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Multiple Access Techniques

Multiple Access Techniques. Multiplexing. channels k i. Multiplexing in 4 dimensions space (s i ) time (t) frequency (f) code (c) Goal: multiple use of a shared medium Important: guard spaces needed !. k 1. k 2. k 3. k 4. k 5. k 6. c. t. c. s 1. t. s 2. f. f. c. t. s 3.

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Multiple Access Techniques

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  1. Multiple Access Techniques

  2. Multiplexing channels ki • Multiplexing in 4 dimensions • space (si) • time (t) • frequency (f) • code (c) • Goal: multiple use of a shared medium • Important: guard spaces needed! k1 k2 k3 k4 k5 k6 c t c s1 t s2 f f c t s3 f 2

  3. 1. Frequency multiplex • Separation of the whole spectrum into smaller frequency bands • A channel gets a certain band of the spectrum for the whole time • Advantages: • no dynamic coordination necessary • works also for analog signals • Disadvantages: • waste of bandwidth if the traffic is distributed unevenly • inflexible • guard spaces k1 k2 k3 k4 k5 k6 c f t 3

  4. 2. Time multiplex • A channel gets the whole spectrum for a certain amount of time • Advantages: • only one carrier in themedium at any time • throughput high even for many users • Disadvantages: • precise synchronization necessary k1 k2 k3 k4 k5 k6 c f t 4

  5. 3. Time and frequency multiplex • Combination of both methods • A channel gets a certain frequency band for a certain amount of time • Example: GSM • Advantages: • better protection against tapping • protection against frequency selective interference • higher data rates compared tocode multiplex • but: precise coordinationrequired k1 k2 k3 k4 k5 k6 c f t 5

  6. 4. Code multiplex • Each channel has a unique code • All channels use the same spectrum at the same time • Advantages: • bandwidth efficient • no coordination and synchronization necessary • good protection against interference and tapping • Disadvantages: • lower user data rates • more complex signal regeneration • Implemented using spread spectrum technology k1 k2 k3 k4 k5 k6 c f t 6

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  9. QPSK modulation 8 9

  10. QPSK receiver 9 10

  11. MSK modulation 11

  12. MSK receiption 11 12

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