Wireless Communications ECE 442
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ECE442 Wireless Communications Lecture 8 Random access Radio Protocols Prof Sudharman K Jayaweera Department of Electrical and Computer Engineering University of New Mexico Medium Sharing Techniques for Multiple access 6 Medium sharing can be classified into two methods a static channelization 0 channel assignment is done in a pre specified way and does not change with time 0 dynamic channelization 0 channel assignment is allocated as needed and thus the allocation changes with time 0 dynamic allocation can be divided also into two methods called scheduled and randomaccess CIaSSIflcatIo of Medium Sharln Techniques Mulnplmum pruluculs Con ictfree Contentionbased Figure 63 Classification of multiple access protocols Random access Protocols 0 Fixed assignment channel access methods a each user is given a share of the channel resources eg a frequency band a timeslot or a code through which to transmit a eg FDMA TDMA and CDMA 9 Make relatively efficient use of radio resources when there is a steady flow of information from the source eg voice a data file a fax 0 However for sources generating short messages at random times this is inefficient o Random access protocols also called the packet radio are of interest in such cases lSOOSI Reference Model Communication system management is organized in layers of responsibility A seven layer reference model has been introduced and widely used for computer networks and is called the International Standards OrganizationOpen Systems Interconnection lSOOSI reference model The lowest three layers physical data link and network layers in this model can be considered as the communication subnetwork The physical layer lowest layer refers to the transmission of data through the physical medium ie via modulationdemodulation ISOOSI Reference Model I Application I Layer 7 I Presentation I Layer 6 I Session I Layer 5 I Transport I Layer 4 I Network I Layer 3 I Data link I Layer 2 I Physical I Layer 1 I l The MAC Sub layer and Random access o The next layer up in the lSOOSI reference model is the data link layer It is responsible for o establishing and maintaining connections a error contro o mediumaccess control MAC 9 Multipleaccess schemes are MAC protocols o The MAC is a sub layer inside the data link layer a The multiple access protocols MAC protocols can be classified into two categories see earlier figure 0 conflictfree o contentionbased o In random access systems a data sequence from a digital source is broken down into smaller pieces which are organized into so called data packets o A data packet is a series of digital symbols with a structure something like below exact details depend on the particular system l TAIL l ERROR CONTROL DATA PAYLOAD ID I HEADER l a To transmit over the wireless channel a data packet may be embedded in a radio packet which also contains further symbols aiding in its demodulation such as synch bits training symbols etc What s in a Data Packet l TAIL l ERROR CONTROL DATA PAYLOAD IID IHEADER l HEADER a sequence that contains information about the source and destination of the packet ID identifies the packet as an element of a group of packets and specifies its place in the order of these packets eg packet 4 of 64 packets DATA PAYLOAD a piece of the source data sequence to be transmitted ERROR CONTROL a sequence of symbols used to determine whether there are errors in the packet cyclic redundancy check CRC TAIL a sequence indicating that the packet is ending only in some cases I l Packet Switching and Routing a In large data networks eg the Internet packets are switched through the network from source to a destination by routers at the nodes ie the points connecting the links of the network 0 You may think of this as a kind of a postal system 0 data packets are like letters a links are like postal routes and transportation routes between major CltleS 9 nodes are like post offices 0 end devices are like mailboxes o Isn39t wireless communications so easy Random Access Protocols Packets are transmitted to a destination through a shared wireless network without any explicit channel assignment they can also be switched through a backbone network Subscribers attempt to access a shared radio channel to transmit packets to a common receiver eg a base station in a minimally coordinated fashion If the packet is correctly received as assessed by the CRC an ACK acknowledgement identifying the received packet is broadcasted back to the subscribers If the receiver detects a collision of two packets or otherwise erroneous reception it broadcasts a NACK a negative acknowledgement o The transmitter must then re send the packet When all packets arrive at the destination safely the payloads are reassembled into the original data sequence I l Contention Protocols Since the channel is shared and users enter and exit channel randomly protocols are needed to ensure the fair and orderly transfer of data A protocol dictates the manner in which packets can be sent originally and how they should be re sent if a NACK is received Such schemes are referred to as contention techniques or contention protocols Two well known basic contention protocols o ALOHA a Carriersense Multiple Access CSMA Key parameters that determine the performance of a contention protocol are 0 Throughput the average number of packets successfully transmitted per unit time 0 Delay the average delay experienced by a typical packet I l LOHA 6 Aloha protocol was developed at the University of Hawaii for bursty low data rate radio communication between the central computer and other data terminals in 197039s 0 Since then there have been various improved versions of the basic algorithm 0 Original algorithm is usually referred to as the pure Aloha algorithm Pure ALOHA o a user transmits as soon as a packet is ready to go o ifa collision occurs ie a NACK received the transmitter waits a random period of time and then retransmits a simple but throughput is low Throughput of ALO HA 0 Suppose that there are an infinite number of users and each user generates packets according to a Poisson process with arrival rate of packatessec o All packets are assumed to have the same fixed length of T sec 9 lfyou consider a particular time instant 1 there are not only new packet arrivals but also those packets that were unsuccessful in getting through and thus are ready to be re transmitted The combination of new and backlogged packets make the scheduling rate g in packetssec to be such that g gt o The scheduling packet process is very difficult 0 model exactly but if the retransmissions from backlogged nodes are random enough then we can approximate the combined process to be also a Poisson process with rate g packetssec 0 Then we can define the normalized offered load G as G gT packets l l Throughput of ALO HA MS 1 packet Time a A packet that is trying to be transmitted at time 1 will be successful if there are no other packets scheduled to be transmitted in the interval from t 7 T7 1 l T Hence the packet success probability is PSUC Prn0 packets in a time interval of 2T 9 ng 0 Hence the normalized throughput of pure ALOHA is TIALOHA GPsuc gTe ZgT Ge ZG packets l l Maximum Throughput of Pure ALOHA o It can be easily shown that the above throughput is maximized if the normalized offered load G 05 and the resulting maximum throughput is 1 TIALOHAmx 27 x 0184 packets e 0 Clearly the throughput of pure ALOHA is not that great We can certainly improve the basic algorithm in several ways Improved Versions of ALOHA Protocol 0 Slotted ALOHA 0 Time axis is discretized into slots The slot size is equal to the packet interval T o Transmissions are allowed only at the beginning of a slot 0 The maximum throughput is doubled compared to pure Aloha o Reservation ALOHA o A transmitter with a long file can reserve slots 0 These schemes may improve throughput at the expense of reduced flexibility andor increased delay Throughput of Slotted ALO HA MS 1 packet Wui for n random lime MSs 2 amp 3 packers l E Collision Retransmission Time o Slotted transmissions help reduce the possible packet collision period from 2T to T compared to pure ALOHA 9 Assuming that the combination of new and backlogged packets is a Poisson process with scheduling rate g in packetssec the packet success probability is Psuc Prn0 packets in a time interval of T e gT I l Throughput and Maximum Throughput of Slotted ALOHA o The normalized throughput of slotted ALOHA is nsiALOHA GPsuc gTe gT Ge G packets where normalized load is G gT o It can be shown that the above throughput is maximized if the normalized offered load is G 1 and the resulting maximum throughput is 1 UsiALOHAJnaX g x 0368 packets hroughput of ALOHA vs Slotted ALOHA 7 T 05 0 4 gt inininin 9368 i 03 7 a 4 Slotted ALOHA J g 02 0184 E i 3 0 1 ALOHA NR NM 00 2 4 6 Traf c load G ALOHA in Work Ericsson MOBITEX System 0 a low data rate data only cellular system 0 dispatch PDAs eg PalmVll etc 0 random access protocol is based on reservation slotted ALOHA Carrier Sense Multiple Access CSMA o Transmitter listens to see if the channel is idle ie no carrier is detected a Only if the channel is found to be idle then transmits according to a fixed protocol 0 Collisions can still occur because of simultaneous transmissions and also because of transmission delay 0 As with Aloha different variations of the basic CSMA algorithm have been proposed and implemented CSMA Protocols 0 1 persistent CSMA a packet is transmitted as soon as the channel is idle 9 Non persistent CSMA a NACK39ed packets are retransmitted only after a random amount of time 6 CSMA with collision detection CSMACD o Transmitter listens while transmitting to see if anyone else is also transmitting listen while tal a Ifan existing transmission is detected transmission is aborted immediately CSMA in Work 0 Ethernet 0 uses CSMACD 9 lEEE 80211 Wireless LAN39s u uses CSMACA collision avoidance 9 Cellular Digital Packet Data CDPD 0 packet service over idle AMPS channels 0 uses a form of CSMACD called digital sense multipleaccess DSMA Higher Layers and Networking Issues 0 MANs WANs and PAN39s have higher order layers to handle routing through the network end to end verification applications etc 0 Some examples of higher level protocols are a Internet Protocol IP 9 Transmission Control Protocol TCP 0 Wireless Application Protocol WAP TCPIP o The TCPIP protocol suit has been defined to provide transfer of data from one network device to another over the Internet It consists of only five layers a The lowest four levels correspond to the those of the lSOOSI model physical data link network and transport 0 The top three layers of the OSI model is collected into one layer named application layer 0 The TCPIP consists of independent protocols that can be mixed and matched depending on the requirements 0 This is in contrast to the lSOOSI model which specifies different functions for different layers 0 TCPIP is the most popular network protocol stack and the Internet uses TCPIP stack as its backbone CPIP Vs OSI OSI layers TCPlP layers Application DNS Application Transport TCP UDP Network 5 Data link I 39 Lowerlevel vendor implementations I 1 Physical Figure 122 TCPIIP protocol stack I l TCP over Wireless 0 TCP protocol defined for wireline networks is inefficient when used in wireless networks a In wireline networks the primary source of errors is the network congestion and the TCP congestionavoidance mechanisms can solve the problem efficiently o In wireless networks however there are unavoidable errors due to the unreliable wireless channe 0 Thus if TCPIP is directly used in wireless channels it can lead to a large number of unnecessary transmissions reducing the efficiency 0 This is because TCPIP congestionavoidance and errorrecovery mechanisms work by re transmitting Modified TCP for Wireless To overcome the inefficiency several modifications to TCP has been proposed for wireless Internet 9 Protocols that modify link layer protocols o In this class of modifications techniques are introduced to ensure reliable link layer protocols o The techniques include error correction using FEC and retransmission of lost packets in response to ARQ messages 0 Split TCP approaches 2 Split the TCP connection between a sender and a receiver into two separate connections at the BS one TCP connections between the sender and the BS and another between the BS and the MS a A specialized TCP protocol is then used over the wireless hop References 0 D P Agrawal and Q Zeng Introduction to Wireless and Mobile Systems Second Edition Thomson 2006 Chapters 6 and 12 o T S Rappaport Wireless Communications Principles and Practices PrenticeHall Second edition 2006 Chapter 9 o J GProakis Digital Communication Fourth Edition McGraw Hill New York 2001 Chapter 15 Current and Emerging Wireless Technologies 0 References D P Agrawal and Q Zeng Introduction to Wireless and Mobile Systems Second Edition Thomson 2006 Chapter 5 ECE442 Wireless Communications Lecture 9 Cellular Systems Prof Sudharman K Jayaweera Department of Electrical and Computer Engineering University of New Mexico Existing Wireless Technologies 0 Cellular telephony 0 Cordless phones 0 Wireless Local Loop WLL systems a Wireless Local Area Networks WLAN39s 0 Wireless Personal Digital Assistants PDA39s a Pagers a Bluetooth Emerging Wireless Tech nologies 0 Mobile satellite services M88 0 Wireless geo location systems E 911 0 Third Generation 3G and beyond 4G etc cellular o WiMaX a Wireless Personal Area networks WPAN39s a Wireless Internet a Mobile ad hoc networks MANET39S and Wireless Sensor Networks WSN39s Cellular Systems Cellular Systems 0 In a wireless cellular system the service area is divided into a collection of geographical areas called cells 0 Each cell has a Base Station BS at the center of the cell usually 0 Wireless mobile telephones are connected to the Base Station via a wireless link a A number of base stations are connected via wire line to a Mobile Switching Center MSC 9 Finally the MSCs are connected via wire line to the Public Switched Telephone Network PSTN Main Elements of a Cellular System Mobile Hadset A portable low power low cost consume item 50 200 Functions 0 voice transponders microphone and speaker 0 radio transceiver Issues 0 size battery life voice quality Main Elements of aiCellular System BaStation BS A high end piece of capital equipment 500 K 1 M Functions 0 radio transceiver 0 cell management 0 communicate with the MSC Issues 0 maintaining links interfacing with MSC multiple access Main Elements of a Cellular System Mobile Switching Center MSC A high end piece of capital equipment Functions 0 call management for a group of cells Issues 0 hand off between cells routing calls to PSTN Public Switched Telephone Network Cell Area 0 The most important factor that determines the performance of a cellular system is the size and shape of a cell 0 A cell can be defined as the radio area covered by a transmitting base station 9 All mobile stations MS in that area are connected to and serviced by the base station Shape of a Cell 0 Ideally the shape of a cell can be denoted as a circle of radius R centered at the base station 0 The actual shape however will be different due to the received signal strength fluctuations 0 These can be due to different signal reflections and refractions caused by elevation of the terrain particles in the air presence of tall buildings andor hills etc 0 Also the ideal shape of circle causes modeling problems 0 We cant cover a given area with nonoverlapping circles c There are many other possible shapes that can be used to represent a cell boundary 0 eg hexagon square and equilateral triangle Shape and Models of Cell Coverage Area R 7 R l R Cull R k FR Hexagonal Cells 0 The most popular and most commonly used cell model is the hexagon o Hexagon is closer to a circle than the other models and multiple hexagons can be used to perfectly cover a given area 0 As the mobile station moves away from the BS the received signal strength goes down v gt Dislauce x from BS 0 Although ideally the signal strength contours should be concentric circles due to atmospheric conditions and topographical contours they could be distorted Hand offs in a Cellular System 0 Consider a MS inside a particular cell and connected to the corresponding base station say 35 0 Since the received signal strength weakens as the MS moves away from the 85 at some point it will have to be served by a different base station say o This changing of mobile station39s radio connection from one BS to another is known as a hand off or a hand over Hand offs SLgnal Signal Sucngzh slrcnglh due uc In BS I my Ping pong Effect in Hand offs o Exactly when to perform a hand off needs to be carefully considered 0 In particular hand offs should not take place too quickly to make the MS change the BS too frequently 9 When this happens it is known as the pingpong effect 0 Ping pong effect can happen if the MS moves back and forth between the overlapped area of two adjacent cells a To avoid this the MS is asked to continue maintaining a radio link with the current BS until the signal strength from BS exceeds that of BS by some pre specified threshold What Determines Hand offs o transmit power of the BS 0 mobility of the MS 0 area and shape of the cell Types of Handoffs in Cellular Systems 0 Hard Handoff 0 Current and nearby base stations monitor the signal from a mobile 0 MSC assigns the mobile to the base with the strongest received signal a Used in AMPStype FDMA systems 9 Mobile assisted Handoff 0 Mobile measures the signals from all surrounding base stations 0 Handoff is initiated when a neighboring base has higher power than the current base a Used in digital TDMAtype systems 9 Soft Handoff o The current and nearby base stations monitor the signal from a mobile 9 When a new base receives sufficiently high power it starts receiving the signal from the mo i o Both old and new bases maintain traffic channels until one is clearly much stronger 0 Used in CDMAtype digital systems I l Capacity of a Cell Offered Traffic Load 0 Offered traffic load of a cell is characterized by two important random parameters 2 Average ca arrival rate A This is the average number of MS39s requesting the service per unit time 0 Average holding time T This is the average length of time the MS39s requiring the service 0 We can define the offered traffic load a in terms of these two parameters as a T o The units of traffic load is Erlang 0 One Erlang is equivalent to a servicing channel that is kept busy for an hour I l Offered Traffic Load An Example 0 Consider a cell with 100 MS39s On average 30 requests are generated during an hour with average holding time of T 360 seconds 0 The average call arrival rate is then 30 requests A 3600 seconds 0 Hence the offered traffic load a is a T 30 requests 7 m x 360 seconds 3 Erlangs I l Radio Channels in a Cellular System 0 Since spectrum is a precious resource a cellular system can only be operated in an allocated frequency band 0 Usually this available frequency band is divided in to a number of channels smaller frequency bands o If W is the total bandwidth allocated for the cellular system and if B is the bandwidth of a duplex channel then the total number of radio channels available is total bandwidth bandwidth of a channel W L total number of radio channels Frequency Reuse in a Cellular System 9 A particular frequency channel used in a cell can be reused in another cell as long as they are far apart so that the signals in the two cells do not interfere with each other a This frequency reuse is one of the key concepts that enables cellular systems o It relies on the fact that EM waves decay attenuate rapidly in a typical wireless channel 9 A set of cells that use non overlapping frequency channels is known as a cluster 0 The total available radio channels L is divided equally among the N cells in a cluster The number of channels available for use in a single cell is thus channels per cell m 7 NNB I l Cluster Sizes 6 Observe that with hexagonal cell shape each cell has exactly 6 equi distant neighbors and the lines joining a center of a cell to each of its neighbors are separated by 60 degrees 0 Hence in order to tessellate the cluster size N can take only certain values In fact the cluster size N should satisfy N i2 l ij j2 for some non negative integers i andj Cluster sizes can thus be 4 7 12 etc Probably the most popular cluster size is N 7 cells per cluster Co channel Neighbors and Frequency Reuse Distance 0 The frequency reuse distance D is defined as the distance between two cells using the same frequency channel see previous figure 0 Given that the cluster sice is N i2 l ij j2 for some non negative integers i and j the nearest co channel cells can be located in the following way 0 First move i cells along any chain of hexagons 9 Next turn 60 degrees counterclockwise and move j cells my Mama Wmm miczl nacelula39sysvemlnrh sxamplaNl9 HE 5 392i lAdduledlmni Oel Gl lEEEl I l Frequency Reuse Factor 0 If R is the radius of each cell then it can be shown that D MR 1 o The reuse factor q is defined as the ratio between reuse distance and the cell radius reuse distance cell radius reuse factor q R 2 0 Hence with hexagonal cells q V3N I l Capacity of a Cellular System 0 If a cluster is replicated M times within the system one measure of the cellular system capacity is given by the total number of duplex channels available in the system W C 7 ML 7 M B where L mN is the total number of radio channels In is the number of channels allocated per cell W is the total bandwidth available and B is the bandwidth of a channel 0 Observe that smaller cluster sizes N lead to larger system capacities if the cell size fixed 0 For a fixed cell size decreasing N would increase the number of clusters M need to cover the same area Thus decreasing N would increase the system capacity C 0 However as cluster size N decreases the co channel cells become closer leading to increased co channel interference 0 Hence the smallest possible cluster size N such that the mobiles and base stations can tolerate the co channel Carrier to lnterference Ratio in a Cellular System 0 As we saw the cluster size is determined by the tolerable co channel interference level 0 Assuming only first order interference coming from the M closest cells the carrier to interference ratio 7 is in C DO I 22210 where Do is the distance from the desired base station to the mobile Dk is the distance from the k th co channel base station to the mobile and n is the path loss exponent Minimum Carrier to lnterference Ratio and the Cluster Size 0 If we assume only the six closest co channel cells and further assume that they are all at a distance D from the mobile and the worst case interference is seen when the mobile is at the boundary of a cell so that Do R we have C 7 DJquot 3 Einsiquot I 7 6D 7 6 D 7 6 0 But recall that with hexagonal cells q x3N Hence C 7 WW2 I 7 6 o If a minimum carrier to interference ratio of lm is required for acceptable quality of service 2n 3W2 gt 6 6llmm 6 7 sz Capacity of a Cellular System 0 Recall that the capacity of a cellular system can be characterized by the number of channels In per cell given by E m NB 9 Assuming a minimum required carrier to interference ratio of l min the radio capacity of a cellular system can be given as 7 channelscell B 6 l l V I min 0 Note that larger the lmquot the capacity will be smaller 4 In general digital systems will lead to higher capacity since the minimum required lTClmm is smaller for digital systems a For example for a narrowband analog system lTClmm 18dB whereas for a typical narrowband digital system llmn12d3 l l Frequency Reuse and Co channel Interference o In a cellular system there are many cells that use the same frequency band 0 Although all the cells that use the same frequency channel are separated at least by the reuse distance and the transmit power levels are carefully controlled there will always be some co channel interference Although EM waves attenuate quickly with distance in a typical wireless channel path loss exponent can be between 2 to 5 it never goes to zero 9 Thus there are many techniques that are used in cell design stage to reduce such interference Methods to Reduce Co channel Interference Two important methods are a cell splitting 0 cell sectoring Cell Splitting 6 It may not be desirable to have the same cell size and same transmit BS power level for all the cells in a system 0 Demand for resources for example frequency channels may depend on the concentration of users MS in a given area 0 In particular over a period of time the number of users in an area could change thus leading to increased traffic 3 A way to cope with this increased traffic is to split a cell into a number of smaller cells Cell Splitting Lurgu u liow immm Silull CL ll min dummy Smullri Kx ll munv Iz39iuilyl 0 When a cell is split additional 8839s are installed at the center of each new cell so that the higher density of calls can be handled efficiently 0 However since the coverage area of the newly split cells are smaller the transmit power levels are lowered and the co channel interference will be reduced Cell Sectoring 6 Omni directional antennas allows transmission of radio signals in all directions with equal power strength 0 Directional antennas covers only an area of angles for example 60 or 120 degrees 3 Cells served by directional antennas are called sectored cells a Usually a set of directional antennas sufficient to cover whole 360 degrees are mounted on a single microwave tower located at the center of a cell Cell Sectori n minim39 I LL3 niii mlu ii 0 Since each antenna in a sectored cell needs to cover only a smaller area a lower transmit power level can be used a this helps reduce the co channel interference o Sectorized cells also improves the spectrum efficiency of the overall system l l Cellular Channels 0 When we discussed the multipleaccess techniques XDMA we were basically referring to cellular traffic channels 0 For proper operation of the network cellular systems also make use of other types of channels like control channels 0 Different types of channels used will depend on the network Uplink Channels Reverse or Mobile to Base Channels 0 Access Channel 0 A shared randomaccess channel a Used by mobile users to register with the base and to signal when they want to make a call 9 Traffic Channels 0 Carry the actual voice or data traffic 0 May also carry other control signals at a lowrate interspersed with voice Downlink Channels Forward or Base to Mobile Channels 0 Pilot Channel Each cell has a pilot channel that provides a carrier reference for coherent detection It also provides power measurements for power control 9 Synch Channel Each cell has one synch channel that is used to broadcast a timing reference to the mobiles o Paging Channels 9 Each cell can have several paging channels that are used to alert mobiles when they have a call a Also provide information needed by the mobiles to set up a traffic channel such as carrier frequency code time slot 0 Traffic Channels Mostly the same as on the uplink Cellular Frequency Allocations in US 0 Cellular 0 Uplink 824 MHZ 849 MHZ a Downlink 869 MHZ 894 MHZ o Frequency Division Duplex FDD Each uplink channel is paired with a downlink channel that is 45 MHZ away 0 Spectrum in each geographical licensing area is divided between few providers 9 Personal Communications Services PCS o Uplink 1850 MHZ 1910 MHZ o Downlink 1930 MHZ 1990 MHZ 9 Frequency Division Duplex FDD Uplink and downlink channels are separated by 80 MHZ Cellular Air Interfaces 0 First Generation Analog Cellular AMPS in US 9 Analog FM using FDMA 30 kHz channels 0 Similar systems worldwide during 1980 s 0 Second Generation 2G Digital Cellular 0 US Digital Cellular or USDC IS54 a 6 slot TDMA within 30 kHz AMPS channels 9 Global System for Mobile GSM a operated in 900 MHz ban d a 8 or 16 slot TDMA within 200 kHz channels with slow frequency hopping a Used throughout Europe Asia Middle East and Africa 9 Code Division Multipleaccess IS95 a 64 code system within 125 MHz subchannels a Used throughout Americas and parts of Asia eg Japan and Korea I l Cellular Air Interfaces ctd o 25G in US PCS SystemsAdvanced Services 0 lS 136 9 Similar to IS54 but dualband PCS and cellular plus some advanced services 9 PCSlQOO a US version of GSM also DCSlBOO and European GSM References 0 D P Agrawal and Q Zeng Introduction to Wireless and Mobile Systems Second Edition Thomson 2006 Chapter 5 o T S Rappaport Wireless Communications Principles and Practices PrenticeHall Second edition 2006 Chapter 3 Existing Cellular Systems GSM and lS 95 0 References D P Agrawal and Q Zeng Introduction to Wireless and Mobile Systems Second Edition Thomson 2006 Chapter 10 ECE442 Wireless Communications Lecture 3 Wireless Channel Prof Sudharman K Jayaweera Department of Electrical and Computer Engineering University of New Mexico An Abstract Model for a Communications System Information m gt gt anne source Modulator Information lt Demodulator lt Destlnatlon Recall from Lecture 1 Challenges in Wireless Communications a Rapidly changing physical channel due to mobility 0 Networking to keep seamless connectivity o Longer battery life for portability a Physical impairments of the wireless channel 0 Providing high data rates to meet the demands of multi media traffic 0 Large network capacities to cover more users and areas 0 Encryption to overcome the vulnerability of the radio link to security problems 0 Efficient resource allocation schemes to meet QoS requirements 9 Global standardization I l