ISHIK UNIVERSITY Faculty of Science Department of Information Technology 2017-2018 Fall Course Name: Wireless Networks
Agenda Lecture 4 Multiple Access Techniques: FDMA, TDMA, SDMA and CDMA 1. Frequency Domain Concept 2. Modulation 3. Analogue Modulation 4. Digital modulation 5. Types of Digital Modulation System 6. Hierarchy of Digital Modulation Techniques 7. Digital Modulation Techniques 8. Signal Vector Representation - Constellation 9. QPSK 10. Multi-level (M-ary) Phase and Amplitude Modulation 11. Bandwidth Efficiency Limits 12. Multiple Access Techniques
1. Frequency Domain Concept Fourier transform of a periodic signal is a set of equally-spaced impulses (of different amplitudes) in the frequency domain.
2. Modulation Digital modulation digital data is translated into an analog signal (baseband) ASK, FSK, PSK differences in spectral efficiency, power efficiency, robustness Analog modulation shifts center frequency of baseband signal up to the radio carrier Motivation smaller antennas (e.g., λ/4) Frequency Division Multiplexing medium characteristics Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) 4
Modulation Modulation of a radio wave can be performed by varying one or more of its signal components amplitude, frequency, or phase while keeping its other signal components constant. analog baseband digital signal data digital analog 101101001 modulation modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data 101101001 radio receiver radio carrier 5
3. Analogue Modulation Amplitude Modulation Amplitude Modulation is a technique where the amplitude of the carrier signal varies in accordance with the instantaneous amplitude of the modulating signal. Disadvantage: An amplitude modulation signal is prone to high levels of noise because most noise is amplitude based and obviously AM detectors are sensitive to it.
Frequency Modulation Frequency Modulation is a technique where the frequency of the carrier wave is varied, in accordance with the instantaneous value of the modulating signal. Advantage: FM can give a better signal to noise ratio than AM when wide bandwidths are used. The amplitude noise can be removed by limiting the signal to remove it.
4. Digital modulation Modulation of digital signals known as Shift Keying 1 0 1 Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference Frequency Shift Keying (FSK): needs larger bandwidth 1 0 1 t t Phase Shift Keying (PSK): more complex robust against interference Many advanced variants 1 0 1 t 8
Why Digital Modulation? More information capacity Compatibility with digital data services Higher data security Better quality communications Digital modulation schemes have greater capacity to convey large amounts of information than analog modulation schemes Constraints are: Available bandwidth Permissible power Inherent noise level of the system
5. Types of Digital Modulation System Coherent (synchronous) detection: process receives signal with a local carrier of same frequency and phase Receiver uses the carrier phase to detect signal Cross correlate with replica signals at receiver Match within threshold to make decision Non coherent (envelope) detection: requires no reference wave Does not exploit phase reference information Less complex receiver, but worse performance
6. Hierarchy of Digital Modulation Techniques
7. Digital Modulation Techniques Amplitude Shift Keying ASK
Frequency Shift Keying FSK
Phase Shift Keying PSK And DPSK
8. Signal Vector Representation - Constellation
Signal Vector Representation - Examples
9. QPSK The only way to achieve high data rates with a narrowband channel is to increase the number of bits/symbol The most reliable way to do this is with a combination of amplitude and phase modulation called quadrature amplitude modulation (QAM) Quadrature Phase Shift Keying is effectively two independent BPSK QPSK systems (I and Q), exhibits the same performance but twice the bandwidth efficiency of that of BPSK. In QPSK Carrier phase is changed by 45 0, 135 0, 225 0, 315 0
Quadrature Phase Shift Keying QPSK 00 10 11 10 01 Basic QPSK constellation QPSK constellation Shifted by 450
10. Multi-level (M-ary) Phase and Amplitude Modulation Amplitude and phase shift keying can be combined to transmit several bits per symbol These modulation schemes are often referred to as linear, as they require linear amplification Amplitude modulation on both quadrature carriers 2^n discrete levels, n = 2 same as QPSK 16-QAM has the largest distance between points, but requires very linear amplification. 16-PSK has less stringent linearity requirements, but has less spacing between constellation points, and is therefore more affected by noise M-ary schemes are more bandwidth efficient, but more susceptible to noise.
16-QAM constellation
11. Bandwidth Efficiency Limits BandwidthEfficiencyLimit =log 2 M WhereMisthenumberofdiscretelevelsinthesignal
12. Multiple Access Techniques The Goal of Multiple Access Techniques is multiple use of a shared medium Frequency division multiple access (FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA) Space division multiple access (SDMA)
Analogy of using shared road with shared communication Channel Industrial City TDM Warehouse s FDM Logistics TDM & FDM Logistics 24
Time multiplex A channel gets the whole spectrum for a certain amount of time Advantages: only one carrier in the medium at any time throughput high even for many users c k 1 k 2 k 3 k 4 k 5 k 6 f Disadvantages: precise synchronization necessary t Sridhar Iyer IIT Bombay 25
Time Division Multiple Access time slots one user per slot buffer and burst method Non-continuous transmission digital data digital modulation
Features of TDMA a single carrier frequency for several users transmission in bursts low battery consumption handoff process much simpler FDD : switch instead of duplexer very high transmission rate high synchronization overhead guard slots necessary
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: k 1 no dynamic coordination c necessary works also for analog signals k 2 k 3 k 4 k 5 k 6 f Disadvantages: waste of bandwidth if the traffic is distributed unevenly t inflexible guard spaces
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 to code multiplex but: precise coordination t required c k 1 k 2 k 3 k 4 k 5 k 6 f
Space Division Multiple Access SDMA (Space Division Multiple Access) is traditionally based in di-viding a particular coverage area into discrete sectors (or cells). In theory, the sectors are non-overlapping. Communications are centralized somehow inside each sector (central or base station: intra-cell management). Communications use different parameters in neighboring cells to ensure low or no interference (this requires coordination at a higher level: inter-cell management). Normally, this is ensured by using different frequency bands.
Space Division Multiple Access This technique is characteristic of cellular systems. Multiple access is not managed at the user level, but for groups of users. Inside the cell, other concurrent medium access technique can be used (e.g. TDMA in GSM networks). This technique is more about planning, architecture, coverage and de- signing protocols (e.g. for handoffs) than about proper medium access. In urban areas, a single base station placement could serve several contiguous cells (S). This is called sectorization, and normally S = 3.
Space Division Multiple Access
Code multiplex Each channel has a unique code All channels use the same k 1 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 (will be covered in next lectures) k 2 k 3 k 4 k 5 k 6 t c f