21-09-2013, 03:33 PM
Mobile Handset Cellular Network
Cellular Network Basics
There are many types of cellular services; before delving into details, focus on basics (helps navigate the “acronym soup”)
Cellular network/telephony is a radio-based technology; radio waves are electromagnetic waves that antennas propagate
Most signals are in the 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz frequency bands
Cellular Network
Base stations transmit to and receive from mobiles at the assigned spectrum
Multiple base stations use the same spectrum (spectral reuse)
The service area of each base station is called a cell
Each mobile terminal is typically served by the ‘closest’ base stations
Handoff when terminals move
Cellular Network Generations
It is useful to think of cellular Network/telephony in terms of generations:
0G: Briefcase-size mobile radio telephones
1G: Analog cellular telephony
2G: Digital cellular telephony
3G: High-speed digital cellular telephony (including video telephony)
4G: IP-based “anytime, anywhere” voice, data, and multimedia telephony at faster data rates than 3G
(to be deployed in 2012–2015)
The Multiple Access Problem
The base stations need to serve many mobile terminals at the same time (both downlink and uplink)
All mobiles in the cell need to transmit to the base station
Interference among different senders and receivers
So we need multiple access scheme
Frequency Division Multiple Access
Each mobile is assigned a separate frequency channel for the duration of the call
Sufficient guard band is required to prevent adjacent channel interference
Usually, mobile terminals will have one downlink frequency band and one uplink frequency band
Different cellular network protocols use different frequencies
Frequency is a precious and scare resource. We are running out of it
Cognitive radio
GSM Services
Voice, 3.1 kHz
Short Message Service (SMS)
1985 GSM standard that allows messages of at most 160 chars. (incl. spaces) to be sent between handsets and other stations
Over 2.4 billion people use it; multi-billion $ industry
General Packet Radio Service (GPRS)
GSM upgrade that provides IP-based packet data transmission up to 114 kbps
Users can “simultaneously” make calls and send data
GPRS provides “always on” Internet access and the Multimedia Messaging Service (MMS) whereby users can send rich text, audio, video messages to each other
Performance degrades as number of users increase
GPRS is an example of 2.5G telephony – 2G service similar to 3G
Base Station Subsystem
Transcoding Rate and Adaptation Unit (TRAU)
Performs coding between the 64kbps PCM coding used in the backbone network and the 13 kbps coding used for the Mobile Station (MS)
Base Station Controller (BSC)
Controls the channel (time slot) allocation implemented by the BTSes
Manages the handovers within BSS area
Knows which mobile stations are within the cell and informs the MSC/VLR about this
Base Transceiver System (BTS)
Controls several transmitters
Each transmitter has 8 time slots, some used for signaling, on a specific frequency
Major LTE Radio Technogies
Uses Orthogonal Frequency Division Multiplexing (OFDM) for downlink
Uses Single Carrier Frequency Division Multiple Access (SC-FDMA) for uplink
Uses Multi-input Multi-output(MIMO) for enhanced throughput
Reduced power consumption
Higher RF power amplifier efficiency (less battery power used by handsets)