Radio Network Planning for Outdoor WLAN-Systems

Transcription

1 Radio Network Planning for Outdoor WLAN-Systems S Postgraduate Course in Radio Communications Jarkko Unkeri 54029P 1

2 Outline Introduction WLAN Radio network planning challenges Objectives for network planning Planning process Coverage planning Capacity planning Frequency planning Summary and discussion 2

3 Introduction Outdoor WLAN-Systems are getting more popular Outdoor Hot-Spots; Train stations, Squares, Campus areas Industry and logistics (e.g. surveillance, harbours) Residential areas City-wide networks; Fixed Wireless Access (FWA), Hot-Spots etc. WLAN outdoor usage is quite challenging IEEE b radio interface is mainly specified for office environment, where distances and number of users are small Unlicenced frequency band Low transmit powers, interference... Need for carefull network planning 3

4 WLAN radio network planning challenges Challenges of WLAN radio technology short distance (LOS: <1km) due to low transmit power medium gain antennas (13 dbi) to get better sensitivity and optimal antenna alignment to use effectively transmit power near-far problem due to lack of transmit power control optimal radiation pattern and alignment of base station antenna (the farer the client, the higher the antenna gain) hidden node problem during the uploading sectorized cell (multiradio site) mimimizes cases, where clients can t hear each other. RTS/CTS handshaking protocol 4

5 Objectives for network planning Capacity There should be enough capacity to support the subscriber traffic with sufficiently low blocking and delay Coverage To obtain the ability of the network to ensure the availability of the service in the entire service area Quality Costs Linking the capacity and the coverage and still provide the required QoS To enable an economical network implementation when the service is established and a controlled network expansion during the life cycle of the network 5

6 Equipment selection Equipment selection is important because e.g. receiver sensitivities might vary a lot between different chipset manufacturers The use of OFDM improves spectral efficiency, improves resistance to frequency selective fading, decreases ISI and lowers multipath distortion. The main advantages of DSSS (802.11b) are that it tolerates noise more efficiently and the client devices are very popular and inexpensive b g a Frequency band (outdoor) [GHz] 2,4-2,483 2,4-2, Maximum data rate [Mbit/s] Physical layer modulation DSSS OFDM OFDM Receiver sesitivity [dbm] 5.5 Mbps: Mbps: Mbps: Mbps: Mbps: Mbps: -77 Maximum transmit power (EIRP) [dbm] Number of channels (outdoor) (Values are based on ETSI regulations) 6

7 Planning process Input Output Requirements for coverage Requirements for capacity Requirements for quality Area type / radio propagation Measured network performance Dimensioning Capacity and coverage planning Network performance visualisation Optimisation -Rough number of base stations and sites -Base station configurations -Site selection -Base station configurations -Cell specific parameters for radios -Capacity and coverage analysis -Quality of service analysis Adjustment of radio parameters 7

8 Capacity Input for the network planning Available backhauls? Offered client end capacity? Population in the service area? Estimated Internet penetration? Load factor? Coverage Area type? Maps, also 3D? Need for 100 % Coverage? Access type (Fixed, mobile)? Frequency Other networks in the area (or other essential interference)? Other Special needs for capacity, coverage or QoS? 8

9 Dimensioning Process 9

10 Coverage planning (1/2) Due to uncertainty of signal levels in NLOS situations, the LOS connection to client is typically needed. For example trees between base station and client shall be avoided, because attenuation can vary tens of decibels depending on weather and season The client end equipments plays important role in WLAN radio network planning (Client antenna gain, receiver sensitivity...) Radio wave propagation depends on environment: Typical path-loss exponents: Free space 2 Urban area cellular Shadowed urban cell Rayleigh fading

11 Coverage planning (2/2) Radio coverage can be simulated based on 3D map using radio network planning tools like Atoll and SitePlanner that are popular in cellular world Simple link budget calculations can be used to estimate the acceptable propagation loss: P r =P t +G t -L kt -L 0 +G r -L kr -M [db] where P t is transmit power, P r is receiver sensitivity, M is fading margin, L o is the free space loss, L kt and L kr are cable losses, G t and G r are antenna gains. L 0 4 π d = 10 log λ where λ is the wavelength and d is the distance between antennas. The link budget should be calculated for both directions (uplink/downlink) 2 11

12 Multipath propagation In a radio connection the signal is often received from different paths reflected, penetrated, diffracted and scattered => Changes in amplitude, phase and polarization in the received signal Signals in phase: Amplified signal Signals out of phase: Distorted signal Figure 5:Multipath propagation 12

13 Base station antenna gain vs. coverage Higher antenna gain results in more directivity narrower beam (horizontal + vertical) larger antenna structure Directional antennas are also used to restrict some of incoming multipath echoes Reasonable omni-directional antenna gain no higher than 6 dbi otherwise elevation beam unreasonable narrow resulting in null sectors Cell size extension by sectorization Antenna gain vs. range 13

14 Sectorized antenna system vs. omni-directional Gives up to four times larger effective coverage per site Decreases probablity for typical WLAN outdoor network problems hidden node, near-far, null sectors Sectorized system gives more even signal strength troughout the cell (down-tilting, shaped beam antennas) Omni antenna cannot be directed (cell size management) 14

15 Capacity planning The use of interference free solutions in transmission network confirms the network operability Wired solutions: Fiber, xdsl etc. Wireless solutions: SDH/PDH, WLL, etc. Bottlenecks should be avoided in transmission network Typical WLAN network topologies 5 GHz (.11a) for wireless links 2.4 GHz (.11b/g) for Client Access SDH/PDH (PtP) Link PtMP WLL/ HotZones WLANs Fiber Corporate WLAN 15

16 throughput In b/g/a the data transmission is half-duplex Effective data rates should be taken into account when planning capacity The maximum number of users/ap depends on the offered client end capacity The throughput of.11g reduces significantly if.11b is used simultaneously (RTS/CTS or CTS overhead) Table 1. Theoretical Maximum Application-level Throughput Number of Non- Interfering Channels Modulation Maximum Link Rate Theoretical Maximum TCP Rate Theoretical Maximum UDP Rate Standard b 3 CCK 11 Mbps 5,9 Mbps 7,1 Mbps g (with b) 3 OFDM/CCK 54 Mbps 14,4 Mbps 19,5 Mbps g (.11g only) 3 OFDM/CCK 54 Mbps 24,4 Mbps 30,5 Mbps a 19* OFDM 54 Mbps 24,4 Mbps 30,5 Mbps a Atheros Turbo Mode 6 OFDM 108 Mbps 42,9 Mbps 52,8 Mbps * 13 non-overlapping channels in United States and up to 19 non-overlapping channels in Europe depending on local regulations 16

17 Frequency planning (1/3) The number of non-overlapping channels sets the limitations for channel planning Isolation between antennas, radio units at the same site and neighbouring cells should be calculated to estimate needed channel difference Isolation consists of things like propagation loss, radiation pattern of antennas and receiver adjacent channel rejection In the worst case, when transmitting (20 dbm) and receiving (-90 dbm) is occuring at the same time, 110 db isolation is needed Isolation can be increased also by using different polarisations (e.g. if g is used for wireless links between b AP s) Antenna type selection has effects also to interference Low sidelobe levels, high High cross polarization discrimination, High front-to-back ratio 17

18 Frequency planning (2/3) Receiver adjacent channel rejection The adjacent channel rejection for.11b and.11g shall be equal to or better than 35 db between any two channels with > 25 MHz separation in each channel group C/I [db] Needed carrier to interference 11 Mbit/s Channel difference 18

19 Frequency planning (3/3) Interference scanning is required in the area Spectrum analyzer is also needed because there might exist some other interference than from other WLAN systems e.g cordless telephones, wireless cameras etc. Centralized management and monitoring of radio channels and interference is needed in large networks (e.g SNMP based tool) 19

20 Summary The outdoor usage of WLAN systems is quite challenging Interference, hidden-node, near-far... With proper radio network planning the effects of these problems can be minimized Base station site selection Antenna type selection Proper capacity calculations Proper frequency planning It should be noticed that WLAN regulations varies from country to another especially at 5 GHz band Centalized management and monitoring of radio parameters are needed in networks with multiple Access Points 20

21 References [1] H. Holma, A. Toskala, WCDMA for UMTS Radio Access for Third Generation Mobile Communications, John Wiley& Sons, [2] M. S. Gast, Wireless Networks, The Definitive Guide, O Reilly, April [3] T.S. Rappaport, Wireless Communications, Upper Saddle River (NJ), Prentice Hall PTR, [4] J. Heiskala, J. Terry, OFDM Wireless LANs: A Theorethical and Practical Guide, Sams Publishing [5] J. Geier, Wireless LANs; Implementing High Performance IEEE Networks, 2nd ed., Sams Publishing [6] Wireless LAN Performance, Atheros Communications Inc., April Available: [7] ORiNOCO AP-4000 Tri-Mode Access Point Technical Specifications, Proxim Corp., Available: 21

22 Homework Calculate using the simple link budget the needed receiver antenna gain to achieve 11 Mbit/s data rate. The connection length is 0,5 km and typical b equipments are used. AP antenna gain: 12 dbi LOS situation with free fresnell-zone 6 db fade margin expexted 5 m RF cable in both ends AP side cable attenuation: 0,12 db/m (1/2 RF) Client end cable attenuation: 1,4 db/m (RG 58) Tx power adjustable in AP side: 7, 10, 13 or 16 dbm Tx power at client end: 15 dbm 22