CHAPTER 1 CELLULAR MOBILE COMMUNICATION AN OVERVIEW

Transcription

1 CHAPTER 1 CELLULAR MOBILE COMMUNICATION AN OVERVIEW

2 Shrikant K. Bodhe : Studies on...microcellular... Hand-off Algorithms. 1.1 TECHNOLOGICAL DEVELOPMENTS. Mobile communication of today is inherently more complex than fixed point communication. As a consequence, the full potentiaf of radio communication is far from being realized. In 1947, D. H. Ring of Bell Labs proposed the revolutionary concept called the cellular approach. The basis was a service area divided into regions called cells, each with its own base station radio equipment. The more populated the area, the smaller the cells. For this cellular concept Federal Communication Commission (FCC) allocated 40 MHz and 150 MHz carrier to support 11 channels for Mobile Telephone Service (MTS) and Improved Mobile Telephone Service (IMTS) MJ. Further allocation of 450 MHz carrier to IMTS MK to accommodate 12 channels was introduced for mobile telephone service. During , the allocation of MHz band was under consideration of FCC. On a detailed proposal of feasibility by Bell Labs, FCC allocated 40 MHz bandwidth for cellular telephones. An additional 10 MHz bandwidth was later sanctioned by FCC to cater to the requirements of the mobile phone users. In late 1978, Bell Labs introduced in the market a working system called Advanced Mobile Phone Service (AMPS), and this was a roaring success. On similar lines, a cellular technology called Total Access Communication System (TACS) was adopted in Britain, which was a modified version of American AMPS. Norway, Sweden and Denmark also developed their own systems. So did Nippon Telegraph & Telephone Public Corporation (NTT) in Japan and Siemens in Germany. Philips and CIT-Alcatel also developed their own systems having some advantages over AMPS. A significant advance in mobile communication was made in 1982 after formation Group Special Mobile (GSM) group of European cellular operators and Government agencies. In the year 1985, after resolving difference of opinion in favour of GSM, the European countries adopted the GSM standard. The first trial of GSM was conducted in Paris in 1987 and thereafter it was finalized for a digital Time Division Multiple Access (TDMA) type of system. Subsequently in the year 1991 the United Kingdom, France, Germany and Italy introduced GSM. 2

3 Slirikant K. Bodhe : Studies on...microcellular... Hand-off Algorithms. The improvement in capacity and the quality of service provided by GSM resulted in a rise in demand for mobile telephones. To meet this demand, various research organizations like European Telecommunication Standards Institute (ETSI), Research and development for Advanced Communication for Europe (RACE), international Telecommunication Union (ITU), proposed to FCC the allocation of Personal Communication Services (PCS) band in the frequency range of 1700 MHz to 2200 MHz. In response FCC allocated a bandwidth of 150 MHz around 1900 MHz carrier. In the year 1993, Digital Cellular System (DCS-1800) was introduced in the frequency range of 1800 MHz with a bandwidth allocation of 150 MHz. The recommendations for this system are specified by ETSI and is the derivative of GSM-900 except for frequency band and power level. The DCS-1800 meets the requirements of high traffic areas and is most suitable for densely populated urban areas. The maximum distance it can support is 10 Km. Many countries have planned to use DCS-1800 system as personal communication network standard. The Digital Enhanced Cordless Telephone (DECT) standard has been specified by ETSI. The main objective of DECT is to provide a specification that will support many applications such as residential cordless telephone, business system, public access network (tele-points), cordless PABX, local loop replacement, area network etc. The specifications provide for transmission of both voice and data at 32 Kbps. The system uses 10 carrier frequencies between MHz. Personal Handyphone System (PHS) is a personal mobile communication system that can provide the use of the same telephone both inside and outside home/office. PHS uses TDMA / TDD (Time Division Duplex) access technology. It is a micro or pico cellular technology. The system provides multi-channel calls with high quality and security due to use of digital technology. PHS has advantages over cordless telephone in that the same mobile phone can be used in home as cordless telephone and outside as mobile telephone, retaining the same telephone number. This has been introduced in Japan after successful trials. 3

4 Shrikant K. Bodhe : Studies on...microcellular... Hand-off Algorithms. Personal Communication Services (PCS) are being planned to be introduced in the USA. PCS has been defined purely as a concept without any reference to any specific technology. PCS will provide terminal mobility and service profile management. It will follow the personal number concept. PCS will provide integration of wireless technology with intelligent network. In its notice of inquiry, the FCC describes PCS as encompassing a broad range of communication services that free individuals from the constraints of the wire line Public Switched Telephone Network (PSTN) and enable them to communicate when they are away from their home or office telephone. The choice of PCS may be based on DCS-1900 and TDMA or Code Division Multiple Access (CDMA). TDMA systems are likely to be deployed first due to availability of equipment. Task group 8/1 of ITU-R is finalizing the recommendations on Future Public Land Mobile Telecom Service (FPLMTS) aiming to provide mobile telecommunication anywhere anytime. FPLMTS is planned to be introduced in the year 2000 AD, subject to market considerations. In Europe, a concept similar to FPLMTS called the Universal Mobile Telecommunication System (UMTS) is currently being defined by ETSI. The research work is pioneered by RACE program under MONET (MObile NETwork). Cellular systems are now using small cells in cities which means that hand sets will receive better service. At the same time cordless telephones are becoming more and more sophisticated. As the technology is supporting the use of wireless EPBX, the future success of mobile communication can be built on solutions that meet the personal requirements of the users. The concept of communication to reach any-where any-time will further development in this area. 1.2 BAND ALLOCATION. Currently the mobile systems use frequencies in the range MHz and MHz. In the most optimistic case, GSM can provide 25 channels with 200 KHz bandwidth and a frequency re-use of 3/9, to carry a traffic around 13.2 erlangs/sector [279], The system for future will require an increase of many orders in the capacity, and this can be achieved by employing much higher 4

5 Shrikant K. Bodlie : Studies on...microcellular... Hand-off Algorithms. propagation frequencies with corresponding increase in bandwidth. This requirement suggests that the carrier frequencies should be in the giga-hertz range. The low giga-hertz region (1-26 GHz) is already occupied by other systems. Moreover frequency bands are unused at the higher frequencies, particularly in the millimeter and sub-millimeter bands and hence this band can be a potential candidate for the mobile communication system of the future. The basic principle of the cellular mobile communication demands that the number of channels to carry the traffic should be sufficiently large and the adjacent channel interference should also be limited. These requirements can be satisfied by various frequency bands that show high absorption of radiated signal due to oxygen molecules in the atmosphere. ATTENUATION DUE TO OXYGEN AND WATER VAPOUR Specific Attenuation (db/km.) 10'* Frequency in GHz. Figure 1.1 5

6 Shrikant K. Bodhe : Studies on...microcelluiar... Hand-off Algorithms. Figure 1.1 shows, resonant absorption lines in the frequency range from about 50 to 70 GHz (A1 band) and 105 to 134 GHz (A2 band ). Peaks in the attenuation of electromagnetic radiation due to water vapor absorption exist at 22 GHz (W1 band) and 200 GHz (W2 band ). Generally, the use of frequencies in the vicinity of water vapor absorption band should be avoided, because the attenuation is dependent on weather conditions. Hence it has been proposed by many researchers that oxygen absorption band A1 will be more suitable for mobile communication since the energy radiated in this frequency band will restrict the coverage area and also limit the co-channel interference [1,2]. The frequency spectrum from 51.4 to 66 GHz has been designated as the oxygen absorption band Al, and some part of this band has been nominated for the mobile communication systems, with a prospective bandwidth of 2 GHz. For the oxygen absorption band the attenuation due to oxygen has been measured as 14 db per Km [2] and is in addition to the rain loss of 9 db per Km and 22 db per Km for outages of 0.2 % and 0.02 % respectively [2,p6], There is also the usual attenuation dependent on distance and frequency. When the propagation frequencies are in the vicinity of 35 GHz, a trough in the water vapor attenuation occurs, accompanied by negligible oxygen absorption. This frequency is therefore useful for rural mobile applications. Window band GHz and GHz can be used for the large cells having relatively few users. 1.3 CELL STRUCTURES. The geographical layout of base stations in cellular mobile radio, the assignment of radio channels and base stations to serve the calls are system requirements that must take into account the total number of channels available to the radio system as also the constraints of propagation, modulation techniques, noise etc. A number of base stations must be assembled into an economically viable system that makes efficient use of radio spectrum while providing the number of channels per unit area required to meet the traffic offered to the customers. Cellular radio communication was designed to increase the teletraffic capacity in the service area. The demand for the service area is growing rapidly, and 6

7 Shrikant K. Bodlie : Studies on...microcellular... Hand-off Algorithms. in the heavily populated areas, the service is reaching its limit. This trend is further enhanced by the increasing penetration of hand held portable units into service. The obvious solution for the system of that vintage is to shrink the cell size and scale the cell clustering pattern. This trend has been ongoing in the heavily populated areas and has resulted in MICROCELLS. Conventionally, cellular systems employ relatively large cell, typically 1-20 Km in diameter, with antennas radiating relatively large powers ( W) from the top of tall buildings. These arrangements are inherently of low capacity. The term microcell has now been used to describe small cells for cellular mobile radio communication for over a decade and became well established with Steele and Prabhu's seminal papers. [1,2] Many advantages have been quoted for microceus in mobile radio applications. It has been claimed that they can greatly increase capacity over conventional cellular systems and, if mobile telephones become truly ubiquitous, use of microceils may be a suitable way to support the resultant increase in teletraffic. Fortunately, there is general agreement amongst research workers regarding the definitions of conventional cells, microcells and picocells. Microcells are the cells achieved when the base station antenna is below the local urban skyline such as, when it is mounted on the side of the buildings or street lamp posts. In this case signal leakage over the top of the building is insignificant and the main propagation mode is along streets and around comers. The shape of such cells is highly constrained by local topography and cell shapes are rarely circular and so the use of term radii to describe coverage area is misleading. Coverage is typically achieved upto m from the base station depending on direction and street pattern. The type of cellular structure employed in a mobile radio system has a profound influence on its performance. There are number of suggestions based on coverage area prediction to include highways, urban area, suburban area, rural areas. The diverse nature of cell shapes can be accommodated with the aid of suitable antennas, correct location of the base station, control of radiated power and the application and order of diversity reception. 7

8 Shnkant K. Bodhe: Studies on...microcellular...hand-off Algorithms. When the propagation frequency is increased to the range of 35 to 70 GHz the Line of Sight (LOS) signal is further attenuated due to the effects of atmosphere, rain etc. and the loss due to increase in frequency. The radiated energy that reaches the receiver by means of reflections from roads and buildings will probably result in a propagation exponent other than the one conventionally used by a system below 10 GHz. The concept of micro-cell, pico-cell has been put into practice to improve the capacity constraints for the existing systems. For the system below 10 GHz and for a micro-cell structure the interference is the limiting factor for capacity. By migrating to 35 to 70 GHz. frequency range one can reduce the inter-cell interference to improve the capacity. 1.4 DIGITAL COMMUNICATION TECHNOLOGY. To improve the capacity of analog cellular mobile telephone systems and to satisfy the demand of data communication, many researchers have studied various aspects of digital communication for mobile communication engineering Modulation Techniques. The choice of modulation technique has a direct impact on the capacity of digital cellular mobile communication system that determines the bandwidth efficiency of a single physical channel in terms of number of bits per second per hertz. A modulation scheme used for mobile environment should utilize the transmitted power and RF channel bandwidth as efficiently as possible. This is because the mobile radio channel is both power and band limited. To conserve power, efficient source encoding schemes are generally used but this is at the cost of bandwidth, whereas to save spectrum in band limited system, spectral efficient modulation techniques are used. The objective of spectral efficient modulation is to maximize the bandwidth efficiency. It is also desirable to achieve the bandwidth efficiency at a prescribed BER with minimum transmitted power. 8

9 Shrikant K. Bodlie : Studies on Microcellular. Hand-off Algorithms. Among the many digital modulation techniques in use, some are responsive primarily to the goal of spectral efficiency. Others focus on the objectives of achieving a narrow power spectral density. The large variation in signal amplitude due to Rayleigh fading encountered, render the digital amplitude modulation schemes almost inoperative. There are two broad modulation strategies emerging which emphasize the use of PSK or MSK derived modulation schemes. Table shows the comparison of bandwidth efficiency and the required S/N (for BER of 10 6) for PSK and MSK modulation systems. Table - 1.1: Comparison of Modulation Systems [3] Modulation Bandwidth Efficiency Required S/N BPSK 1b/s/Hz 11.1 db. QPSK 2b / s / Hz 14.0 db PSK (16 level) 4b / s / Hz 26.0 db MSK (2 level) lb / s/hz db MSK (4 level) 2b / s / Hz db TABLE Bandwidth Efficiency of Digital Cellular and Cordless Telephone Systems. [3] System Modulation Channel BW Data Rate Bandwidth Efficiency JDC p/4 QPSK 25 KHz 42.0 Kbps 1.68 b/s/hz ADC p/4 QPSK 30 KHz 48.6 Kbps 1.62 b/s/hz GSM GMSK 200 KHz Kbps 1.35 b/s/hz CT-2 GMSK 100 KHz 72.0 Kbps 0.72 b/s/hz DECT GMSK 1728 KHz 1572 Kbps 0.67 b/s/hz 9

10 Shrikant K. Bodhe : Studies on...microoellular... Hand-off Algorithms. Table summarizes various modulation schemes adopted in second generation cellular and cordless telephone systems. It is observed that though linear modulation schemes offer better bandwidth efficiency, GMSK is also considered as a promising modulation scheme Coding Techniques. Coding Techniques allow reduction in the amount of information which must be transmitted on the communication channel, typically by removing redundant information, thereby reducing the overall bandwidth. Figure 1.2 shows trends in digital voice coding techniques. SPEECH CODING TRENDS [4] Line A represents the bit rate reduction for toll quality speech as used over telephone networks, and line B shows rates necessary for 10

11 Shrikant K. Bodlie : Studies on...microcellular... Hand-off Algorithms. communication quality coders which produce slightly degraded speech for applications such as dispatch. In many cases, these speech coding systems have initially increased the occupied bandwidth due to relatively unsophisticated coding schemes, while noticeably degrading the audio quality. More recently, advanced digital signal processors, have contributed to the development of improved speech coding algorithms. For example, the coder proposed for US digital cellular (8.0 Kbps VSELP shown in the Figure 1.2) provides near toll quality audio with a bandwidth comparable to analog speech[4]. The mobile radio channel is subjected to severe signal degradation due to shadowing and multi-path fading. This makes the recovery and regeneration of the source information difficult, complex fading mitigation techniques such as error correcting codes and diversity can be used effectively. The carrier modulation and coding are logically independent processes but are strongly interrelated. The improvement in either of them is towards achievement of a common goal of a higher bandwidth efficiency. Higher level modulation offers higher bandwidth efficiency but higher SNR is required to achieve a given BER objective which is difficult to achieve in mobile environments. The required S/N can be reduced by using low bit rate voice coding and efficient error correction techniques prior to modulation. The criteria for selection of a coding technique is simply to achieve the desired voice quality by adding minimum overhead while protecting the information to fight the channel noise. This strategy is assisted by retaining those bits within a message which decide the speech quality, separately from those which are less critical. LPC-RPE speech coder used in GSM cellular system operates at 13 Kbps data rate. The most sensitive bits are protected by a CRC code with rate 1/2 convolutional code with constraint length 5. The overall coded bit rate per speech signal is 22.8 Kbps. The speech coding technique adopted for the American Digital Cellular (ADC) system is VSELP operating at 7.95 Kbps. The channel coding used is convolutional code with constraint length of 6. The overall coded bit rate is 13 Kbps only. ll

12 Shrikant K. Bodhe : Studies on... Microcellular... Hand-off Algorithms. Traditional LPC coders are capable of handling very low bit rates ( Kbps ), but tend to be extremely vulnerable to errors. RELP coder operating at 8-16 Kbps and sub-band coders operating at 16 Kbps have also proved to be fairly robust. The implementation of these complex recovery techniques can be achieved using DSP ICs. 1.5 MULTIPLE ACCESS TECHNIQUES. Ever since the analog cellular system started to face its capacity limitation in 1987, vigorous efforts for development of digital cellular systems for increasing capacity have been made. Multiple access is an indispensable technique for efficient radio spectrum utilization in mobile radio systems because a large number of subscribers are connected only when they require a radio channel. A multiple access protocol is needed for both the up-link and downlink channels. There are many kinds of multiple access protocols that can be divided into three groups. The first group is frequency division multiple access (FDMA), where the total band is divided in to narrow frequency sub-bands, and each of these channels is allocated to one and only one user for the period of communication. Existing radio telephones use FDMA, and two narrow band channels are used by each mobile for up-link and down-link transmissions. In the second group of protocols called time division multiple access (TDMA) all the users have access to the total band, and simultaneous communications are possible when the users transmit during different time slots. Such techniques are widely used in wire packet switching networks, and they have already been applied to some mobile systems. The third group of multiple access protocol is called code division multiple access (CDMA) where the transmitters simultaneously use all the available frequency bands. In order to obtain distinguishable radio signals, the users must have different possibilities of encoding digital information into modulated wave- 12

13 Shrikant K. Bodlie : Studies on...microcellular... Hand-off Algorithms. forms. In theory it does not matter whether the spectrum is divided into frequencies, time slots or codes, the capacity provided from these three multiple access schemes is the same. However, in cellular system, one may prove better than the other FDMA. In radio communication if there are multiple users then the frequency assigned to individual users is different from each other with guard band between the two. For cellular mobile communication the concept is to use the same frequency band in different geographical areas, then the available bandwidth is divided amongst the number of cells in a cluster. This type of usage of radio channels is similar to the concept used in satellite communication called FDMA. The concept of FDMA scheme in mobile communication is realized by assigning single channel per carrier (SCPC). For high capacity SCPC-FDMA scheme, the cost of base station increases with complex channel control unit. A highly stabilized oscillator is required to achieve the efficient spectrum utilization and the increase in number of carriers results in increased interference TDMA. In satellite communication a new multiple access scheme was proposed to avoid the problem of intermodulation of FDMA. In this scheme, instead of assigning a frequency band to each up-link, specific time intervals are assigned to the station and the station transmits only during its allotted interval. This type of operation was referred to as time division multiple access (TDMA). To accommodate many users, TDMA time intervals must necessarily be short, requiring burst-type transmissions and the time interval of all users must be properly and accurately synchronized, requiring several levels of timing control. Experimental TDMA systems were first reported in These early systems proved that time-interleaved, short-interval communications were in fact technically feasible. Later systems established that advanced operational TDMA 13

14 Shrikant K, Bodhe : Studies on...microeellular... Hand-off Algorithms. concepts, such as high accuracy, fast acquisition, synchronization and high capacity data formats were possible and advantagous in satellite communication. When the analog systems started facing the crunch of capacity the researchers proposed the well established concept of TDMA in satellite communication for the mobile communication systems. TDMA showed the potential of reducing the cost. It also has another main advantage over FDMA, that it is capable of handling hand-offs more quickly and with higher accuracy. This is because of the possibility of using idle time frames in the mobile to make measurements on the radio channel. This feature makes it considerably easier to handle extremely small cells than in analog and digital FDMA systems. TDMA can theoretically be expected to achieve subscriber densities about ten times the current systems. Comparisons between analog systems with different channel spacing (30, 25, 12.5 KHz) have shown that in critical situations the narrower spacing does not improve spectrum efficiency. The increased number of channels is counteracted by a higher number of required channel groups. This statement is valid for areawide homogeneous systems. For certain cases with high peak geographical demand the increased number of channels gives more freedom in frequency management, which is a definite advantage. Table 1.3 shows the comparison of various multiple access systems with conventional FDMA. TABLE -1.3 Comparison of Multiple Access Schemes. Digital Narrowband Wideband TDMA FDMA TDMA Capacity Improvement Cost A conclusion from the table 1.3 is that a TDMA system is the natural choice for capacity enhancement. 14

15 Shrikant K. Bodlie: Studio on.microcellular... Hand-off Algorithms CDMA. In 1977, Cooper and Nettelton proposed a spread-spectrum mobile radio system using frequency-hopped multiple access, Hadamard coding for error correction and differential phase shift keyed modulation and claimed higher spectral efficiency than FD-FM systems. Goodman et.al. suggested an alternative modulation scheme in 1980, using multilevel frequency shift keying (MFSK) and achieved a 30% higher efficiency. Spreading techniques in modulation are generally used in military systems for anti-jamming purpose. In spread-spectrum two general techniques used are direct sequence (DS) and frequency hopping (FH). The first DS experiment was carried out in 1949 by DeRosa and Rogoff, who established a link between New Jersey and California. In DS, each information bit is symbolized by a large number of coded bits called chips. The spectrum spreading in DS is measured by the processing gain (PG) in decibels and is given by the formula PG = 10 log (Bss/ B) db. Bss is total spread bandwidth and B the information bandwidth. An FH receiver would equip N frequency channels for an active call to hop over those N frequencies with a determined hopping pattern. The total hopping frequency channels are called chips. There are two basic hopping patterns; one called fast hopping which makes two or more hops for each symbol. The other called slow hopping which makes two or more symbols for each hop. In FH the PG = 10 log N db. The use of signals all occupying the same frequency band at the same time results in a number of operational characteristics that can be very advantagous in a mobile communication system. The advantages are increased user density, reduction in the degradation in performance that normally arises from rapid fading, no hard limit on the number of active users that can be handled simultaneously by the system. Since each potential user of the system is assigned a unique signal set, message privacy is achieved as a fringe benefit. Besides each user retains his unique signal set permanently and there is no channel switching or address changes as the user moves from cell to cell. Hence, the particularly objectionable characteristic of FM systems known as forced termination, which 15

16 Shrikant K. Bodhe : Studies on... Microcellular... Hand-off Algorithms. occurs when a mobile crosses a boundary into a cell in which no channel is available, will not occur in this system. The desirable characteristics noted above cannot be achieved without cost. Some of the disadvantages of spread-spectrum approach are the requirement of effective power control, more complex message encoding and decoding. The spread-spectrum approach does not appear to be particularly attractive for large-cell systems from the stand-point of cost effectiveness. When the number of active users exceeds the design value, the result is a degradation of performance for all users rather than denial of access. This is usually referred to as graceful degradation. Widespread use of CDMA for personal wireless communication is likely to occur over the next several years. CDMA field trials are currently underway by major cellular and personal communications companies using existing cellular spectrum in the MHz and GHz bands. 1.6 CONCLUDING REMARKS. Ever since the inception of Cellular Mobile Communication Systems (CMCS), the demand for the higher spectrum allocation never ended. The trend of development was to improve the service provided to the user. The change in the technology is towards a common global standard for the CMCS. As CMCS target to reach any-time any-where type of wireless communication system, micro-cellular concept was proposed several years ago as a means of increasing the capacity. However, due to the ever increasing demand by the users, operators are still facing the capacity crunch. As suggested by Prof. Dr. R. Steel [1,2] and Dr. W. C. Y. Lee [279] the capacity demand can be very well fulfilled by migrating to GHz frequency band. 16

17 Shrikant K. Bodhe : Studies on...microcellular...hand-off Algorithms. 1.7 ORGANIZATION OF THE THESIS Chapter 2 takes a review of literature related to digital communication techniques used in CMCS, capacity of CMCS, hand-off process and propagation models. Chapter 3 defines the problem of the present work. In chapter 4 the development of propagation model is carried out for generating data of RSS for hand-off performance evaluation. Chapter 5 analyses a CMCS for estimating the capacity of a CMCS operating in GHz frequency band. Chapter 6 introduces the concepts of tele traffic modelling required for hand-off analysis. Chapter 7 gives a brief introduction to the hand-off process in CMCS. In Chapter 8 model for hand-off analysis is developed using three RSS based handoff algorithms for the CMCS working in the GHz band. The discussion about the present work is contained in chapter 9. Chapter 10 deal with the conclusion of the present work. The simulation will be carried out using MATLAB. 17