networks and Mobility

Cellular Network Architecture and Components

Cellular networks form the backbone of mobile communications. They divide large geographic areas into smaller regions called cells, each served by its own base station. This architecture allows networks to reuse radio frequencies across non-adjacent cells, dramatically increasing the number of users a network can support. Understanding this structure is essential for grasping how mobility, handoffs, and network evolution all fit together.

Architecture of cellular networks

A cellular network has two fundamental components:

• Base stations (BSs) provide wireless coverage to a specific area (a cell). Each base station is equipped with antennas and radio transceivers that communicate with mobile devices. The base station connects back to the core network through a backhaul link, which carries aggregated traffic from all users in that cell.
• Mobile stations (MSs) are user devices like smartphones, tablets, and IoT sensors. They communicate with the nearest base station to access network services and can move between cells while maintaining connectivity through a process called handoff.

The geographic area is divided into cells, each served by one base station. In practice, cells aren't perfect hexagons like textbook diagrams suggest. Their size and shape depend on terrain, building density, and coverage requirements. Urban areas typically use smaller cells, while rural areas use larger ones.

Factors in cell coverage

Cell coverage refers to the geographic area a single base station can reliably serve. Several factors determine how large or small a cell is:

• Frequency band: Lower frequencies (e.g., 700 MHz) propagate farther and penetrate obstacles better, enabling larger cells. Higher frequencies (e.g., 3.5 GHz, mmWave) offer more bandwidth but cover shorter distances.
• Transmit power: Increasing transmit power extends range, but it also increases interference with neighboring cells that reuse the same frequencies.
• Antenna height and type: Taller antennas and directional (sector) antennas can extend coverage and focus capacity where it's needed. Omnidirectional antennas radiate in all directions, while sector antennas divide a cell into wedge-shaped regions.
• User density and traffic demand: Dense areas like urban centers or stadiums need smaller cells (microcells, picocells, femtocells) to provide enough capacity per user.
• Terrain and obstacles: Mountains, buildings, and foliage all attenuate signals and create coverage gaps.

The tradeoff is straightforward: smaller cells provide higher capacity per unit area but require more base stations (and more infrastructure cost) to cover the same region.

Cellular Network Evolution and Standards

Generations of cellular technology

Each generation of cellular technology introduced major changes in how data is encoded, transmitted, and managed:

• 1G (First Generation)
  • Analog voice communication only
  • Primary standard: Advanced Mobile Phone System (AMPS)
  • Very limited capacity, no data services, and poor security (calls could be easily intercepted)
• 2G (Second Generation)
  • Shifted to digital voice, which improved capacity and call quality
  • Introduced low-speed data services like SMS and MMS
  • Major standards: GSM (dominant globally) and CDMA (primarily in North America)
  • Added encryption for voice calls and enabled international roaming
• 3G (Third Generation)
  • Designed for mobile broadband, with peak data rates up to about 2 Mbps
  • Standards: UMTS (based on W-CDMA, the 3G evolution of GSM) and EV-DO (the 3G evolution of CDMA)
  • Enabled multimedia applications like video calling, mobile web browsing, and streaming
• 4G (Fourth Generation)
  • All-IP architecture, meaning both voice and data travel as IP packets
  • Peak theoretical rates up to 1 Gbps (stationary) and 100 Mbps (mobile), though real-world speeds are lower
  • Primary standard: LTE (and later LTE-Advanced)
  • Significant improvements in spectral efficiency and latency, enabling HD video streaming and real-time gaming
• 5G (Fifth Generation)
  • Targets up to 20 Gbps peak throughput and latency as low as 1 ms
  • Three main use cases: enhanced mobile broadband (eMBB), massive machine-type communications (mMTC for IoT), and ultra-reliable low-latency communications (URLLC for autonomous vehicles, remote surgery)
  • Introduces network slicing (creating virtual, purpose-built networks on shared infrastructure), edge computing, and heavy use of mmWave spectrum

Cellular network standards

• GSM (Global System for Mobile Communications)
  • The dominant 2G standard, developed by ETSI
  • Uses a combination of TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access) to share the radio channel
  • Supports voice, SMS, and data up to 9.6 kbps (later extended to ~384 kbps with EDGE)
• CDMA (Code Division Multiple Access)
  • Developed by Qualcomm, used in 2G (cdmaOne) and 3G (CDMA2000/EV-DO)
  • All users share the same frequency band simultaneously, distinguished by unique spreading codes
  • Offers better capacity under heavy load and inherent resistance to narrowband interference compared to TDMA-based systems
• LTE (Long Term Evolution)
  • The dominant 4G standard, developed by 3GPP
  • Uses OFDMA (Orthogonal Frequency Division Multiple Access) on the downlink and SC-FDMA on the uplink, combined with MIMO (Multiple Input Multiple Output) antenna techniques
  • Real-world download speeds typically range from 5 to 100 Mbps, with theoretical peaks up to 300 Mbps (and higher with LTE-Advanced carrier aggregation)

Mobility Management in Cellular Networks

Mobility management is the set of mechanisms that let a user move freely while staying connected. It has two core problems to solve: transferring active connections when you move between cells (handoff), and knowing where to find a user when a new call or session arrives (location management).

Handoff (handover)

A handoff transfers an ongoing call or data session from one base station to another as the user crosses a cell boundary. The goal is to make this transition seamless so the user doesn't notice.

The handoff process generally works like this:

1. The mobile station continuously measures signal strength from its serving base station and neighboring base stations.
2. When the signal from a neighbor becomes stronger (or the current signal drops below a threshold), the mobile station or network triggers a handoff.
3. The network allocates resources on the target base station and redirects the connection.

There are two main types:

• Hard handoff: The connection to the old base station is dropped before the new one is established. This is a "break-before-make" approach. It's simpler but risks a brief interruption. GSM and LTE use hard handoffs.
• Soft handoff: The mobile station communicates with both the old and new base stations simultaneously during the transition, then drops the old one. This is "make-before-break" and provides smoother transitions. CDMA networks support soft handoffs because multiple base stations can use the same frequency.

Location management

When someone calls your phone, the network needs to know which base station you're near. Location management solves this.

• The network groups cells into location areas. Your phone sends a location update whenever it enters a new location area, so the network always knows your approximate position.
• When an incoming call arrives, the network uses paging: it broadcasts a message across all cells in your last known location area, asking your phone to respond. Your phone replies, and the network routes the call to the correct base station.

The tradeoff here is between update frequency and paging cost. Smaller location areas mean fewer cells to page (faster call setup) but more frequent location updates from the mobile station (more signaling overhead). Larger location areas reduce updates but increase paging load.

Challenges in mobility management

• Handoff latency: Minimizing the delay during handoff is critical for real-time applications like voice and video. Even brief interruptions can cause noticeable drops.
• Signaling overhead: Every location update and handoff generates control messages. In dense networks with many users, this signaling load can become significant.
• Vertical handoff: Moving between different network technologies (e.g., from Wi-Fi to LTE) requires a handoff across heterogeneous systems, which is more complex than switching between two cells of the same type.
• Privacy and security: Location tracking must be handled carefully. Networks use temporary identifiers (like TMSI in GSM) and encryption to protect user identity and location from eavesdroppers.