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Wireless Transmission

Wireless Transmission. f (Hz) 10 0 10 2 10 4 10 6 10 8 10 10 10 12 10 14 10 16 10 18 10 20 10 22 10 24. Radio. Microwave. Infrared. UV. X-ray. Gamma Ray. Visible Light. Twisted Pair. Fiber Optics. Satellite. Coax. Terrestrial Microwave. AM Radio. FM Radio.

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Wireless Transmission

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  1. Wireless Transmission

  2. f (Hz) 100 102 104 106 108 1010 1012 1014 1016 1018 1020 1022 1024 Radio Microwave Infrared UV X-ray Gamma Ray Visible Light Twisted Pair Fiber Optics Satellite Coax Terrestrial Microwave AM Radio FM Radio Maritine TV Band LF MF HF VHF UHF SHF EHF THF Electromagnetic Spectrum f (Hz) 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 V: very U: ultra S: super E: extremely T: tremendously L: low M: medium H: high

  3. General Frequency Ranges 指向型天線 • Microwave frequency range • 1 GHz to 40 GHz • Directional beams possible • Suitable for point-to-point transmission • Used for satellite communications • Radio frequency range • 30 MHz to 1 GHz • Suitable for omnidirectional applications • Infrared frequency range • Roughly 3x1011 to 2x1014 Hz • Useful in local point-to-point multipoint applications within confined areas 全向型天線 ISM (industrial, scientific, and medical) bands? Meanings?

  4. ISM Bands • ISM and U-NII (Unlicensed National Information Infrastructure) bands used in the US by wireless devices

  5. What Frequency Bands Are Used by Your Smartphone?

  6. Non-Line-of-Sight (Non-LOS) Conditions • Most mobile transmissions are characterized by non-LOS conditions • Reflection(反射) • Diffraction (衍射) • Refraction (折射) • Scattering (散射) BS: base station MU: mobile user

  7. Non-LOS Conditions:圖例

  8. Non-LOS Conditions • Reflection of electromagnetic waves at a boundary • Occurs when signal encounters a surface that is large relative to the wavelength of the signal

  9. Non-LOS Conditions • Diffraction of electromagnetic waves at the edge of a building • Occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave

  10. Non-LOS Conditions • Scattering of electromagnetic waves • Occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

  11. Non-LOS Conditions • Signal reaches the receiver through reflection and diffraction

  12. Non-LOS Conditions • Signal reaches the receiver through reflection and scattering, as well as via a direct path

  13. Non-LOS Conditions • The most general case of signal receptionconsists of a direct path, a reflected path, a scattered path, and a diffracted path

  14. Challenges with Wireless Communications • Path loss • Shadow fading • Multipath fading • Delayed spread • Co-channel interference • Adjacent channel interference Tx: transmit Rx:receive

  15. Free Space Loss (Ideal Isotropic Antenna) 理想的等向天線 • Pt = signal power at transmitting antenna • Pr = signal power at receiving antenna •  = carrier wavelength • d = propagation distance between antennas • c = speed of light (≈ 3 x 108 m/s) where d and  are in the same units (e.g., meters)

  16. Free Space Loss Considering Gain of Antennas • Gt = gain of transmitting antenna • Gr = gain of receiving antenna • At = effective area of transmitting antenna • Ar = effective area of receiving antenna

  17. 無線通道模型 (1/2) O. Awoniyi and F. A. Tobagi. “Effect of Fading on the Performance of VoIP in IEEE 802.11a WLANs,” IEEE International Conference on Communications, Vol. 6, pp. 3712–3717, June 2004. • 設傳送端(transmitter)和接收端(receiver)距離為d，則receiver收到transmitter傳送資料的SNR值為： • Pt: transmit power; Pr: receive power • Gt: transmitterantenna gains • Gr: receiverantenna gains • Pnoise: receive noise power • Im: implementation margin • Pr(d):發射與接收天線距離為d時的receive power Xs: 期望值為0、標準差為s的高斯(Gauss)隨機變數 Ploss(d): 發送端與接收端距離為d時的訊號路徑損失(path loss)

  18. 無線通道模型 (2/2) l: carrier wavelength L: system loss factor (可設為1) • 上述模型需多個參數，在某些情境下，不易得知所有參數。為簡化考量，以收訊強度(RSS, received signal strength，單位dBm)簡化考量作為替代方案 • RSS(d)and RSS(d0) represent the RSS at distance of d and reference distance d0 • Xs : a zero-mean Gaussian random variable with standard deviation σ (typical value of σ is 6 to 8 dB) that represents the statistical variation in RSS(d) caused by shadowing S. Mohanty, “A New Architecture for 3G and WLAN Integration and Inter-system Handover Management”, Wireless Networks, 12(6):733—745, Nov. 2006

  19. 無線涵蓋 • Radio coverage under different degrees of irregularity (DOI)

  20. 註：Zero-Mean Gaussian Distribution • 例：平均值m=0、標準差s=3的鐘形(bell-shape)分佈 • 常態分佈 • 數值落點雖不同，但其中68.2%的數值落於[m-s,m+s]=[-3,3]的區間內

  21. 註：基本統計 • Mean (平均、均數), arithmetic mean (or average) • 例：Given numbers 10, 12, 18, 6, 4, 5, and 15 • Variance (變異數，常以s2 表示) • Var(X )= 上例之變異數為： • Standard deviation (標準差，常記為s) • 稱為 X 的標準差 • Var(X ) 表示 X 的分散程度；Var(X) 越小 (即 s 愈小) 則 X越集中於平均值E(X)；反之， Var(X)越大 (即 s 愈大) 則 X越離散

  22. Path Loss (訊號路徑損失) • Path loss is a measure of attenuation based only on the distance to the transmitter • Ratio of transmit power to receive power in decibels • Typically the attenuation is much faster than predicted by inverse square law • Generally, Pr d -v = d -v •  = constant • Free space propagation models are not suited to calculate the attenuation v

  23. Path Loss • Received power for different values of loss parameter v (v=2 corresponds to free space) • Increased loss is seen as v goes up

  24. 知道RSS與Path Loss，又如何？有啥用處呢？

  25. 利用“收訊強度”可用於定位

  26. Multipath Fading d+ Transmitter Receiver d • If =/2, these signal will exactly cancel each other • If /2 = 3 inches at 2GHz • Small movements cause large variations in signal strength

  27. Delay Spread • Differences exist in the arrival times of the multipath signal • Speed of light = 1 ft/nsec (= 3  108 m/sec) R 50 ft T 100 ft

  28. Techniques for Wireless Signal Propagation • Cyclic coding for error detection • Convolutional coding or forward error correction • Interleaved data transmission • Frequency hopping

  29. Multiplexing Multiplexing is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource.

  30. Multiplexing (多工) • Capacity of transmission medium usually exceeds capacity required for transmission of a single signal • Multiplexing: carrying multiple signals on a single medium • More efficient use of transmission medium

  31. Multiplexing Techniques • Frequency-division multiplexing (FDM) • Advantage: useful bandwidth of the medium exceeds the required bandwidth of a given signal • Time-division multiplexing (TDM) • Advantage: achievable bit rate of the medium exceeds the required data rate of a digital signal

  32. Duplex Operations • Simplex (單工) • Communication that sends information in one direction only • E.g., radio, TV • Half-Duplex(半雙工) • A communications channel thatoperates in one direction at a time, but that may be reversible • E.g., walkie-talkie (無線對講機) • Time-Division Duplex (TDD) • Full-Duplex(全雙工) • Two-way communication • E.g., cellular phone • Frequency-Division Duplex (FDD)

  33. Multiple Access Methods in Wireless Networks • Frequency Division Multiple Access (FDMA) • Time Division Multiple Access (TDMA) • Code Division Multiple Access (CDMA) • CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) • 例：WiFi • 將於第三大單元說明

  34. guard band . . . Frequency-Division Multiple Access (FDMA) • Each user is assigned a different radio frequency (channel) • Channel information may be transmitted in a dedicated frequency • Simple but inefficient Frequency

  35. guard time frame . . . . . . . . . Time Time-Division Multiple Access (TDMA) • Users are assigned time slots to share a physical channel • Cellular systems always overlays TDMA on top of an FDMA structure

  36. Time-Division Multiple Access (TDMA) • TDMA是一種數位的多工技術 • 兩個以上的訊號或資料流可以同時在一條通訊線路上傳輸，其表現為同一通訊頻道的子頻道。將頻道使用時間由使用者輪流利用，又可以分成同步分時多工(Synchronous TDM)與非同步分時多工(Asynchronous TDM)，其區別在於各個頻道時間的分配方式，若分配時間完全相同就是同步分時多工，反之若可能考慮使用量的多寡而加以修改則是非同步分時多工

  37. TDMA Over FDMA Frequency . . . . . . . . . . . . . . . . . . . . . Time

  38. Code Division Multiple Access (CDMA) • CDMA allows each station to transmit over the entire frequency spectrum all the time • Multiple simultaneous transmissions are separated using coding theory • Analogy: an airport lounge with many pairs of people conversing • TDM: pairs of people in the room taking turns speaking • FDM: pairs of people speaking at different pitches, some high-pitched and some low-pitched such that each pair can hold its own conversation at the same time as but independently of the others • CDMA: each pair of people talking at once, but in a different language. The French-speaking couple just hones in on the French, rejecting everything that is not French as noise

  39. Code Division Multiple Access (CDMA) • Frequency-hopping CDMA • Each station transmits at a particular frequency for a short duration, then hops to another frequency • Direct-sequence CDMA • Each bit of the transmitter is replaced by a codeword, i.e., a sequence of bits (chips) • Orthogonal codewords must be chosen to prevent too much confusion at a receiver • Spread spectrum: uses a wider spectrum that is strictly necessary for carrying the transmitter’s signal

  40. 1 -1 -1 1 -1 1  1 -1 -1 1 -1 1 Decoding function 1 1 1 1 1 1 =6 -1 1 1 -1 1 -1  1-1 -1 1 -1 1 -1-1 -1-1 -1 -1 =-6 CDMA: Basic Principle • Break each bit into k chips • According to a fixed pattern specific to each user • Bit data rate D kD chips per second • Example: the code for A: cA=1,-1,-1,1,-1,1 k =6 1 1,-1,-1,1,-1,1 User A 0 -1,1,1,-1,1,-1 (complement)

  41. CDMA: Multiple Users User A 1 -1 -1 1 -1 1 cA User B 1 1 -1 -1 1 1 cB User C cC 1 1 -1 1 1 -1 1 1 -1 -1 1 1 1, 1,-1,-1, 1, 1 User B  1 -1 -1 1 -1 1 cA Decoding with cA 1 -1 1 -1 -1 1 =0 -1 1 1 -1 1 -1 -1, 1, 1,-1, 1,-1 User A  1 1 -1 -1 1 1 cB Decoding with cB -1 1 -1 1 1 -1 =0 cA and cB are orthogonal

  42. CDMA Example: k = 8 A: (-1 -1 -1 +1 +1 -1 +1 +1) B: (-1 -1 +1 -1 +1 +1 +1 -1) C: (-1 +1 -1 +1 +1 +1 -1 -1) D: (-1 +1 -1 -1 -1 -1 +1 -1) C B+C A+B’ A+B’+C A+B+C+D A+B+C’+D S1= (-1 +1 -1 +1 +1 +1 -1 -1) S2 = (-2 0 0 0 +2 +2 0 -2) S3 = (0 0 -2 +2 0 -2 0 +2) S4 = (-1 +1 -3 +3 +1 -1 -1 +1) S5 = (-4 0 -2 0 +2 0 +2 -2) S6 = (-2 -2 0 -2 0 -2 +4 0) S1 C = S1 C / k = 1 S2 C = S2 C / k = 1 S3 C = S3 C / k = 0 S4 C = S4 C / k = 1 S5 C = S5 C / k = 1 S6 C = S6 C / k = -1

  43. Theoretical Foundation • A  C = 0 and A  C’ = 0 if A and C are orthogonal • X  (A+B+C’+D) = X  A + X  B + X  C’ + X  D • If X = C then X  A = X  B = X  D = 0 • X  (A+B+C’+D) = X  C’

  44. CDMA把多個訊息合而為一，單一訊息給多人接收，大家各取所需、從中萃取各自感興趣之資訊CDMA把多個訊息合而為一，單一訊息給多人接收，大家各取所需、從中萃取各自感興趣之資訊 底下是延伸應用範例

  45. 架構

  46. 工作原理

  47. 新成員加入時的處理程序

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