Any radio telephone capable of operating while moving at any speed, battery operated and small enough to be carried by a person comes under the mobile communication systems. These communication systems may have different facilities. The different types of mobile communication systems are mobile two-way radio, public land radio, mobile telephone and amateur (HAM) radio.
Mobile two-way radios are one-to-many communication systems that operate in half-duplex mode, i.e., push to talk. The most common among this type is citizen band (CB) radio, which uses amplitude modulation (AM). It operates in the frequency range of 26-27.1 MHz having 40 channels of 10 kHz. It is a non-commercial service that uses a press-to-talk switch. It can be amplitude-modulated having double-sideband suppressed carrier or single-sideband suppressed carrier.
Public land mobile radio is a twoway FM radio system, used in police, fireand municipal agencies. It is limited to small geographical areas.
Mobile telephones offer full-duplex transmission. These are one-to-one systems that permit two simultaneous transmissions. For privacy, each mobile unit carries a unique telephone number.
Amateur (HAM) radios cover a broad frequency band from 1.8 MHz to above 30 MHz. These include continuous wave (CW), AM, FM, radio teleprinter, HF slow-scan still picture TV, VHF or UHF slow-scan or fast-scan TV, facsimile, frequency-shift keying and amplitude-shift keying.
Present and past of mobile communications
Before I narrate the journey from 1G to 4G, let me explain the important technologies behind the phenomenal growth of mobile communication systems. Since the commercial introduction of advanced mobile phone system (AMPS) service in 1983, mobile communication systems have witnessed an explosive growth. The most important breakthrough was the cellular concept.
Cellular concept. The advent of cellular operation brought frequency reuse capabilities. Advances in wireless access, digital signal processing, integrated circuits, increased battery life, etc led to exponential growth of personal communication services.
Cellular system works as follows: An available frequency spectrum is divided into discrete channels, which are assigned in groups to geographic cells covering a service area. The discrete channels are capable of being reused in different cells with diameters ranging from 2 to 50 km. The service area is allotted a radio frequency (RF) transmitter, whereas adjacent cells operate on different frequencies to avoid interference.
Cellular telephones began as a simple two-way analogue communication system using frequency modulation for voice and frequency-shift keying for transporting control and signaling information. Other cellular systems are digital cellular system, cordless telephony, satellite mobile and paging. Analogue cellular systems fall in the first-generation(1G) category and digital cellular low-power wireless fall in the second-generation (2G) category.
Analogue cellular phone. In 1970, Bell Labs in New Jersey proposed a cellular telephone concept as advanced mobile telephony system (AMPS). AMPS is a standard cellular telephone service placed into operation on October 13, 1983 by Illinois Bell. It uses narrow-band FM with a usable audio frequency band of 300-3 kHz and maximum frequency deviation of ±12 kHz for 100 per cent modulation. According to Carson’s rule, this corresponds to 30 kHz.
AMPS uses frequency-division multiple access (FDMA), where transmissions are separated in the frequency domain. Subscribers are assigned a pair of voice channels (forward and reverse) for the duration of their call. Analogue cellular channels carry both voice using FM and digital signaling information using binary FSK.
Digital cellular system. It provides improvements in both capacity and performance. FDMA uses a frequency canalisation approach to spectrum management, while time-division multiple access (TDMA) utilises a time-division approach. The entire available cellular RF spectrum is sub-divided into narrow-band radio channels to be used as a one-way communication link between cellular mobile units and base stations.
Multiple access technologies for cellular systems
Generally, a fxed amount of frequency spectrum is allocated to a cellular system. Multiple access techniques are deployed so that the users can share the available spectrum in an efficientmanner.
For wireless communication, multiplexing can be carried out in three dimensions: Time (TDMA), frequency (FDMA and its variation OFDMA) and code (CDMA).
In TDMA the available spectrum is partitioned into narrow frequency bands or frequency channels, which, in turn, are divided into a number of time slots. In case of North American digital cellular standard IS-136, each frequency channel (30 kHz) is divided into three time slots, whereas in European digital cellular system GSM each frequency channel (200 kHz) is divided into eight time slots. Guard bands are needed both between frequency chan-nels and time slots.
In FDMA, users share the available spectrum in a frequency band called trafficchannel. Different users are assigned different channels on demand basis. The user’s signal power is concentrated in a relatively narrow frequency band. All the analogue cellular systems used FDMA system.
OFDM is a multi-cellular transmission technique where a data stream is carried with many lower-rate subcarrier tones. It has been adopted in mobile communications to combat hostile frequency-selective fading and has been incorporated into wireless network standards.OFDM is a multi-cellular transmis-sion technique where a data stream is carried with many lower-rate sub-carrier tones. It has been adopted in mobile communications to combat hostile frequency-selective fading and has been incorporated into wireless network standards.
OFDM combines the advantages of coherent detection and OFDM modulation and has many merits that are critical for future high-speed transmission systems. By using up/down conversion, electrical bandwidth requirement can be greatly reduced for the OFDM transceiver, which is extremely attractive for high-speed circuit design where electrical signal bandwidth dictates the cost. Lastly, signal processing in the OFDM transceiver can take advantage of efficient algorithm of fat Fourier transform (FFT)/inverse FFT, which suggests that OFDM has superior scalability over channel dispersion and data rate.
Digital modulation keying
Communication systems often involve modulation of a carrier, which results in a bandpass waveform. A digital signal can be used to modulate the amplitude, frequency or phase of a sinusoidal carrier producing three different forms of digital modulation: amplitude-shift keying (ASK), frequency-shift keying (FSK) and phase-shift keying (PSK). In addition to these basic techniques, there are some modulation schemes that employ a combination of amplitude and phase modulation. It may be noted that unlike ASK signal, PSK transmission is polar. At the same time, ASK is a linear modulation scheme, whereas PSK is a non-linear modulation scheme. PSK has a superior performance over ASK.
Quadrature phase-shift keying (QPSK). Digital modulation techniques mentioned above are spectrally inefficientin the sense that the available channel bandwidth is not fully used. Spectral efficiencycan be improved by using QPSK. It is a system for two message sources. In this system modulation carriers in phase quadrature are combined to form the output waveform. In QPSK the amplitude of the modulator waveform and modulator gains are made as nearly equal as possible.
Differential phase-shift keying (DPSK). DPSK is a modification of PS that avoids the need to provide synchronous carrier required for detection of PSK signals. It is an ingenious technique whereby the carrier reference is derived from the received waveform in the preceding bit interval by use of a 1-bit delay. In essence, the received waveform delayed by 1-bit duration serves as its own reference.
Data transmission using packet switching
This is done by supplying various addressed packets, which are interconnected to have the conversation. New dedicated paths are created for sending the data. From the multiple paths to the destination, any path can be used to send data. Cellular digital packet data was designed for optimal operation with an analogue cellular system, especially AMPS.
Short message service. Short message service is the most common packet service that is supported on digital cellular networks like GSM, IS-136, EDGE and PDC (packet data service). It is a store-and-forward/packet mode service that provides inter-working with the various applications and services within a fixednetwork. For message transfer between relevant network entities, control and signalling channels (instead of normal traffic channels) are generally used for data transmission.
General packet radio service (GPRS). GPRS essentially represents add-on capabilities to the basic voice-optimised cellular network that nevertheless maintain the essential characteristics of radio-access technology. You can use these using GPRS or GSM modules.
Enhanced data rates for GSM evolution (EDGE). In order to enhance the data handling capabilities of 2G service, radio-access portion had to be modified. This modification was evolved in Europe in the form of EDGE. EDGE also supports a link adaptation mechanism that selects the best combination of modulation and encoding schemes based on the timevarying link quality.
EDGE concept applies to both circuit-mode and packet-mode data and is sufficiently generic for application to other digital cellular systems. It works in the 200kHz bandwidth with one or more high-level modulation schemes and a range of efficient coding methods. Modulation schemes are offset QPSK and offset 16 QAM.
It is a special communication technique that purposefully uses much more RF bandwidth than necessary to transmit a signal. This helps in improving the signal-to-noise (S/N) ratio. The main advantages of this technique are secure communication and resistance to intentional jamming. There are 75 channels in the 2400-2483.3MHz band.
There are two methods of perform-ing spread spectrum:
Frequency hopping. This technique spreads the narrow-band signal as a function of time. The transmitted frequency is changed to a different pre-assigned channel several times per second (hopped). The order in which the pre-assigned channels are selected is ‘pseudo random.’ In other words, the channel order is seemingly random but actually repeats itself at a definedinterval. The specificorder in which frequencies are occupied is a function of code sequence and the rate of hopping from one frequency to another is a function of information rate.
Direct sequence. This technique spreads a signal by expanding the signal over a broadband portion of the radio band. It uses a locally generated pseudo noise (PN) code to encode digital data to be transmitted. The most practical all-digit version is direct sequence. Binary phase-shift keying is the simplest and most often used modulation technique.
One of the most important features of spread-spectrum signals is that these contain a large number of very different signaling formats, used for communicating data symbols. It means that the receiver which detects one of these formats cannot detect any other format within a single message. The number of formats used in a spread-spectrum system is called multiplicity factor of the communication link and amounts to thousands.
CDMA. CDMA is a form of direct-sequence spread-spectrum technology that allows many users to occupy the same time and frequency allocations in a given band/space. CDMA assigns each user a unique spreading code to spread the baseband data before transmission, in order to help differentiate signals from various users in the same spectrum. It is the platform on which 2G and advanced 3G services are built.
After speech, the codec converts voice into digital signal. CDMA spreads the voice stream over the full 1.25MHz bandwidth of the CDMA channel, coding each stream separately. The receiver uses a correlator to despread the wanted signal, which is passed through a bandpass filter.Unwanted signals are not despread and not passed through the filter.
The rate of the spreading signal is known as the ‘chip rate’ as each bit in the spreading signal is known as ‘chip.’ All 2G networks support only single-user data rates of the order of 10 kbps, which is too slow for rapid e-mail and Internet browsing.
CDMA provides more than ten times the capacity of the analogue AMPS and fivetimes the calling capacity of GSM and TDMA systems. It requires fewer cell sites than GSM and TDMA.
Personal communication system
Personal communication system (PCS) is a new class of cellular telephone system such as AMPS. PCS systems are a combination of cellular telephone network and intelligent network, which is the entity of super-simple transfer (SST) inter-office protocol tha distinguishes physical components of the switching network such as signal service point, signal control point and signal transfer point from the services provided by SST network.
In essence, PCS is the North American implementation of European GSM standard. GSM utilised its own TDMA access methods and provided expanded capacity and unique services such as caller ID, call forwarding and short messaging. A critical feature was seamless roaming, which allowed subscribers to move across provider boundaries. The effort was directed towards second-generation cellular systems.
In 1990, a second frequency band was specified. This band included twodomains—1710-1785 MHz and 1805-1880 MHz, i.e., twice 75 MHz; three times as much as the primary 900MHz band.
Digital enhanced cordless telecommunication (DECT). DECT is a type of PCS system. DECT standard was developed by European Telecommunication Standards Institute (ETSI) for wireless PABX data LAN applications that represent closed environments requiring minimal open cordless access, since it was essential that products from different vendors not only coexist but interwork with each other.
DECT system has a TDMA/TDD frame structure with 24 slots that are equally allocated for downlink and uplink operation. DECT specifiesboth simplex (half-slots) and duplex (full slot) operation. Higher data rates are achieved by utilising multilevel modulation. The basic modulation scheme is a two-level Gaussian filled frequency-shif keying (GFSK), which is supplemented with 8-level modulation scheme leading to as high as 2.88 Mbps per carrier.
Global system for mobile communications (GSM) was developed by the Groupe Special Mobile, which was an initiative of the Conference of European Post and Telecommunications (CEPT) administrations. GSM was firs devised as a cellular system in a specific 900MHz band called the primary band. This primary band includes two sub-bands of 25 MHz each, 890-915 MHz and 935-960 Mhz.
GSM systems like Iridium, Globalstar and ICO use constellations of low-earth orbit (LEO) or medium-earth orbit (MEO) satellites and operate as overlay networks for existing cellular and PCS networks. Using dual-mode, these extend the coverage to any and all locations on the earth’s surface.
International Mobile Telecommunication-2000 (IMT-2000) is a standard developed by ITU for 3G. It ensures global mobility in terms of global seamless roaming and service delivery. An appreciation of the role of numbering and identities in mobility management, international roaming, call delivery, and billing and charging is important in understanding the operation of mobile and personal communication networks.
Personal communication satellite service (PCSS) uses LEO satellite repeaters incorporating QPSK modulation and both FDMA and TDMA.
The main advantages of GSM are international roaming (in harmony with ISDN principles assuring inter-working between ISDN and GSM) and features like privacy and encryption, frequency hopping, discontinuous transmission and short message service. Other facilities include call forwarding, barring, waiting, hold and teleconferencing.
The basic architecture comprises a network sub-system, base station sub-system, mobile stations, and system interworking and interfaces.
A subscriber identity module (SIM) is required to activate and operate a GSM terminal. The SIM may be contained within the mobile station or it may be a removable unit that can be inserted by the user in his mobile set.
New developments along the way
Before we proceed to evolution from 1G to 4G, let me touch upon the new developments that took place in 1G to 4G.
Global positioning system (GPS). GPS is a reliable navigational aid available anywhere on the earth, operating in all weather conditions 24 hours a day. It can be used by marine, airborne and land users. GPS technology was developed in 1983.
GPS consists of three segments:
Space segment. GPS consists of 24 NAVSTAR satellites along with three spare satellites orbiting at 20,200 km above the earth’s surface in six circular orbital planes with a 12-hour orbital period each. These satellites operate at L1 band (1.575 GHz) continuously broadcasting navigational signals called coarse acquisition code. These codes can be received by anyone for decoding and findingnavigational parameters like longitude, latitude, velocity and time.
Control segment. It consists of a master control station (MCS) and a number of smaller earth stations called monitoring stations located at different places in the world. Monitoring stations track satellites and pass on the measured data to the MCS. The MCS computes satellite parameters (called ephemeris) and sends them back to the satellite, which, in turn, broadcasts to all GPS receivers.
User segment. The user segment consists of all moving and stationary objects with GPS receivers. A GPS receiver is a multi-channel satellite receiver that computes every second its own location and velocity.
Bluetooth. Compared to WLAN technologies, Bluetooth technology aims at so-called ad-hoc piconets, which are local-area networks with a very limited coverage and without the need for an infrastructure. The term ‘piconet’ is a collection of Bluetooth devices that are synchronised to the same hopping sequence. One device in the piconet can act as master and all other devices connected to the master act as slaves. The master determines the hopping pattern and the slaves have to synchronise to this pattern. The hopping pattern is determined by the device ID—a 48-bit worldwide unique identifier.The phase in the hopping pattern is determined by the master’s clock. All active devices are assigned a 3-bit active member address.
All parked devices use an 8-bit parked member address. Devices in standby mode do not need an address. The goal for Bluetooth development was to use a single-chip, low-cost, radio-based wireless network technology for laptops, notebooks, headsets, etc.
Bluetooth operates in the 2.4GHz ISM band. However, MAC, physical layer and the offered services are completely different. Bluetooth transceivers use Gaussian FSK for modulation and are available in three power classes: Class 1 (max. power 100 mW), class 2 (max. power 2.5 mW) and class 3 (max. power 1 mW).
Journey from 1G to 4G
1G system. 1G specifications were released in 1990 to be used in GSM. 1G systems are analogue systems such as AMPS that use FDM to divide the bandwidth into specificfrequencies that are assigned to individual calls.
2G system. These second-generation mobile systems are digital and use either TDMA or CDMA method. Digital cellular systems use digital modulation and have several advantages over analogue systems, including better utilisation of bandwidth, more privacy, and incorporation of error detection and correction.
2.5G system. It was introduced mainly to add latest bandwidth technology to the existing 2G generation. It supports higher-data-rate transmission for Web browsing and also supports a new browsing format language called wireless application protocol (WAP). The different upgrade paths include high-speed circuit-switched data (HSCSD), GPRS and EDGE.
HSCSD increases the available application data rate to 14.4 kbps as compared to 9.6 kbps of GSM. By using four consecutive time slots, HSCSD is able to provide a raw transmission rate of up to 57.6 kbps to individual users.
GPRS supports multi-user network sharing of individual radio channels and time slots. Thus GPRS supports many more users than HSCSD but in a bursty manner. When all the eight time slots of a GSM radio channel are dedicated to GPRS, an individual can achieve as much as 171.2 kbps. But this has not brought any new evolution.
EDGE introduces a new digital modulation format called 8-PSK (octal phase-shift keying). It allows nine different air interface formats, known as multiple modulation and coding schemes, with varying degree of error control and protection. These formats are automatically and rapidly selectable. Of course, the covering range is smaller in EDGE than in HSCSD or GRPS.
3G system. To overcome the short-comings of 2G and 2.5G, 3G has been developed. It uses a wideband wireless network that offers increased clarity in conversations. Countries throughout the world are currently determining new radio spectrum bands to accommodate 3G networks. ITU has established 2500-2690MHz, 1700-1855MHz and 806-960MHz bands. Here the target data rate is 2 Mbps. The data is sent through packet switching. Voice calls are interpreted through circuit switching.
3G W-CDMA (UMTS). Universal Mobile Telecommunication System (UMTS) or W-CDMA assures backward compatibility with 2G and 2.5G TDMA technologies. W-CDMA, which is an air interface standard, has been designed for always-on packet-based wireless service, so that computers and entertainment devices may all share the same wireless network and connect to the Internet anytime, anywhere.
W-CDMA supports data rates of up to 2.048 Mbps if the user is stationary, thereby allowing high-quality data, multimedia, streaming audio, streaming video and broadcast type services to consumers. With W-CDMA, data rates from as low as 8 kbps to as high as 2 Mbps can be carried simultaneously on a single W-CDMA 5MHz radio channel, with each channel supporting between 100 and 350 simultaneous voice calls at once, depending on antenna sectoring, propagation conditions, user velocity and antenna polarisation.
Time slots in W-CDMA are not used for user separation but to support periodic functions. (This is in contrast to GSM where time slots are used to separate users). The bandwidth per W-CDMA channel is 4.4 to 5 MHz.
Since the global standard was diffiult to evolve, three operating modes have been specified:A 3G device will be a personal, mobile, multimedia communication device (e.g., TV provider redirects a TV channel directly to the subscriber’s phone where it can be watched). Second, it will support video conferencing, i.e., subscribers can see as well as talk to each other. Third, it will also support location-based services, where a service provider sends localised weather or trafficconditions to the phone or the phone allows the subscriber to findnearby businesses or friends.
3.5G. It supports a higher through-put and speed at packet data rates of 14.4 Mbps, supporting higher data needs of consumers.
4G system. It offers additional features such as IP telephony, ultrabroadband Internet access, gaming services and HDTV streamed multimedia. Flash-OFDM, the 802.16e mobile version of WiMax (also known as WiBro in South Korea), can support cellular peak data rates of approx. 100 Mbps for high-mobility communications such as mobile access and up to 1 Gbps for low-mobility communications such as nomadic/local wireless access, using scalable bandwidths of up to 40 MHz. The infrastructure for 4G is only packet-based (all-IP).
However lately, another system is making rounds. The 5th generation of mobile communication. Read about it here
The author is director (R&D) at Global Institute of Technology, EPIP Sitapura, Jaipur