M OBILE W IRELESS C OMMUNICATIONS I NTRODUCTION C OMMUNICATIONS ‐ I NTRODUCTION Aditya K. Jagannatham Indian Institute of Technology Kanpur Indian Institute of Technology Kanpur Commonwealth of Learning Vancouver MOOC on M4D 2013
2G Wireless Systems 2G Wireless Systems Gener Standard Rate Services ation 2G GSM ( Global System for Mobile 10 Kbps Voice calls Communications ) 2G CDMA ( Code Division for Multiple 10 Kbps Voice calls Access ) 2.5G GPRS ( General Packet Radio 50 Kbps Internet/ e ‐ Service ) mail access 2.5G EDGE ( Enhanced Data Rates for 200 Kbps Internet/e ‐ GSM Evolution ) mail access MOOC on M4D 2013
3G Wireless Systems 3G Wireless Systems Gener Gener Standard Standard Rate Rate Services Services ation 3G 3G WCDMA (Wideband WCDMA (Wideband 384 Kbps 384 Kbps Video Video CDMA)/UMTS (Universal Telephony, Mobile Telecommunication video System) streaming 3G CDMA 2000 384 Kbps Video Telephony, video streaming streaming 3.5G 3 5G HSDPA (High Speed Downlink HSDPA (High Speed Downlink 5 ‐ 30 Mbps 5 30 Mbps Online Online Packet Access)/HSUPA (High Gaming, HD MOOC on M4D 2013 Speed Uplink Packet Access) Streaming
4G Wireless Systems 4G Wireless Systems Gene Standard Rate Services ratio ti n 4G LTE (Long Term Evolution) ( ) 100 ‐ 200 Mbps Mobile TV, Multiplayer Gaming Gaming 4G WiMAX (Worldwide 100 Mbps Mobile TV, Interoperability for Interoperability for Multiplayer Multiplayer Microwave Access) Gaming MOOC on M4D 2013
Wireless Signal Propagation Wireless Signal Propagation MOOC on M4D 2013
Wireless Channel Fading Wireless Channel Fading • The wireless signal can reach the receiver via direct and multiple scattered paths. – Multipath Propagation – As a result, the receiver sees the superposition of multiple copies of the transmitted signal. • These signal copies experience different g p p – Attenuations – Delays Delays MOOC on M4D 2013
Wireless Channel Fading Wireless Channel Fading • Results in interference , amplifying or attenuating the signal power seen at the Rx. – This phenomenon is termed as fading . • Strong destructive interference is referred to as a St o g dest uct e te e e ce s e e ed to as a deep fade . MOOC on M4D 2013
Wireless Channel Fading Wireless Channel Fading Fading Channel Magnitude vs Time 0 -2 -4 4 nitude -6 annel magn -8 cha -10 10 -12 Deep fade Deep fade -14 -16 16 0 20 40 60 80 100 120 140 160 180 200 t (ms) MOOC on M4D 2013
Multipath Delay Characterization Multipath Delay Characterization • Each replica of the wireless signal s ( t ) is delayed, Each replica of the wireless signal s ( t ) is delayed, attenuated, phase shifted. T T d MOOC on M4D 2013
Multipath Delay Characterization Multipath Delay Characterization • T is the symbol duration. • T d is the delay spread . T d is the delay spread . MOOC on M4D 2013
Multipath Delay Characterization Multipath Delay Characterization T T d d • If T d > T , i.e. delay spread is larger than symbol time , there is ISI (Intersymbol Interference) . there is ISI (Intersymbol Interference) . MOOC on M4D 2013
WSSUS Channel Variables Delay WSSUS Channel Variables ‐ Delay • Typical wireless channel delay spreads are of the Typical wireless channel delay spreads are of the order of 1km/3 x 10 5 kms ‐ 1 ~ 3 μ s . ~ Km ~ Km ~ Km MOOC on M4D 2013
WSSUS Channel Variables ‐ Delay WSSUS Channel Variables Delay • Therefore, to avoid ISI, T > T d = 3 μ s. • It is immediately clear the maximum symbol rate in outdoor channels is, 1 R 333 Kbps p max max 6 6 3 3 10 10 15 MOOC on M4D 2013
Frequency Domain Interpretation h ( t ) H ( f ) B c f 0 T d • Let H ( f ) denote the FT of the wireless channel ( f ) impulse response h ( t ) • The bandwidth of the channel response is The bandwidth of the channel response is termed the coherence bandwidth B c . MOOC on M4D 2013
Frequency Domain Interpretation B c f 0 T d T d B c f 0 T d • As the delay spread T d increases coherence As the delay spread T d increases , coherence bandwidth B c decreases . B c ~ 1/ T d MOOC on M4D 2013
Implication of Coherence Bandwidth Implication of Coherence Bandwidth • If B c is greater than the wireless signal bandwidth B s , g g c s there is NO distortion . B s B c f f 0 0 • Such a wireless channel is also known as a Flat Fading channel. MOOC on M4D 2013
Implication of Coherence Bandwidth Implication of Coherence Bandwidth If B c is smaller than the wireless signal bandwidth B s , • g s , c there is distortion . B s B c f f f f 0 0 • Such a wireless channel is also known as a Frequency Selective channel. MOOC on M4D 2013
Coherence bandwidth Coherence bandwidth • Coherence bandwidth of the channel is defined in terms of delay spread as, 1 B c c T T d • For outdoor channels, T d ~ 3 s as seen earlier. – Hence, the coherence bandwidth B c is given as, , g , c 1 6 333 333 KHz KHz B B c 3 10 MOOC on M4D 2013
Back to Time Domain • If the coherence bandwidth is lesser than the wireless signal bandwidth wireless signal bandwidth B B c c s s 1 1 T T d d T T d d • Hence, ISI in time domain is equivalent to Frequency Selective distortion in frequency domain. MOOC on M4D 2013
Doppler Effect in Wireless Channels Doppler Effect in Wireless Channels • Relative motion between transmitter (Base station) and receiver (mobile) causes a shift in the frequency of the received signal. f h i d i l – This change in frequency is known as the Doppler shift f d . MOOC on M4D 2013
Doppler Effect in Wireless Channels Doppler Effect in Wireless Channels • The bandwidth corresponding to the maximum Doppler shift is known as the Doppler spread B d . MOOC on M4D 2013
Doppler Effect in Wireless Channels Doppler Effect in Wireless Channels • The Doppler shift f d for a path is given by the The Doppler shift f d for a path is given by the expression below. v v cos f f d c c MOOC on M4D 2013
Doppler Effect in Wireless Channels Doppler Effect in Wireless Channels • f is the carrier frequency. f c is the carrier frequency. • v is the relative velocity of motion between transmitter and mobile receiver transmitter and mobile receiver. • is the angle between the incident wave and the direction of observer motion direction of observer motion. • At 60 Km/Hr, 2 GHz and = 0, f d ~ 55 Hz. 5 1 9 f f 60 60 2 2 10 10 55 55 Hz Hz d 8 18 3 10 MOOC on M4D 2013
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