ECE 476/ECE 501C/CS Wireless Communication Systems Winter Lecture 9: Multiple Access, GSM, and IS-95

Transcription

1 ECE 476/ECE 501C/CS Wireless Communication Systems Winter 2003 Lecture 9: Multiple Access, GSM, and IS-95 Outline: Two other important issues related to multiple access space division with smart antennas and power control for CDMA A little more detailed look at GSM and IS-95 standards Chapters 9 and 11 Will depend on class presentations for more details on some of these issues. (Nothing on Chapters 8 or 10) I. Multiple Access Techniques Chapter 9 Multiple Access Techniques for Wireless Communications What is the purpose of multiple access techniques? Four options FDMA TDMA CDMA SDMA Space Division Multiple Access Can use combinations of these four (GSM uses both FDMA and TDMA) Lecture 9, Page 1 of 8

2 FDMA (Frequency Division Multiple Access) A session is given a dedicated frequency band to use for the duration of the call 1 user per channel (pair of channels for duplex operation) FDMA Features : 1) FDMA channel relatively narrow ~ 30 khz for AMPS idle when not in use wasted resource 2) FDMA Mobile Systems: can support analog signals lower channel utilization (1 channel per user pair) TDMA (Time Division Multiple Access) A session is given a time slot to use for the duration of the call Multiple sessions use the same frequency band, but only transmit during their regularly occurring time slot Given a frame of N time slots, rotating among all time slots every x seconds Figure 9.4, page 454 Transmit for x/n seconds Wait x(n-1)/n seconds to transmit again TDMA Features : 1) TDMA channel : user data communication done in periodic bursts must use digital modulation idle when not in use wasted resource 2) Bandwidth on demand: can allocate multiple time slots per frame for a higher rate user as is done the GPRS for GSM Lecture 9, Page 2 of 8

3 CDMA (Code Division Multiple Access) PN sequence converts narrowband signal to wideband signal Many users with unique PN codes share same RF channel As # users system BW efficiency but also the noise floor Perfect power control needed to keep PN noise from single user from dominating base station Rx for all other users near/far effect Strongest mobile (near to base) captures base Rx This one mobile appears as a lot of noise at the base station to interfere with other signals farther away. In the extreme, the base station can only hear one mobile, either as a signal or as dominating noise. Power control done by each cell base station (not MSC) Want each mobile to contribute same level of base Rx regardless of distance from base Base station must send control commands to mobile very frequently SDMA (Space Division Multiple Access) Direct radiated energy to each individual user in space Figure 9.8, page 461 Uses adaptive multi-beam antenna arrays multi-beam serve different groups of users by location adaptive must follow mobile units Sectoring primitive non-adaptive form of SDMA How does SDMA improve performance? Lecture 9, Page 3 of 8

4 Use narrow Rx (not Tx) antenna beam at base station to focus in on mobile users base station "hears" very well from one direction decreases ACI & CCI from all other directions by significant amount (10-15 db) acts as spatial filter (only receives signals well from certain points in space) How does it work? Antenna + Digital Signal Processing (DSP) Technology Antenna array (many individual antenna elements) required to have : multiple beams can create a focused capability by combining the signals from the multiple antennas in a weighted fashion, high directivity can be accomplished adaptive change pattern width & direction vs. time requires significant DSP solutions SDMA technology is currently being deployed To further and in some cases greatly increase capacity of existing systems Common terminology Smart antennas Adaptive antenna arrays Chapter 11 Wireless Systems and Standards III. Global System for Mobile (GSM) System History : First created by Groupe Spécial Mobile committee First cellular system to use digital modulation (which makes it a second generation technology) First cellular system to specify network features and services (like caller ID which were not available with first generation analog systems) Most widely used standard for cellular systems in the world Lecture 9, Page 4 of 8

5 Quite complicated standard Operates in European 900 MHz band PCS version of GSM: DCS 1800 for GHz U.S. PCS band (e.g. T-Mobile) Services and Features : Teleservices: mobile telephony, FAX Data services: packet switched protocols (IP traffic), computer-tocomputer communication 300 to 9600 bps Call forwarding, closed user groups, caller ID, short messaging service (SMS) (160 character alphanumeric paging messages) Remarkable feature from user perspective: Subscriber Identity Module (SIM) memory module that stores the brains of the phone service features, privacy keys, ID, user-specific info can be used on any GSM phone SIM's can be smart cards or plug-in modules Another remarkable feature: Digital encryption secure communication System Characteristics : Uses combination of FDMA and TDMA 200 khz FDMA channels Each FDMA channel is shared among as many as 8 subscribers using TDMA 8 timeslots per frame 3 timeslot spacing between Tx/Rx on forward/reverse links user channel BW = 25 khz (200 khz/8 users) Data rate: kbps (1625/6 kbps) in 200 khz RF BW Lecture 9, Page 5 of 8

6 0.3 GMSK modulation on forward/reverse links Special type of continuous phase frequency shift keying Baseband pulse shaping to improve spectral efficiency Shaped to a Gaussian shape Total GSM Capacity (125 channels in 25 MHz) * (8 timeslots per channel) = 1000 channels per cell Actual implementation only uses 24.8 MHz (100 khz guard band on edges of spectrum), so 124 channels * 8 TS = 992 channels Complex signaling and control system provides great flexibility and wide range of user services significant advantage over other systems Speech Coding uses vocoding analyzes & extracts key parameters using knowledge of speech characteristics - vowels, different types of consonants in contrast to simply digitizing the analog waveform Produces very low data rates (~ 5 to 15 kbps) for GSM, full rate = 13 kbps voice activity detection (VAD) so mobile Tx operates in discontinuous transmission mode (DTX) and only uses power when it has something to send. improves battery life and reduces interference IV. CDMA Digital Cellular Standard (IS 95) System History : Developed for AMPS band as alternative to Digital AMPS (USDC) Each user within a cell can use the same radio channel, as can users in adjacent cells No need for frequency planning Each IS-95 channel uses 1.25 MHz (about 40 AMPS channels) Theoretical capacity advantages over TDMA technology Lecture 9, Page 6 of 8

7 Macroscopic spatial diversity advantages RAKE Rx (forward link) and Soft Handoff (reverse link) GHz version (J STD 8) available for U.S. PCS band (Sprint PCS) System Characteristics : AMPS band, 45 MHz Forward/Reverse channel separation 1.25 MHz RF channel BW (after spreading) 10 IS-95 RF channels in 12.5 MHz of AMPS spectrum per provider User data rate changes in real time depending on voice activity 1200 to 9600 bps low data rate for silent period (but not zero) data repeated multiple times for low rates to obtain constant 19.2 kbps Forward link Up to 63 forward traffic channels per 1.25 MHz FDM channel (630 possible voice channels per cell). Uses 1/2 rate error control coding Spread by 64 orthogonal spreading sequences (Walsh functions) Also scrambled by a pseudorandom sequence of length 2 15 to prevent interference between users in adjacent cells. QPSK modulation base station simultaneously transmits all data for mobile units in a cell users share common channel unique PN sequence for each user spreading sequence chip rate = Mcps = 128 x 9600 bps spreading factor of 128 Lecture 9, Page 7 of 8

8 pilot code also embedded in transmission for timing for power signal measurement to determine if handoff is needed for coherent Rx demodulation in mobile Rx RAKE Rx exploits macroscopic spatial diversity by combining signals from multiple base stations if present power control commands included in frames for rapid power control sent every 1.25 msec instruct mobile to raise or lower transmitted power 1 db steals 800 bps from speech data tightly control power to eliminate near/far problem Reverse link Rate 1/3 coding. variable data rate transmission from 1200 to 9600 bps cycle Tx on/off (DTX) for lower data rate transmission reduces PN interference for other mobile users All 3 rd Generation (3G) proposed wireless standards have some form of CDMA as integral component flexible user BW for data services Next and final lecture , 3G standards, and Ultra Wideband Lecture 9, Page 8 of 8