GSM Overview1 Outline Frequency Concepts Frequency Specifications Wavelength Frequency Allocation – US Bandwidth Channels Duplex Distance Carrier Separation Capacity Frequency Reuse Transmission Rate Modulation Method Access Method

GSM Overview2 Outline Analog and Digital Transmission Analog and Digital Signals Advantages of Using Digital Signals Transmission Problems Path Loss Shadowing Carrier to Interference C/I Ratio Carrier to Adjacent C/A Ratio Multipath Fading Rayleigh Fading Time Dispersion Time Alignment Combined Signal Loss

GSM Overview3 Outline Solutions to Transmission Problems Cell Planning Channel Coding Interleaving Antenna Space Diversity Antenna Polarization Diversity Adaptive Equalization Frequency Hopping Time Advance

GSM Overview4 Outline GSM Transmission Process Analog to Digital Conversion Sampling Quantization Coding Speech Coding Channel Coding Adaptive Multi Rate AMR Interleaving Ciphering / Encryption Burst Formatting Modulation and Transmission

First Generation The first generation of mobile cellular telecommunications systems appeared in the 1980s. The first generation was not the beginning of mobile communications, as there were several mobile radio networks in existence before then, but they were not cellular systems either. The capacity of these early networks was much lower than that of cellular networks, and the support for mobility was weaker. In mobile cellular networks the coverage area is divided into small cells, and thus the same frequencies can be used several times in the network without disruptive interference. This increases the system capacity. The first generation used analog transmission techniques for traffic, which was almost entirely voice.

Second Generation The second-generation (2G) mobile cellular systems use digital radio transmission for traffic. Thus, the boundary line between first- and second generation systems is obvious: It is the analog/digital split. The 2G networks have much higher capacity than the first-generation systems. One frequency channel is simultaneously divided among several users (either by code or time division). Hierarchical cell structures—in which the service area is covered by macrocells, microcells, and picocells—enhance the system capacity even further. There are four main standards for 2G systems: Global System for Mobile (GSM) communications and its derivatives; digital AMPS (Advanced Mobile Phone Service) (D-AMPS); code division multiple access (CDMA) IS-95; and personal digital cellular (PDC). GSM is by far the most successful and widely used 2G system.

Second Generation The basic GSM uses the 900-MHz band, but there are also several derivatives, of which the two most important are Digital Cellular System 1800 (DCS-1800; also known as GSM-1800) and PCS-1900 (or GSM-1900). The latter is used only in North America and Chile, and DCS-1800 is seen in other areas of the world. The prime reason for the new frequency band was the lack of capacity in the 900-MHz band. The 1,800-MHz band can accommodate a far greater user population, and thus it has become quite popular, especially in densely populated areas. The coverage area is, however, often smaller than in 900-MHz networks, and thus dual band mobiles are used, where the phone uses a 1,800-MHz network when such is available and otherwise roams onto a 900-MHz network.

Generation 2.5 “Generation 2.5” is a designation that broadly includes all advanced upgrades for the 2G networks. These upgrades may in fact sometimes provide almost the same capabilities as the planned 3G systems. The boundary line between 2G and 2.5G is a hazy one. It is difficult to say when a 2G becomes a 2.5G system in a technical sense. Generally, a 2.5G GSM system includes at least one of the following technologies: a) High-speed Circuit-Switched Data (HSCSD), b)General Packet Radio Services (GPRS), c)Enhanced Data rates for Global Evolution (EDGE). An IS-136 system becomes 2.5G with the introduction of GPRS and EDGE, and an IS-95 system is called 2.5G when it implements IS-95B, or CDMA2000 1xRTT upgrades.

Generation 2.5 The biggest problem with plain GSM is its low air interface data rates. The basic GSM could originally provide only a 9.6-Kbps user data rate. Later, 14.4-Kbps data rate was specified, although it is not commonly used. Anyone who has tried to Web surf with these rates knows that it can be a rather desperate task. HSCSD is the easiest way to speed things up. This means that instead of one time slot, a mobile station can use several time slots for a data connection. In current commercial implementations, the maximum is usually four time slots. One time slot can use either 9.6-Kbps or14.4-Kbps speeds. The total rate is simply the number of time slots times the data rate of one slot. This is a relatively inexpensive way to upgrade the data capabilities, as it requires only software upgrades to the network (plus, of course, new HSCSD-capable phones), but it has drawbacks. The biggest problem is the usage of scarce radio resources.

3 rd Generation 3G is the third generation of wireless technologies. It comes with enhancements over previous wireless technologies, like high-speed transmission, advanced multimedia access and global roaming. 3G is mostly used with mobile phones and handsets as a means to connect the phone to the Internet or other IP networks in order to make voice and video calls, to download and upload data and to surf the net. How is 3G Better?: 3G has the following enhancements over 2.5G and previous networks: a)Several times higher data speed; b)Enhanced audio and video streaming; c)Video-conferencing support; d)Web and WAP browsing at higher speeds; e)IPTV (TV through the Internet) support. GSM Overview10

4 th Generation 4G wireless is the term used to describe the fourth-generation of wireless service. 4G is a step up from 3G, which is currently the most widespread, high-speed wireless service. 4G is only available in limited areas. While all 4G service is called "4G," the underlying technology is not the same. Sprint, for example, uses WiMax technology for its 4G network, while Verizon Wireless uses a technology called Long Term Evolution, or LTE. No matter what technology is behind it, 4G wireless is designed to deliver speed. On average, 4G wireless is supposed to be anywhere from four to ten times faster than today's 3G networks. Sprint says its 4G WiMax network can offer download speeds that are ten times faster than a 3G connection, with speeds that top out at 10 megabits per second. Verizon's LTE network, meanwhile, can deliver speeds between 5 mbps and 12 mbps. WiMax is a wireless broadband technology that delivers WiFi-like speeds to wide areas GSM Overview11

Frequency Concepts

GSM Overview13 Frequency Specifications Frequency specifications of various GSM systems

GSM Overview14 Frequency Specifications The above diagram displays the frequencies used by major mobile standards The frequency of a wave is the number of times the wave oscillates per second

GSM Overview15 Wavelength Wavelength is the length of one complete signal oscillation (λ) in meters Frequency and wavelength are related via the speed of propagation (c = λ * f) The higher the frequency, the shorter the wavelength Lower frequencies (longer wavelengths) are suitable for transmission over long distances Higher frequencies are more suitable for transmission over short distances

GSM Overview16 Frequency Allocation - US FCC auctions licenses to mobile network operators (Federal Communications Commission) FCC specified six blocks within the frequency band Three 30 MHz duplex blocks: A, B and C Three 10 MHz duplex blocks: D, E and F Major Trading Areas (MTAs) and Basic Trading Areas (BTAs)

GSM Overview17 Bandwidth Amount of frequency range allocated to one application Depends on available frequency spectrum Important factor in determining capacity of mobile systems Capacity is the number of simultaneous calls that can be handled

GSM Overview18 Channels A channel is a set of frequencies allocated for transmission and reception of information Types of communication channels are listed in the above table

GSM Overview19 Channels The direction from the MS to the network is the uplink The direction from the network to the MS is the downlink

GSM cellular technology The GSM cellular technology had a number of design aims when the development started: a)It should offer good subjective speech quality b)It should have a low phone or terminal cost c)Terminals should be able to be handheld d)The system should support international roaming e)It should offer good spectral efficiency f)The system should offer ISDN compatibility GSM Overview20

GSM Overview21 Duplex Distance Uplink and downlink channels must be separated by duplex distance This prevents both channels from interfering with each other

GSM Overview22 Carrier Separation Carrier separation is distance on frequency band between channels being transmitted in same direction Prevents overlapping of information in one channel into an adjacent channel Dependent on amount of information to be transmitted within the channel

GSM Overview23 Capacity Number of frequencies in a cell determines the capacity Each company is allocated a limited number of frequencies Frequencies are distributed throughout cells in the network

GSM Overview24 Frequency Reuse Frequencies must be reused several times to provide sufficient capacity Same frequencies cannot be reused in neighboring cells

GSM Overview25 Transmission Rate Amount of information transmitted over a radio channel per period of time Expressed in bits per second In GSM, transmission rate over air interface is 270 kbps

GSM Overview26 Modulation Method Modulation technique used in GSM is GMSK (Gaussian Minimum Shift Keying : MSK + Gaussian filter) GMSK is a form of phase shift keying Carrier bandwidth is 200 KHz GMSK provides : – high interference resistance level – Quite insensitive to non-linearities of power amplifier – Robust to fading effects – But moderate spectral efficiency

GSM Overview27 Multiple Access Method GSM uses TDMA to transmit and receive speech signals over air interface With TDMA, one carrier is used to carry a number of calls at designated periods of time TDM frame consists of 8 time slots

Analog and Digital Transmission

GSM Overview29 Analog & Digital Signals Analog signal is continuous waveform which changes smoothly over time Digital signal consists of a set of discrete values Analog Signal Digital Signal

GSM Overview30 Advantages of Digital Signals Digital signals provide better quality for transmission than analog signals: Error detection and correction Data Compression Ciphering / Encryption