Thus received power p r at distance d a e receiver

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Thus, received power P r at distance d : A e = receiver antenna effective area G t = transmitter antenna antenna gain 2 4 d P AeG t t π = Pr
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35 Free Space Path loss 2 4 = c df L c p π After some manipulations we obtain the expression of Path loss L: Note: path loss increases with carrier frequency. Lp = path loss (average propagation loss over wide area): Determined by distance , carrier frequency , land profile , Free space path loss in decibels can be written as: ) ( log 20 ) ( log 20 45 . 32 10 10 km d MHz f L c f + + =
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36 Free-space path loss (Cont) 1500 MHz 150 MHz ) ( log 20 ) ( log 20 45 . 32 10 10 km d MHz f L c f + + =
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37 Wireless Link Power Budget Analysis When operating at 3 Mbps, a WLAN system has transmission power of 15 dBm, and the receiver power must be at least –72 dBm. The communication power budget is therefore: ( 29 dB 87 dBm 72 dBm 15 (dBm) P (dBm) P (dB) budget Power receiver trans = - - = - = To find how transmission distance and frequency affect the power budget, we can calculate the free space isotropic loss. This is the path loss incurred by an electromagnetic wave as it propagates in a straight line through a vacuum from one isotropic antenna to another. At any point around the antenna, the power density is: ( 29 2 4 r P r total ISO π ρ =
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38 Wireless Link Power Budget Analysis (cont) The power received is the product of the receiving antenna’s effective area and the power density: ( 29 ( 29 2 2 2 2 4 4 4 4 , = = = = r f c P r P r P A r P d transmitte d transmitte d transmitte eff ISO received π π λ π λ π φ θ ρ The free space isotropic loss is simply the inverse of the power, and is usually expressed in decibels: dB 4 log 20 : decibels 4 2 = = c r f L c r f L ISO ISO π π r : distance in meters λ : wavelength in meters f : frequency in Hertz m/second. 8 10 3 = c
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39 Signal Loss Chart Obstruction Additional Loss (dB) Effective Range Approximate Range Open Space 0 100 % 300 m Window (non-metallic) 3 70 % 215 m Window (metallic tint) 5-8 50 % 150 m Wall (dry wall) 5-8 50 % 150 m Wall (wood) 10 30 % 100 m
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40 802.11 Wireless LAN Configurations BS To send an IP packet over the WLAN, Make 802.11 compatible with Ethernet above the DLL . 1. Ad-Hoc Networking 2. Peer-to-peer Networking
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41 CSMA (Carrier Sense Multiple Access) CSMA : listen before transmit. If channel is sensed busy , listens again. ( Kleinrock, 1975 ) Persistent CSMA : retry immediately when collide (this may cause instability) Non persistent CSMA : retry after random interval Note: collisions may still exist, since two stations may sense the channel idle at the same time In case of collision, transmission time is wasted CSMA with Collision Avoidance (CA) access schemes used in wireless LANs. Here sensing the carrier combined with a back-off scheme. (MACA)
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42 Differences with Ethernet Ethernet operates with CSMA/CD protocol.
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