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

4. 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

5. 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)

6. 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

7. 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

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

9. FDMA
ch1
ch2
ch3
ch4 …
frequency
time

10. 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)

11. TDMA
ch1
ch2
ch3
ch4 …
frequency
time

12. 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)

13. 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

14. GPRS
SGSN
Internet
GGSN IP
Base
Station
MSC
PSTN

15. 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

16. 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

17. 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

18. CDMA code key3 key2 … key1 frequency wide band (1.25 Mhz) time ch3 ch2

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