Wireless Communications Engineering Cellular Fundamentals Definitions Wireless Communication

Wireless Communications Engineering Cellular Fundamentals

Definitions – Wireless Communication n What is Wireless Communication? n Ability to communicate via wireless links. Mobile Communication = + ?

Wireless Communication n Wireless Communication are of two types: n n Fixed Wireless Communication Mobile Wireless Communication.

Mobile Wireless Communication n Mobile Wireless Communication (Infrastructured Network) Single Hop Wireless Link to reach a mobile Terminal. Mobile Communication = + ?

Mobile Ad Hoc Networks n Infrastructureless or Adhoc Network Multihop Wireless path from source to destination.

Mobile Radio Environment

Mobile Radio Environment n n n The transmissions over the wireless link are in general very difficult to characterize. EM signals often encounter obstacles, causing reflection, diffraction, and scattering. Mobility introduces further complexity. We have focused on simple models to help gain basic insight and understanding of the wireless radio medium. Three main components: Path Loss, Shadow fading, Multipath fading (or fast fading).

Free Space loss n Transmitted signal attenuates over distance because it is spread over larger and larger area n This is known as free space loss and for isotropic antennas Pt = power at the transmitting antenna Pr = power at the receiving antenna λ = carrier wavelength d = propagation distance between the antennas c = speed of light

Free Space loss n For other antennas Gt = Gain of transmitting antenna Gr = Gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of receiving antenna

Thermal Noise n Thermal noise is introduced due to thermal agitation of electrons n n Present in all transmission media and all electronic devices a function of temperature uniformly distributed across the frequency spectrum and hence is often referred to as white noise amount of noise found in a bandwidth of 1 Hz is N 0 = k T N 0 = noise power density in watts per 1 Hz of bandwidth k = Boltzman’s constant = 1. 3803 x 10 -23 J/K T = temperature, in Kelvins N = thermal noise in watts present in a bandwidth of B = k. TB where

Free Space loss n Transmitted signal attenuates over distance because it is spread over larger and larger area n This is known as free space loss and for isotropic antennas Pt = power at the transmitting antenna Pr = power at the receiving antenna λ = carrier wavelength d = propagation distance between the antennas c = speed of light

Free Space loss n For other antennas Gt = Gain of transmitting antenna Gr = Gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of receiving antenna

Thermal Noise n Thermal noise is introduced due to thermal agitation of electrons n n Present in all transmission media and all electronic devices a function of temperature uniformly distributed across the frequency spectrum and hence is often referred to as white noise amount of noise found in a bandwidth of 1 Hz is N 0 = k T N 0 = noise power density in watts per 1 Hz of bandwidth k = Boltzman’s constant = 1. 3803 x 10 -23 J/K T = temperature, in Kelvins N = thermal noise in watts present in a bandwidth of B = k. TB where

Data rate and error rate n Bit error rate is a decreasing function of Eb/N 0. n n If bit rate R is to increase, then to keep bit error rate (or E b/N 0) same, the transmitted signal power must increase, relative to noise Eb/N 0 is related to SNR as follows B = signal bandwidth (since N = N 0 B)

Doppler’s Shift n n n When a client is mobile, the frequency of received signal could be less or more than that of the transmitted signal due to Doppler’s effect If the mobile is moving towards the direction of arrival of the wave, the Doppler’s shift is positive If the mobile is moving away from the direction of arrival of the wave, the Doppler’s shift is negative

Doppler’s Shift where fd =change in frequency X due to Doppler’s shift v = constant velocity of the mobile receiver λ = wavelength of the transmission S θ Y

Doppler’s shift f = fc + fd where f = the received carrier frequency fc = carrier frequency being transmitted fd = Doppler’s shift as per the formula in the previous slide.

Multipath Propagation n Wireless signal can arrive at the receiver through different paths n LOS n Reflections from objects n Diffraction n Occurs at the edge of an impenetrable body that is large compared to the wavelength of the signal

Multipath Propagation (source: Stallings)

Mobile Radio Channel: Fading

Limitations of Wireless n n n Channel is unreliable Spectrum is scarce, and not all ranges are suitable for mobile communication Transmission power is often limited n n Battery Interference to others

Advent of Cellular Systems n n n Noting from the channel model, we know signal will attenuated with distance and have no interference to far users. In the late 1960 s and early 1970 s, work began on the first cellular telephone systems. The term cellular refers to dividing the service area into many small regions (cells) each served by a low-power transmitter with moderate antenna height.

Cell Concept n Cell A cell is a small geographical area served by a singlebase station or a cluster of base stations n Areas divided into cells n n Each served by its own antenna Served by base station consisting of transmitter, receiver, and control unit Band of frequencies allocated Cells set up such that antennas of all neighbors are equidistant

Cellular Networks

Cellular Network Organization n n Use multiple low-power transmitters Areas divided into cells n n Each served by its own antenna Served by base station consisting of transmitter, receiver, and control unit Band of frequencies allocated Cells set up such that antennas of all neighbors are equidistant

Consequences n Transmit frequencies are re-used across these cells and the system becomes interference rather than noise limited n n the need for careful radio frequency planning – colouring in hexagons! a mechanism for handling the call as the user crosses the cell boundary - call handoff (or handover) increased network complexity to route the call and track the users as they move around But one significant benefit: very much increased traffic capacity, the ability to service many users

Cellular System Architecture

Cellular Systems Terms n Mobile Station n n Base Station (BS) n n n users transceiver terminal (handset, mobile) fixed transmitter usually at centre of cell includes an antenna, a controller, and a number of receivers Mobile Telecommunications Switching Office (MTSO) /Mobile Switch Center (MSC) n n n handles routing of calls in a service area tracks user connects to base stations and PSTN

Cellular Systems Terms (Cont’d) n Two types of channels available between mobile unit and BS n n n Control channels – used to exchange information for setting up and maintaining calls Traffic channels – carry voice or data connection between users Handoff or handover n process of transferring mobile station from one base station to another, may also apply to change of radio channel within a cell

Cellular Systems Terms (Cont’d) n Downlink or Forward Channel n n Uplink or Reverse Channel n n radio channel for transmission of information (e. g. speech) from mobile station to base station Paging n n radio channel for transmission of information (e. g. speech) from base station to mobile station a message broadcast over an entire service area, includes use for mobile station alert (ringing) Roaming n a mobile station operating in a service area other than the one to which it subscribes

Steps in an MTSO Controlled Call between Mobile Users n n n Mobile unit initialization Mobile-originated call Paging Call accepted Ongoing call Handoff

Frequency Reuse n n n Cellular relies on the intelligent allocation and re–use of radio channels throughout a coverage area. Each base station is allocated a group of radio channels to be used within the small geographic area of its cell Neighbouring base stations are given different channel allocation from each other

Frequency Reuse (Cont’d) n If we limit the coverage area within the cell by design of the antennas n n we can re-use that same group of frequencies to cover another cell separated by a large enough distance transmission power controlled to limit power at that frequency to keep interference levels within tolerable limits n the issue is to determine how many cells must intervene between two cells using the same frequency

Radio Planning n n Design process of selecting and allocating channel frequencies for all cellular base stations within a system is known as frequency re-use or frequency planning. Cell planning is carried out to find a geometric shape to n n n tessellate a 2 D space represent contours of equal transmit power Real cells are never regular in shape

Two-Dimensional Cell Clusters n n Regular geometric shapes tessellating a 2 D space: Square, triangle, and hexagon. ‘Tessellating Hexagon’ is often used to model cells in wireless systems: n n n Good approximation to a circle (useful when antennas radiate uniformly in the x-y directions). Also offer a wide variety of reuse pattern Simple geometric properties help gain basic understanding and develop useful models.

Coverage Patterns

Cellular Coverage Representation

Geometry of Hexagons Hexagonal cell geometry and axes

Geometry of Hexagons (Cont’d) n n D = minimum distance between centers of cells that use the same band of frequencies (called co-channels) R = radius of a cell d = distance between centers of adjacent cells (d = R√ 3) N = number of cells in repetitious pattern (Cluster) Reuse factor Each cell in pattern uses unique band of frequencies

Geometry of Hexagons (Cont’d) n n n The distance between the nearest cochannel cells in a hexagonal area can be calculated from the previous figure The distance between the two adjacent co-channel cells is D=√ 3 R. (D/d)2 = j 2 cos 2(30) + (i+ jsin 30)2 = i 2 + j 2 +ij = N D=Dnorm x √ 3 R =(√ 3 N)R In general a candidate cell is surrounded by 6 k cells in tier k.

Geometry of Hexagons (Cont’d) n n Using this equation to locate co-channel cells, we start from a reference cell and move i hexagons along the uaxis then j hexagons along the v-axis. Hence the distance between co–channel cells in adjacent clusters is given by: D = (i 2 + ij + j 2)1/2 n n n where D is the distance between co–channel cells in adjacent clusters (called frequency reuse distance). and the number of cells in a cluster, N is given by D 2 N = i 2 + ij + j 2

Hexagon Reuse Clusters

3 -cell reuse pattern (i=1, j=1)

4 -cell reuse pattern (i=2, j=0)

7 -cell reuse pattern (i=2, j=1)

12 -cell reuse pattern (i=2, j=2)

19 -cell reuse pattern (i=3, j=2)

Relationship between Q and N

Proof

Cell Clusters since D = SQRT(N)

Co–channel Cell Location n n Method of locating co–channel cells Example for N=19, i=3, j=2

Cell Planning Example n Suppose you have 33 MHz bandwidth available, an FM system using 25 k. Hz channels, how many channels per cell for 4, 7, 12 cell re-use? n n n total channels = 33, 000/25 = 1320 N=4 channels per cell = 1320/4 = 330 N=7 channels per cell = 1320/7 = 188 N=12 channels per cell = 1320/12 = 110 Smaller clusters can carry more traffic However, smaller clusters result in larger cochannel interference

Remarks on Reuse Ratio

Co-channel Interference with Omnidirectional Cell Site

Propagation model

Cochannel interference ratio

Worst-case scenario for cochannel interference

Worst-case scenario for cochannel interference

Reuse Factor and SIR

Remarks n n n SIGNAL TO INTERFERENCE LEVEL IS INDEPENDENT OF CELL RADIUS! System performance (voice quality) only depends on cluster size What cell radius do we choose? n n Depends on traffic we wish to carry (smaller cell means more compact reuse or higher capacity) Limited by handoff

Adjacent channel interference n n n So far, we assume adjacent channels to be orthogonal (i. e. , they do not interfere with each other). Unfortunately, this is not true in practice, so users may also experience adjacent channel interference besides co-channel interference. This is especially serious when the near-far effect (in uplinks) is significant n Desired mobile user is far from BS n Many mobile users exist in the cell

Near-Far Effect

Near-Far Effect (Cont’d)

Reduce Adjacent channel interference n n n Use modulation schemes which have small out-of-band radiation (e. g. , MSK is better than QPSK) Carefully design the receiver BPF Use proper channel interleaving by assigning adjacent channels to different cells, e. g. , for N=7

Reduce Adjacent channel interference (Cont’d) n n Furthermore, do not use adjacent channels in adjacent cells, which is possible only when N is very large. For example, if N =7, adjacent channels must be used in adjacent cells Use FDD or TDD to separate the forward link and reverse link.

Improving Capacity in Cellular Systems n n Adding new channels – often expensive or impossible Frequency borrowing (or DCA)– frequencies are taken from adjacent cells by congested cells Cell splitting – cells in areas of high usage can be split into smaller cells (microcells with antennas moved to buildings, hills, and lamp posts) Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels

Sectoring n Co-channel interference reduction with the use of directional antennas (sectorization) n Each cell is divided into sectors and uses directional antennas at the base station. n Each sector is assigned a set of channels (frequencies).

Site Configurations

Sectorized Cell Sites

Worst case scenario

Sectorizd Cell Sites

Worst case scenario

Illustration of cell splitting 1

Illustration of cell splitting 2

Illustration of cell splitting 3

Cell Splitting

Design Tradeoff n n Smaller cell means higher capacity (frequency reused more). However, smaller cell also results in higher handoff probability, which also means higher overhead n Moreover, cell splitting should not introduce too much interference to users in other cells

Handoff (Handover) Process n n Handoff: Changing physical radio channels of network connections involved in a call, while maintaining the call Basic reasons for a handoff n n n MS moves out of the range of a BTS (signal level becomes too low or error rate becomes too high) Load balancing (traffic in one cell is too high, and shift some MSs to other cells with a lower load) GSM standard identifies about 40 reasons for a handoff!

Phases of Handoff MONITORING PHASE n - n measurement of the quality of the current and possible candidate radio links - initiation of a handover when necessary HANDOVER HANDLING PHASE - determination of a new point of attachment - setting up of new links, release of old links - initiation of a possible re-routing procedure

Handoff Types n Intra-cell handoff – narrow-band interference => change carrier frequency – controlled by BSC n Inter-cell, intra-BSC handoff – typical handover scenario – BSC performs the handover, assigns new radio channel in the new cell, releases the old one n Inter-BSC, intra-MSC handoff – handoff between cells controlled by different BSCs – controlled by the MSC n Inter-MSC handoff – handoff between cells belonging to different MSCs – controlled by both MSCs

Handoff Types (cont’d)

Handoff Strategies n n n Relative signal strength with threshold Relative signal strength with hysteresis and threshold Prediction techniques

Intra-MSC Handoff (Mobile Assisted)

Handover Scenario at Cell Boundary

Handoff Based on Receive Level How to avoid ping-pong problem?

Handoff – 1 G (Analog) systems n n Signal strength measurements made by the BSs and supervised by the MSC BS constantly monitors the signal strengths of all the voice channels Locator receiver measures signal strength of MSs in neighboring cells MSC decides if a handover is necessary

Handoff – 2 G (Digital) TDMA n n n Handoff decisions are mobile assisted Every MS measures the received power from surrounding BSs and sends reportsto its own BS Handoff is initiated when the power received from a neighbor BS begins to exceed the power from the current BS (by a certain level and/or for a certain period)

Handoff – 2 G (Digital) CDMA n n n CDMA uses code to differentiate users. Soft handoff: a user keeps records of several neighboring BSs. Soft handoff may decrease the handoff blocking probability and handoff delay

Avoiding handoff: Umbrella cells

Mixed Cell Architecture

Handoff Prioritization n The idea of reserving channels for handoff calls was introduced in the mid 1980 s as a way of reducing the handoff call blocking probability Motivation: users find calls blocked in midprogress a far greater irritant than unsuccessful call attempts. The basic idea is to reserve a certain portion of the total channel pool in a cell for handoff users only.

Performance Metrics n n Call blocking probability – probability of a new call being blocked Call dropping probability – probability that a call is terminated due to a handoff Call completion probability – probability that an admitted call is not dropped before it terminates Handoff blocking probability – probability that a handoff cannot be successfully completed

Performance Metrics (Cont’d) n n Handoff probability – probability that a handoff occurs before call termination Rate of handoff – number of handoffs per unit time Interruption duration – duration of time during a handoff in which a mobile is not connected to either base station Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur

Summary n n n n n cellular mobile uses many small cells hexagonal planning, clusters of cells cell repeat patterns 3, 7, 12 etc. . . re-uses frequencies to obtain capacity is interference not noise (k. TB) limited S/I is independent of cell radius choose cell radius to meet traffic demand N=7 is a good compromise between S/I and capacity. handoff