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Basics of Wireless Networks – Ch. 2 (pp 6-14)

Basics of Wireless Networks – Ch. 2 (pp 6-14). ENTC 455: Wireless Transmission Systems Ana Goulart Assistant Professor Texas A&M University. Existing Wireless Technologies (p.6. Mobile Networks Originator and/or Recipient are in motion Fixed Wireless Networks

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Basics of Wireless Networks – Ch. 2 (pp 6-14)

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  1. Basics of Wireless Networks – Ch. 2 (pp 6-14) ENTC 455: Wireless Transmission Systems Ana Goulart Assistant Professor Texas A&M University

  2. Existing Wireless Technologies (p.6 • Mobile Networks • Originator and/or • Recipient are in motion • Fixed Wireless Networks Known as RLL – radio in the loop FRA – fixed radio access WLL – wireless local loop Subscribers are stationary Fixed cellular systems Proprietary fixed radio access Affects communication: channel behavior, system performance

  3. Examples of WLL • Emerging economies • Remote places with no infrastructure • Rural communities • To connect subscribers to the Public Switched Network • Or to connect subscribers to the Internet (e.g., voice-over-IP)

  4. Analog Cellular Systems – First Generation • AMPS • Analog FM radio system • Frequency re-use • 800 MHz range, 25KHz carrier spacing • Hexagonal cells • To increase capacity => instead of omnidirectional antennas, use 3 sets of directional antennas • Re-use factor: K = 7 x 3 = 21

  5. Cellular Digital Packet Data (CDPD) (with AMPS) • Packet services on an existing cellular telephone network • 30 KHz channel spacing • Assign some channels for data only • Bit rates up to 19.2 kbps

  6. FDMA • Based on frequency division multiplexing (FDM) • Digitally encoded speech signal modulate the carrier (digital FDMA) • Single chanell <=> user • First generation cellular (AMPS)

  7. FDMA ch1 ch2 ch3 ch4 … frequency time

  8. TDMA • 800 MHz and 1.9 GHz markets • TDMA-136 (1988) • 30 KHz carrier spacing in a 3-slot TDMA solution: 3 users access a single radio channel (frequency) without interference, each user has a unique time-slot • Current TDMA systems => 6 slots/channel, each user two slots (TDD) • 3 to 1 gain over AMPS • Examples of TDMA systems: Japan Digital Cellular (JDC), North American Digital Cellular (NADC), Global System for Mobile Communications (GSM)

  9. TDMA ch1 ch2 ch3 ch4 … frequency time

  10. GSM – Second Generation • 1991 – Europe • Leading second generation standard (2G) • 900 MHz band, also 1800 MHz (DCS 1800 in UK) • 8-slot TDMA system • 200 KHz carrier spacing • Packet services – GPRS (General Packet Radio Service)

  11. GPRS • Data rates around 170 kbps (using all 8 time slots at the same time) • Connectivity to IP • Overlaying a packet-based air interface on the existing circuit-switched GSM network

  12. GPRS SGSN Internet GGSN IP Base Station MSC PSTN

  13. Comparing Wireless Packet With Circuit Switched Data Circuit switching data Packet switching (GPRS, CDPD) • Call setup • Channel is allocated even if no data is sent • Example: HSCSD = High Speed Circuit Switched Data • No call setup (always on) • Radio resources are used only when users have data to send • Not a dedicated channel to a mobile user • Packets may take different paths, may be lost and corrupted • Retransmission, and data integrity schemes add delays

  14. CDMA • Spread-spectrum technology • At the transmitter: every digital packet is encoded with a key • At the receiver: receiver responds only to that key • Key = Walsh codes • Diverse reception – frequency, spatial, time, and path diversity • 1993: CDMA IS-95 standard or cdmaOne (2G) • Channels are 1,250 KHz

  15. CDMA • Different approach to frequency re-use (theoretical value is 1) • Simplified system planning through the use of the same frequency in every sector of every cell • Subscriber’s power control is very important to control interference • Capacity increases of 8 to 10 times than an AMPS systems, and 4 to 5 times than of a GSM system

  16. CDMA code ch3 key3 ch2 ch5 key2 … ch1 ch 4 key1 frequency wide band (1.25 Mhz) time

  17. CDMA • 2G: IS-95 (narrrowband CDMA) • 3G: new flavors of CDMA

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