无线通信原理与应用第五章

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5.4 SSMA
Spread
Spectrum modulation spreads signal bandwidth to a bandwidth that is several orders of magnitude wider than the signal bandwidth
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TDD system
Mobile Station Transmitter Receiver
Base Station Transmitter
BPF F1
BPF F1 Synchronous Switches
Receiver
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Multiple Access
– Frequency (f) – Time (t) – Code (c) – Space (si) – Packet Radio Goal: multiple use of a shared medium Multiple Access Techniques PR FDMA TDMA CDMA SDMA
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TDMA
Utilized only in digital systems!!! Number of time slots (TS) depends on technology
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Leabharlann Baidu10
TS1
TS2
TS3
TS4
TS5
TS6
Call 1
Call 2 Call 3
BS Antenna
Power Spectral Density (PSD)
Narrowband Waveform
Spread Waveform Noise Level Frequency
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FHMA
Discrete changes of carrier frequency
sequence
of frequency changes determined via pseudo random number sequence Advantages
Since
different slots are used for T and R, duplexers are not required. Equalization is required, since transmission rates are generally very high as compared to FDMA channels.
Spreading (secondary modulation)
spreading sequence (spreading code)
Power Density
10110100
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Radio Frequency
Tx 19
Demodulating
you know the correct spreading sequence (code) ,
not
as robust as DSSS
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DSSS (CDMA)
Each channel has a unique code
All channels use the same spectrum at the same time –bandwidth efficient –no coordination and synchronization necessary Implemented using spread spectrum technology
• ISI is low since the symbol time is large compared to average delay spread No equalization is required.
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FDMA
• Since FDMA is a continuous transmission scheme, fewer bits are needed for overhead purposes (such as synchronization and framing bits) as compared to TDMA. • The FDMA mobile unit uses duplexers. • FDMA requires tight RF filtering to minimize adjacent channel interference. • Nonlinear Effects in FDMA: Intermodulation frequency
you can find the spreading timing which gives the maximum detected power, and
00000000 11111111 00000000
Accumulate for one bit duration
Demodulated data
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FDMA
CDMA TDMA
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Multiple access technologies used in different wireless systems
Cellular Systems
AMPS ( Advanced Mobile Phone system ) GSM ( Global System for Mobile ) IS – 95 ( U.S Narrowband Spread Spectrum )
0 1 0 Base-band Frequency
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• Unlike TDMA or FDMA, CDMA has a soft capacity limit.
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TDMA
TDMA
shares the single carrier frequency with several users, where each user makes use of non-overlapping timeslots.
Data
Transmission for user of TDMA system is discrete
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5.3 TDMA
TDMA systems divide the radio spectrum into time slots and each user is allowed to either transmit or receive in each time slots. Each user occupies a cyclically repeating time slots. Disadvantages: • precise synchronization necessary
(a) The time duration of a bit, (b) The time duration of a slot, Tslot = 156.25 x Tb = 0.577 ms. (c) The time duration of a frame, Tf= 8 x Tslot= 4.615 ms. (d) A user has to wait 4.615 ms, the arrival time of a new frame, for its next transmission.
If GSM uses a frame structure where each frame consists of eight time slots, and each time slot contains 156.25 bits, and data is transmitted at 270.833 kbps in the channel, find (a) the time duration of a bit, (b) the time duration of a slot, (c) the time duration of a frame, and (d) how long must a user occupying a single time slot wait between two successive transmissions. Solution
bursts
• The result is low battery consumption, power save.
• Handoff process is simpler, since it is able to listen for other base stations during idle time slots.
MA Technique FDMA / FDD TDMA / FDD CDMA / FDD CDMA / FDD,CDMA/TDD
WCDMA
TD-SCDMA Cdma2000
CDMA / TDD CDMA / FDD
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5.2 FDMA
Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time
frequency
selective fading and interference limited to short period
simple uses
implementation
only small portion of spectrum at any time
Disadvantages
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• Each user is allocated a unique frequency band or channel. These channels are assigned on demand to users who request service.
• The bandwidth of FDMA channels is narrow (30 KHz) since it supports only one call/ carrier.
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TDMA&FDMA
Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM
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Example
Chapter 5: Multiple Access Techniques for Wireless Communications
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5.1多址技术概述 Duplexing
Frequency domain techniques FDD Time domain techniques TDD
Power Density
received signal
TIME
10110100 01001011 10110100
spreading sequence (spreading code) Radio Frequency
10110100 10110100 10110100
gathering energy !
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How to spread in a CDMA
system?
Direct Sequence (DS)
Power Density
user data
TIME Modulation (primary modulation) Base-band Frequency data rate
Two
spectrum spread methods Hopping Spread Spectrum
•Frequency •Direct
Sequence Spread Spectrum
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Spread Spectrum
Shannon’s capacity equation
C Blog2 (1 S / N )