Small scale multipath fading 16dB Multipath fading due to constructive and destructive interference of the transmitted waves at very high carrier frequency Wireless Networks 1 Example Transmit antenna d Wall r Receive antenna Difference in phases of direct and reflected waves where is the wavelength of the signal Movement of 4 goes from a peak to a valley this is 0 3m at frequency 900 MHz Wireless Networks 2 1 Fading is also Frequency Selective Why in wavelength depends on frequency When f changes by c 4 d r valley becomes peak Wireless Networks 3 Multipath Channel as LTI System Wireless channels can be modeled as LTI systems where ai i are the gain and delay of path i The impulse response is with frequency response H f Wireless Networks 4 2 How to turn unreliable physical channel into a reliable link 16dB If a bit is sent when the channel is in deep fade it will be lost Wireless Networks 5 Diversity Let probability in deep fade p say 0 2 unacceptable Provide L independent looks at the information bit As long as at least one of the looks are not in deep fade then information can be recovered Probability of error reduced to pL The independent looks can be at different points in space time or frequency Wireless Networks 6 3 Spatial Diversity via Antennas Receive Transmit Both Antennas should be at a wavelength or more apart Wireless Networks 7 Spatial Diversity via Motion aka Time Diversity Can get diversity if we send the same symbol at times separated by approximately v seconds apart For 900 MHz cellular and vehicular speed of 100 km hr v 10 milliseconds If the delay constraint is D milliseconds then we can send the symbol D 10 times so as to get D 10 fold diversity 8 Wireless Networks 4 Time Diversity via Interleaving GSM Example Amount of time diversity limited by delay constraint and how fast channel varies In GSM delay constraint is 40ms voice Wireless Networks 9 Frequency Diversity Multipath wireless channels are frequency selective By repeating the same information bit at different frequencies frequency hopping we get frequency diversity Wireless Networks 10 5 Glamorous History of Frequency Hopping Hedy LaMarr inventor of frequency hopping Wireless Networks 11 Example OFDM in 802 11a 48 sub carriers over a 20 MHz channel time domain signal sub carriers s Information is conveyed in the frequency domain and converted into time domain signal by IDFT Wireless Networks 12 6 OFDM in 802 11a Channel bandwidth determines the total symbol rate number of sub carriers x symbol rate of each sub carrier Aggregate data rates range from 6 9 12 18 24 36 48 54kbps depending on how many bits are modulated into each transmitted symbol on each carrier When channel strength is strong the number of possible levels each symbol can take on is larger conveying more bits per symbol eg 2 4 or 8 levels By coding and interleaving over the sub carriers frequency diversity is achieved Repeating the same symbol across different sub carriers is the simplest form of coding repetition coding Higher spectral efficiency can be achieved by more efficient coding Wireless Networks 13 7
View Full Document
Unlocking...