RFCellular Communication Basics: A Tutorial
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We will explore the following topics:
- Cellular System Architecture (with GSM example)
- Cellular Coverage Expansion Techniques (Cell splitting, Cell Sectoring, Frequency Re-use)
- Access Techniques (FDMA, TDMA, CDMA)
- Handover concept in Cellular communication
- Next generation cellular technologies
Cellular communication has become a replacement for wired communication, initially used for voice. Nowadays, cellular infrastructure handles both voice and data. The increasing use of internet applications has driven the development of cellular technologies like LTE and LTE-Advanced to support high-speed internet access.
Cellular System Architecture
The basic cellular system comprises three key elements:
- Mobile terminal or phone
- Cellular Tower (i.e. base station)
- Backbone infrastructure
Different wireless standards have evolved to support varying bandwidths and coverage areas in the cellular domain. The mobile phone connects to the cellular tower. The cellular tower facilitates two-way communication with other mobiles in the system. The backbone infrastructure connects base stations to each other and to other systems (internet, ISDN, PSTN, other cellular technologies, etc.). These connections between subsystems are called interfaces.
Figure 1: GSM Network Architecture
Mobile and base stations communicate using radio frequency (RF) or electromagnetic waves. Specific RF frequencies are planned based on regional needs. For example, GSM uses the 900 MHz band. Two-way communication requires a frequency pair: one for the uplink (mobile to base station) and one for the downlink (base station to mobile). In GSM, 890 to 915 MHz is used for the uplink, and 935 to 960 MHz is used for the downlink. These bands are further divided into frequency channels with specific bandwidths, using a method called FDMA (Frequency Division Multiple Access), as explained below.
Access Techniques for Resource Sharing
The 890 to 915 MHz band has a bandwidth of approximately 25 MHz. This is divided into 124 channels, each with a bandwidth of 200 KHz. To accommodate more users, each frequency channel is further divided into eight time slots. These time slots are assigned to users using TDMA (Time Division Multiple Access). Therefore, GSM uses FDMA/TDMA for resource allocation.
CDMA (Code Division Multiple Access) is another technique used in IS-95, CDMA-2000, and WCDMA-based cellular technologies. In CDMA, unique codes are assigned to both mobile users and base stations to differentiate them within the system.
Cellular Coverage Expansion
Figure 2: Cellular Coverage Expansion
Before providing cellular service, towers and infrastructure must be installed. Connectivity between cellular systems, using fiber optic, microwave, or satellite links, is essential for long-distance calls.
In cellular network deployment, an area is divided into clusters, which are further divided into cells. Each cell is equipped with a base station (BTS). Cells are typically hexagonal, but their shape varies due to terrain and infrastructure.
Frequency is a valuable resource, so RF engineers developed the concept of frequency reuse. This allows the same frequency to be used in other cells without interference. To prevent co-channel interference, frequency reuse is employed in different clusters. Neighboring cells in different clusters can use the same frequency channels.
Early on, base stations used high transmission power to reach subscribers in the farthest areas. To address the increasing number of users in urban areas, cells are divided into smaller cells. Base stations then transmit at lower power in these areas to avoid RF interference. This is known as cell splitting.
Figure 2 shows three clusters, each with seven cells. Cell 1 in each cluster can use the same frequency pair.
Handover Concept
Handover (or handoff) refers to the process of a mobile phone user moving from one cell to another. The type of handover depends on the speed of the movement. With the variety of cellular systems available today, handover is not limited to a single technology. It can occur between technologies, such as GSM to LTE, GSM to CDMA, CDMA to GSM, and LTE to GSM, to provide uninterrupted service.
To support the demand for higher data bandwidth, cellular offloading has also been developed. In cellular data offloading, data traffic is transferred from the cellular system to WLAN-based indoor systems using cellular-to-WLAN handover.
Next Generation Cellular Technologies
The invention of OFDM (Orthogonal Frequency-Division Multiplexing) allows for more data bits to be transmitted simultaneously over multiple carriers.
Cellular technologies such as Mobile WiMAX, LTE, and LTE-A use OFDM and OFDMA (Orthogonal Frequency-Division Multiple Access) to meet the high data rate demands of users. MIMO (Multiple-Input Multiple-Output) and beamforming techniques also provide higher data rates and better coverage.
The system architectures, access techniques, RF frequencies, and bandwidths vary for different wireless technologies, but the basic system elements remain the same.
