n n 3 x 108 ms 300 000

速度，波長，周波数 n n 光速： 3 x 108 m/s = 300, 000 km/s 光速 = 波長 x 周波数 n n n AC current: 60 Hz = 5, 000 km FM radio: 100 MHz = 3 m Cellular: 800 MHz = 37. 5 cm Ka band satellite: 20 GHz = 15 mm Ultraviolet light: 1015 Hz = 3 x 10 -7 m Tip: …MHz x …m = 300 2

Decibel (d. B) n n n A ratio is expressed in Decibels (d. B) by computing Ratio(d. B) = 10 log 10(Ratio) ex. 0. 5 = -3 d. B 0. 00000000001 = -200 d. B If a signal is amplified 100 -fold, the gain equals 20 d. B 0 d. BW = 1 Watt -3 d. BW = 0. 5 Watt -10 d. BW = 0. 1 Watt 3

シャドウイングとマルチパス tx antenna rx antenna multipath reflections shadow rx antenna 11

遅延スプレッド RMS Delay spread ( ) Power(d. B) Mean excess delay Typical values for : Indoor: 10 -100 ns Outdoor: 0. 1 -10 s Noise threshold Delay 14

マルチパス干渉 n n Signals arrive out-of-phase Destructive interference 160° phase differential + = 15

Received Power フェージング Flat Fade Deep Fades Distance 16

ドップラー効果 receding object perceives lower frequency approaching object perceives higher frequency 24

Stop-and-Wait ARQ I+3 ACK ACK I+1 ACK i ACK 3 e I+ ACK lost fram 3 retransmit e I+ 2 e I+ frame lost fram 2 e I+ 1 e I+ fram ei fram receiver time out I+2 time out sender retransmit & duplicate discard 27

Go-back-N ARQ 6, 7 and 8 retransmitted 9 K 6 NAC ACK 6 4 ACK e 8 7 and 8 will be discarded fram e 6 e 7 fram e 8 error fram e 6 e 7 fram e 5 e 4 fram e 3 fram e 2 fram e 1 fram receiver ACK sender 28

ACK 10 6 K 6 NAC ACK e 10 fram e 9 fram e 6 4 Only 6 is retransmitted sender fram e 8 fram e 4 e 5 e 6 e 7 fram e 3 error receiver fram e 2 fram e 1 fram ACK Selective-repeat ARQ 29