ece 6332, spring, 2012 wireless communications zhu han department of electrical and computer...
Post on 12-Jan-2016
215 Views
Preview:
TRANSCRIPT
ECE 6332, Spring, 2012
Wireless Communications
Zhu Han
Department of Electrical and Computer Engineering
Class 22
April 23th, 2012
OutlineOutline Chapter 13 CDMA, Spread Spectrum
– FHSS
– DSSS
– Multiuser case
spread-spectrum transmission spread-spectrum transmission
Three advantages over fixed spectrum – Spread-spectrum signals are highly resistant to noise and
interference. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background.
– Spread-spectrum signals are difficult to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter.
– Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.
Pseudo Random Sequence GeneratorPseudo Random Sequence Generator Pseudorandom sequence
– Randomness and noise properties
– Walsh, M-sequence, Gold, Kasami, Z4
– Provide signal privacy
Example: Linear Congruential Generators
Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum
Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.
Military
Frequency Hopping ExampleFrequency Hopping Example
Who is Bluetooth?Who is Bluetooth?
Harald Blaatand “Bluetooth” II– King of Denmark 940-981 AC
– Harald Bluetooth was first Christian king of Denmark
– He united Denmark under his rule in the mid-900's
– Similarly, Bluetooth seeks to unite personal computing devices wirelessly
In 1994 – need for low power consumption wireless devices to substitute for cable
Ericsson – driving force behind Bluetooth– Pre-Cell phone– 1998, Ericsson, Nokia, IBM, Toshiba, Intel formed the
Bluetooth Special Interest Group (SIG)– 1999 – Release of Bluetooth protocol– 2002 – IEEE adopted Bluetooth standard, 802.15
working group
Adaptive Frequency-hopping spread Adaptive Frequency-hopping spread spectrum In Bluetoothspectrum In Bluetooth
Works like this …– During a connection, radio transceivers hop from one channel to another– One packet is sent on a channel, two devices then retune their frequencies (hop) to
send the next packet on a different channel. So, if one frequency channel is blocked, limited disturbance to the
Bluetooth communication– Allows several Bluetooth networks to run concurrently without interrupting one
other– Link rate: 1 Mbps, but with overhead, this reduces to 721 kbps– Range for Bluetooth:
10m, can reach up to 100m depending on the power class of the device
Bluetooth version 2.0 + EDR uses an enhanced technology called: Adaptive Frequency Hopping (AFH)
– AFH allows Bluetooth devices to measure quality of wireless signal– Determines if there are bad channels present on specific frequencies due to interference from
other wireless devices.– If bad channels present on a specific frequency, Bluetooth device will adjust its hopping
sequence to avoid them– As a result, the Bluetooth connection is stronger, faster, and more reliable
S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks 10
Bluetooth radio and baseband parameters
Topology Up to 7 simultaneous linksModulation Gaussian filtered FSK
RF bandwidth 220 kHz (-3 dB), 1 MHz (-20 dB)RF band 2.4 GHz ISM frequency band
RF carriers 79 (23 as reduced option)Carrier spacing 1 MHzAccess method FHSS-TDD-TDMAFreq. hop rate 1600 hops/s
S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks 11
Frequency hopping spread spectrum (1)
Bluetooth technology operates in the 2.4 GHz ISM band, using a spread spectrum, frequency hopping, full-duplex
signal at a nominal rate of 1600 hops/second.
2.4000 GHz 2.4835 GHz
Time1 MHz
The signal hops among 79 frequencies (spaced 1 MHz apart) in a pseudo-random
fashion.
83.5 MHz
S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks 12
The adaptive frequency hopping (AFH) feature (from Bluetooth version 1.2 onward) is designed to reduce
interference between wireless technologies sharing the 2.4 GHz spectrum.
2.4000 GHz 2.4835 GHz
Time Interference e.g. due to microwave oven => this frequency in the hopping sequence should be
avoided.
Frequency hopping spread spectrum (2)
S-72.3240 Wireless Personal, Local, Metropolitan, and Wide Area Networks 13
In addition to avoiding microwave oven interference, the adaptive frequency hopping (AFH) feature can also avoid
interference from WLAN networks:
2.4 GHz 2.48 GHz
79 FHSS frequencies
2.4 GHz 2.48 GHz
WLAN channel
... ...
22 MHz (802.11b)16.5 MHz (802.11g)
Frequency hopping spread spectrum (3)
Direct Sequence (DS)-CDMADirect Sequence (DS)-CDMA
It phase-modulates a sine wave pseudo-randomly with a continuous string of pseudo-noise code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.
It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.
System Block DiagramSystem Block Diagram
Jammer/Noise/Interferencej(t)
BPSKModulator
BPSKMatched
Filter
Channel
PseudorandomSequence Generator
PseudorandomSequenceGenerator
SourceData
OutputData
(to detector)
b(t)
c(t)
s(t) x(t) u(t)
c(t)
y(t) rn
y(t) = j(t) + x(t)u(t) = s(t) + j(t)c(t)
rn = bn + jammer projection
s(t) = b(t)cos(wot)x(t) = s(t)c(t)
Unique code to differentiate all users
Sequence used for spreading have low cross-correlations
Allow many users to occupy all the frequency/bandwidth allocations at that same time
Processing gain is the system capacity– How many users the system can support
Spreading & DespreadingSpreading & Despreading
Spreading– Source signal is multiplied by a PN signal
Processing Gain:
Despreading– Spread signal is multiplied by the spreading code
Polar {±1} signal representation
DataRate
ChipRate
T
T
T
TG
b
c
c
bp
1
1
Direct Sequence Spread Spectrum ExampleDirect Sequence Spread Spectrum Example
CDMA Example – transmission from two sourcesCDMA Example – transmission from two sources
1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0
CodeData
0 1 0 0 1 1 0 1 0 0 1 10 1 0 0 1 1 0 1 0 0 1 1
1 0 1 1 0 0 0 1 0 0 1 1 1 0 1 1 0 0
0 0 1 0
1 0 1 0 1 0
1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 0 1 0 1 0 1
1 0 1 1
1 0 1 1 0 0
TransmittedA+B
Signal
A Data
A Codeword
B Data
B Codeword
CodeDataA Signal
B Signal
CDMA Example – recovering signal A at the receiverCDMA Example – recovering signal A at the receiver
0 1 0 0
A+BSignal
received
A Codeword
atreceiver
CodeB)(A
IntegratorOutput
ComparatorOutput
Take the inverse of this to obtain A
CDMA Example – recovering signal B at the receiverCDMA Example – recovering signal B at the receiver
1 1 0 1
A+BSignal
received
B Codeword
atreceiver
CodeB)(A
IntegratorOutput
ComparatorOutput
Take the inverse of this to obtain B
CDMA Example – using wrong codeword at the receiverCDMA Example – using wrong codeword at the receiver
X 0 1 1 Noise
A+BSignal
received
Wrong Codeword
Used atreceiver
IntegratorOutput
ComparatorOutput
Wrong codeword will not be able to decode the original data!
Wideband Interference SuppressionWideband Interference Suppression
Transmitter
Receiver
Narrowband Interference SuppressionNarrowband Interference Suppression
Transmitter
Receiver
CDMACDMA
Rake Receiver
Road MapRoad Map
1XRTT/3XRTT
cdma2000CDMA
(IS 95 A) IS 95 B
GSM
TDMA EDGE UWC-136
GPRS W-CDMA
3X3X3X3X
No 3XNo 3XNo 3XNo 3X
cdmaOnecdmaOneIS-95AIS-95A
cdmaOnecdmaOneIS-95AIS-95A
1999 2000 2001 2002
1X1X1X1XIS-95BIS-95BIS-95BIS-95B
2G 2.5G 3G Phase 1 3G Phase 2
2G: IS-95A (1995)2G: IS-95A (1995)
Known as CDMAOne
Chip rate at 1.25Mbps
Convolutional codes, Viterbi Decoding
Downlink (Base station to mobile):– Walsh code 64-bit for channel
separation
– M-sequence 215 for cell separation
Uplink (Mobile to base station):– M-sequence 241 for channel
and user separation
Standard IS-95, ANSI J-STD-008
Multiple Access CDMA
Uplink Frequency 869-894 MHz
Downlink Frequency
824-849 MHz
Channel Separation 1.25 MHz
Modulation Scheme BPSK/QPSK
Number of Channel 64
Channel Bit Rate 1.25 Mbps (chip rate)
Speech Rate 8~13 kbps
Data Rate Up to 14.4 kbps
Maximum Tx Power
600 mW
2.5G: IS-95B (1998)2.5G: IS-95B (1998)
Increased data rate for internet applications– Up to 115 kbps (8 times that of 2G)
Support web browser format language– Wireless Application Protocol (WAP)
3G Technology3G Technology
Ability to receive live music, interactive web sessions, voice and data with multimedia features
Global Standard IMT-2000– CDMA 2000, proposed by TIA– W-CDMA, proposed by ARIB/ETSI
Issued by ITU (International Telecommunication Union) Excellent voice quality Data rate
– 144 kbps in high mobility– 384 kbps in limited mobility– 2 Mbps in door
Frequency Band 1885-2025 MHz Convolutional Codes Turbo Codes for high data rates
3G: CDMA2000 (2000)3G: CDMA2000 (2000)
CDMA 1xEV-DO– peak data rate 2.4 Mbps– supports mp3 transfer and video conferencing
CDMA 1xEV-DV– Integrated voice and high-speed data multimedia service up to 3.1
Mbps
Channel Bandwidth: – 1.25, 5, 10, 15 or 20 MHz
Chip rate at 3.6864 Mbps Modulation Scheme
– QPSK in downlink – BPSK in uplink
3G: CDMA2000 Spreading Codes3G: CDMA2000 Spreading Codes
Downlink – Variable length orthogonal Walsh sequences for channel separation
– M-sequences 3x215 for cell separation (different phase shifts)
Uplink– Variable length orthogonal Walsh sequences for channel separation
– M-sequences 241 for user separation (different phase shifts)
3G: W-CDMA (2000)3G: W-CDMA (2000)
Stands for “wideband” CDMA
Channel Bandwidth: – 5, 10 or 20 MHz
Chip rate at 4.096 Mbps
Modulation Scheme– QPSK in downlink
– BPSK in uplink
Downlink – Variable length orthogonal sequences for channel separation
– Gold sequences 218 for cell separation
Uplink– Variable length orthogonal sequences for channel separation
– Gold sequences 241 for user separation
Near/Far ProblemNear/Far Problem
Performance estimates derived using assumption that all users have same power level
Reverse link (mobile to base) makes this unrealistic since mobiles are moving
Adjust power levels constantly to keep equal
1k
top related