Mobile Communications-1

A Paper Presentation On Mobile Communications Submitted by : Renuka. V.T. Reg. No: 3PG01EC044 PDIT College, Hospet, Bellary (Dist), Karnataka. e-mail : [email protected]

INTRODUCTION What will computers look like in ten years in the next Century? Most computers will certainly be portable. How will users access networks with the help of computers or other communication devices? Definitely by wireless. How will people spend much of their time at work, during vacation? Many people will be ‘Mobile’ – One of the key characteristics of today’s society. Modern aircraft already offers limited network access to passengers and aircraft of the next generation will offer easy Internet access. Furthermore, think of cars with Internet access and billions of embedded processors that have to communicate with for instance cameras, mobile phones, CD-players, headsets, keyboards, intelligent traffic signs and sensors. The trends mentioned above create an ever-lasting demand for well – educated communication engineers who understand the developments & possibilities of mobile communication. The future will see more and more mobile devices, merging of classical voice and data transmission technologies and extension of today’s internet application on the mobile and wireless devices. New application and new mobile network will bring ubiquitous computing to the mass market. A Short History of Wireless Communication: For a better understanding of today’s wireless systems and development, a short history of it is essential. This cannot cover all inventions but highlights that have contributed fundamentally to today’s systems. The use of Smoke and light signals for communication is mentioned by Polybius, Greece, as early as 150 BC. Wired communication started with the I commercial telegraph line between Washington & Baltimore in 1843 and invention and marketing of the telephone in 1876. The success of wireless communication is certainly with Marconi who gave the first demonstration of wireless telegraphy in 1895 . The first radio broadcast took place in 1906. In 1915 the first wireless voice was set up between New York and San Francisco just commercial Radio Station Started in 1920. One of the I mobile transmitters was on board a Zeppelin in 1911. As early as 1926 the first TP in a train was available on Berlin – Hamburg line. The first radio for cars was commercially available in 1927 indeed in 1922 . 1928 was the year of many field trials for TV broadcasting. In 1983 the us STD advanced mobile phone S/m (AMPS) at 850 mHz. Telephones (TP) at home went wireless with the std wireless TP in 1984. The early 1990’s marked the beginning of fully digital systems. In 1991 ESTI adopted the std digital European Cordless Telephone (DECT) for digital cordless telephony (ETSI, 1998) GSM was standardized in a document of more than 5,000 pages in 1991 although

coming later than HIPERLAN in 1997 the IEEE std 802.11 looks like a winner for LAN. 1998 finally marked the beginning of mobile communication using Satellite with Iridium S/m. Iridium marks the beginning of small and truly portable mobile satellite TP including data service. Basically there are two different kinds of mobility user mobility and device

portability. User mobility Ex call forwarding solution known from the telephone or computer desktops supporting roaming. Device portability, Ex is mobile phone system. A Communication device can thus exhibit one of the following characteristics. •

Fixed & Wired : Desktop Computer in an office

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Mobile & Wired : Loptops users carry the laptop from one hotel to the next, reconnecting to the company’s network via the TP network and a modem.

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Fixed & wireless : In historical buildings, installing network to avoid damage by installing wires

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Mobile & wireless : No cable restricts the user, who can roam between different wireless network.

Mobile Communication Basics. (Signals, Antennas and signal propagation) Radio transmission starts at several KHz, the very low freq range (VLF). There are very long waves. Waves in low freq range are used by submarines. The medium freq (MF) & high freq (HP) ranges are typical for transmission of hundreds of radio stations either as AM or FM. Short wave are used for radio transmission around the world enabled by rejection at the ionosphere conventional TV is transmitted in the range of VHF and UHF bands. VHF & UHF allow for small antennas and relatively reliable connections for mobile telephony. Super high frequency (SHF) are used for directed µ wave links and fixed satellite services in C-band, Ka band, extremely high frequency (EHF) are used by some other systems. The next step into higher frequencies involves optical transmission which is not used only for fibre optic links but also for wireless communication. The most widespread IR technology infrared data associations (IDA) uses wavelengths of 850 – 900nm to connect laptops PDAS etc.

Signals: Signals are the physical representation of data. If users of a communication system want to exchange data, this is only possible through the transmission of signals. Signals are functions of time and location. Signal parameters represent the data values. The most interesting types of sgs for radio transmission are periodic signals, sine waves as carriers. A typical way to represent signals is in the time domain here amplitude of a signal is shown verses time. Representations in time domain are problematic if a signal consists of many different frequencies. Better representation of a signal is the frequency domain, here the amplitude of a certain frequency part of the signal is shown verses the frequency. A third way to represent signals is the phase domain. This representation also called phase state diagram shows the amplitude M of a sign and its phase in polar co-ordinates. Antennas: Coupling the energy from the transmitter to the outside world and in reverse, from the outside world to the receiver. This is exactly what antenna’s do. Antennas couple Electromagnetic energy to and from space to from a wire or co-axial cable. Real antennas all exhibit directive effects i.e., intensity of radiation is not same in all directions from the antenna. The simplest real antenna is a thin centre fed dipole, also called Hertzian dipole. If an antenna is positioned eg in a valley or between buildings, an omnidirectional radiation pattern is not very useful. directional atennas with certain fixed preferential transmission and reception directions can be used. Directed antenas are typically applied in cellular systems.

A more advanced solution is provided by smart antenna’s which

combine multiple antenna elements with signal processing to optimize the rad / reception pattern in response to the slg environment. Signal Propagation:Wireless communication networks also have senders and receivers of signals. In wireless network the signal has no wire to determine the direction of propagation. For wireless transmission the predictable behaviour is only valid is a vacuum having certain parameters like Transmission range, Detection range, interference range etc. This simple and ideal scheme led to the notion of cells around a transmitter. Radio transmission has to contend with out atmosphere, mountains, buildings, moving senders and receivers etc.

Multiplexing:Multiplexing describes how several users can share a medium with minimum or no interference. For wireless communication, multiplexing can be carried out in four dimensions: Space, time, frequency and code. The task of multiplexing is to allign space, time, frequency and code to each communication channel with a minimum of interference and a max of medium utilization. The term communication channel here only refers to an association of senders and receivers that went to exchange data. The code division multiplexing is well suited for present wireless transmission at it is good protection against interference and tapping. Different codes have to be alligned, but code space is huge compared to the frequency space and thus assigning individual codes to each sender typically does not cause any problems. Modulation: Digital modulation is required if digital data has to be transmitted over a medium that only allows for analog transmission. In wireless network the binary bit stream has to be translated into an analog signal first. The three basic translation methods are amplitude shift keying, frequency shift keying and phase shift keying. Apart from the translation of digital data into analog signals wireless transmission requires an additional modulation, an analog modulation, that shifts the centre frequency of the base band signal generated by the digital modulation up to the radio carries. Spread Spectrum:These techniques involve spreading the bandwidth needed to transmit data. Spreading of a narrow band signal is achieved using a special code. Each channel is allotted its own code, which the receivers have to apply to recover the signal without knowing the code the signal cannot be recovered and behaves like background noise. Apart from military uses, the combination of spread spectrum and CDM is becoming more and more attractive for everyday applications. Direct sequence and frequency hopping are the two methods of spread spectrum commonly used. Spread spectrum make a transmission more robust against narrowband interference as the signal is spread over a

larger bandwidth and thus narrowband interference only influences a small fraction of signal. Cellular Systems: Cellular systems for mobile communications implement space division multiplexing. Each transmitter, typically called a base station, covers a certain area, a cell. Cell radio can very from tens of metres in buildings and hundreds of metres in cities, up to tens of kilometers in the country side. The shape of cells are never perfect circles or hexagons but depend on the environment, on weather conditions and sometimes even on system load. Typical systems using this approach are mobile communications systems. Where a mobile stations within a cell around a base station communicates with this base station and vice versa. Advantages of cellular systems: •

Higher capacity: Implementing SDM allows frequency reuse. If one transmitter is far away from another, outside the interference range, it can reuse the same frequencies. As most mobile phone systems assign frequencies to certain users, this frequency is blocked for other users. But frequencies are a scarce resource and thus, the no of concurrent users per cell is very limited. Huge cells do not allow for more users.

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Less transmission Power: While power aspects are not a big problem for base stations, they are indeed problematic for mobile stations. A receiver far way from a base station would need much more transmit power than the current few watts.

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Local Interference Only: With small cells, Mobiles stations and base stations only have the deal with local interference.

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Robustness : Cellular systems are decentralized and thus more robust against failures of single components. If one antenna fails, this defect only influences communication within a small area. Two possible models to create cell patterns with minimal interference, cells are

combined in clusters- one the left three cells form a cluster, on the right side seven cells form a cluster. All cells within a cluster uses set F1 another cell F2 and the third cell F3. To reduce interference even further sectorized antennas can be used cells with more traffic

are dynamically allotted more frequencies. This scheme is known as borrowing channel allocated (BCA) Medium Access Control:-

Medium access control comprises all mechanisms that

regulates user access to a medium using SDM, TDM, FDM or CDM. MAC is thus similar to traffic regulations in the highway. MAC belongs to layer 2, the data link control layer (DLC). Layer 2 is subdivided into the logical link control (LLC) layer 2b, and the MAC layer 2a. the task of DLC is to establish a reliable pt to pt or pt or multi pt connection between different devices over a wired or wireless medium. Following are the basic MAC mechanisms. 1. Motivation for a specialized MAC a. Hidden and exposed terminals. b. Near and far terminals. 2. SDMA 3. FDMA 4. TDMA-

Fined TDM Classical Aloha Slotted Aloha CSMA Demand assigned multiple access PRMA Reservation TDMA Multiple access with collision avoidance Polling. Inhibit Sense Multiple access 5. CDMA- Spread Aloha Multiple access. 1. Motivation for a specialized aloha: The strength of a signal decreases proportionally to the square of the distance to the sender. The CSMA / CD Mac scheme from wired network fails in Wireless network. This can be shown by considering Hidden and exposed terminals and Near and far terminals. .

a) Hidden and exposed terminals. Consider a scenario with 3 mobile phones. The transmission range of A reaches B, but not C. The transmission range of C reaches B but not A. The transmission range of B reaches A and C i.e., A cannot detect C and vice versa.

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A starts sending to B, C does not receive this transmission. C also wants to send something to B and senses the medium. The medium appears to be free, the carrier sense fails. Thus A is hidden for C and vice-versa. While hidden terminals cause collision, the next effect only causes unnecessary delay. Now consider the situation that B sends something to A and C wants to transmit data to some other mobile phone outside the interference ranges of A and B. C senses the carrier and detects that the carrier is busy. Thus, C postpones its transmission. But as A is outside the interference range of C waiting is not necessary causing a ‘collision’ at B does not matter because the collision is too weak to propagate to A. In this situation C is exposed to B . b)

Near and Far Terminals: Consider the situation as shown above A and B are both sending with same

transmission power. As the signal strength decreases proportionally to the square of the distance B’s signal draws the A’s signal. As a result, C cannot receive A’s transmission.

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The near / far effect is a severe problem of wireless network using CDM. All signals should arrive at the receiver with more or less some strength. Precise power control is needed to receive all senders with the same strength at the Rx. 2. SDMA : Space Division Multiple Access:SDMA is used for allocating a seperate space to users in wireless Networks. A typical application involves assigning an optimal base station to a mobile phone user. The mobile phone may receive several base stations with different quality SDMA is never used in isolation but always in combination with one or more or other schemes. The basis for the SDMA algorithm is formed by cells and sectorized antennas which constitute the infrastructure space division multiplexing.

3. FDMA: Frequency division Multiple Access:FDMA comprises all algorithms allocating frequencies to transmission channels according to the frequency division multiplexing scheme channels can be assigned to the same frequency at all time i.e., pure FDM or FDMA combined with TDMA. The latter example is the common practice for many wireless systems to circumvent narrow band interference at certain frequencies, known as frequency hopping sender and receiver have to agree on a hopping pattern, otherwise the receiver could not tune to the right frequency.

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4. TDMA: Time division Multiple access:TDMA offers much more flexible scheme, which comprises all technologies that allocates certain time slots for communication i.e controlling TDMA. Demand Assigned Multiple Access with explicit reservation

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A) Fixed TDM: The simplest algorithm for using TDM is allocating time slots for channels in the fixed pattern. This results in a fixed bandwidth and is the typical solution for wireless phone systems. MAC is quite simple, as the only crucial factor concerns accessing the reserved time slot at the right moment. If this is assured, each mobile station knows its turn and no

interference will happen. The fixed pattern can be assigned to the base station, where competition between different mobile stations that want to access the medium is solved.

B) Classical Aloha: What happens if TDM is applied without controlling access? This is exactly what the classical Aloha scheme does, a scheme which was invented at the university of Hawai and was used in the ALOHANET for wireless connection of several stations. Each station can access the medium at one time. This is random access scheme, without a central arbiter controlling access the medium and without co-ordination among the stations. If two or more stations access the medium at the same time, a collision occurs and the transmitted data is destroyed. C) Slotted ALOHA: In this case, all senders have to be synchronized, transmission can only start at the begin of a time slot. still access is not co-ordinated. Under the assumption stated above the introduction of slots raises throughput from 18% to 36% i.e slotted doubles the through put.

D) Carrier Sense Multiple Access: This is improvement on basic Aloha i.e CSMA. Sensing the carrier and accessing the medium only if the carrier is idle decreases the probability of a collision. If a hidden terminals transmit at the same time as another sender a collision might occur at the receiver. Several versions of CSMA exist. They are non persistent,1-persistent and Ppersistent CSMA. In non-persistent CSMA, stations sense the carrier and start sending immediately if the medium is idle. If the medium is busy the station pauses a random amount of time before sensing the medium again and repeating this pattern. In P-persistent CSMA system nodes also sense the medium, but only transmit with a probability of P. with the station deferring to the next slot with the probability 1-P, i.e access is slotted in addition. In 1 persistent CSMA system, all stations wishing to transmit access the medium at the same time, as soon as it becomes idle. To create some fairness for stations waiting for a longer time, back off algorithms can be introduced, which are sensitive to waiting time as this is done for standard Ethernet. E) Demand Assigned Multiple Access: These schemes typically have a reservation period followed by a transmission period. During the reservation period, stations can reserve further slots in the transmission period. While, depending on the scheme, collisions may occur during the reservation

period, the transmission period can be accessed without collision. These schemes cause higher delay under a light load, but allow higher throughput. Demand Assigned Multiple Access with explicit reservation C

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F) PRMA: Packet reservation multiple access: An implicit reservation scheme is packet reservation multiple access here slots can be reserved implicitly. A certain member of slots forms a frame. The frame is repeated in time is a fixed TDM pattern is applied. R

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G) Reservation TDMA Multiple access with collision avoidance (MACA) presents a simple scheme that solves the hidden problem. Does not need a base station and is still a random access Aloha scheme but with dynamic reservation. Remember A and C both want to send to B A has already started the transmission, but is hidden for C, C starts with its transmission, thereby causing a collision at B. with MACA, A does not start its transmission at once, but sends a request to send (RTS) first. B receives the RTS that contains the name of sender and receiver, as well as the length of the future transmission. This RTS is not heard by C and N

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triggers an acknowledgment from B called clear the send (CTS). The CTS again contains the names of sender and receiver and the length of the future transmission. The CTS is now heard by C and the medium for future use by A is now reserved for the duration of the transmission. After receiving a CTS, C is not allowed to send anything for the duration indicated in the CTS towards B. Thus a collision

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transmission and hidden terminal problem is solved provided that the transmission conditions remain same.

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Similarly exposed terminal problem can also be solved by using MACA.

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H) Polling: Polling is a strictly centralized A scheme withB one master station C and several slave stations. The master can poll the slaves according to many schemes: round robin, randomly, according to reservations etc. The master could also establish a list of stations wishing to transmit during a contention phase. After this pahse, the station polls each station on the list. I) Inhibit sense multiple access: A scheme which is used for packet data transmission service cellular digital packet data (CDPD) in the AMPS mobile phone system also known as digital sense multiple access(DSMA). Here, the base station only signals a busy a medium via a busy tone on the downlink. After the busy tone stops, accessing the uplink is not co-ordinated any further. The base station acknowledges successful transmissions, a mobile station detects a collision only via the missing +ve acknowledgement. In the case of collisions, additional back-off and retransmission mechanisms are implemented.

5. DMA: Code Division Multiple Access: CDMA System use exactly these codes to separate different users in code space and to enable access to a shared medium without interference.  A sender “A” want to send data CDMA assigns the following key sequences: Key Ak = 010011, Sender A Wants to send the bit Ad = 1. Let us assume that we code a binary 0 as – 1, a binary 1 as +1. We can then apply the standard addition and multiplication rules.  Sender spreads the signal using its key as chipping sequence. Sender then sends the signal as = Ad * Ak = +1 * (-1, +1, -1, -1, +1, +1) = (-1, +1, -1, -1, +1, +1).  Signal is then transmitted. The signal C is received at a receiver  The receiver now wants to receive data from sender A and, therefore tunes in to the code A, i.e As applies code for despreading : C*Ak = (-2,0,0,-2,+2,0)*(-1,+1,-1,-1, +1,+1) = (2+0+0+2+2+0)=6. As the result is larger than 0. the receiver detects a binary 1. a) Spread Aloha Multiple Access: This Scheme is a Combination of CDMA and Medium access of Aloha and also TDMA SAMA works as follows.

Each sender uses the same spreading code. The std case for Aloha access is shown in the upper part of the figure. S

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APPLICATIONS Particular application are listed below: 1. Vehicles: Cars can build a local ad hoc network for fast information exchange and maintain safe distance and speed and avoid accidents. In case of accidents occurring emergency procedures may be speeded up 2. Emergencies: Emergencies like accidents, heart attacks fires can be better dealt by advance exchange of information. 3. Business: Today’s typical travelling salesman can access databases at distance places also. 4. Replacement of wired Networks: wireless networks can also be used to replace wired networks. 5. Infotainment and more: Wireless networks can provide up-to-date information eg., travel guide, concert and games. 6. Location dependent Services:  Follow on Services  Location Aware Services:  Privacy:  Information Services:  Support Services: 7. Mobile and Wireless devices: Currently, laptops are considered to be the upper end of the mobile device range. Some of the mobile and wireless devices graded by increasing performance are as below:

 Sensor Embedded Controllers keyboards, mice, headsets, washing machines, coffee machines, hair dryers.  Pager  Mobile Phones  Personal Digital Assistent  Palmtop / Pocket Computer  Note Book / Laptop