Systematic Design of Space-Time Trellis Codes for Wireless ...

EE 551/451, Fall, 2006

Communication Systems

Zhu Han

Department of Electrical and Computer Engineering

Class 24

Nov. 14th, 2006

EE 541/451 Fall 2006

OutlineOutline CDMA

– Frequency Hopping

– Direct Sequence

– CDMA one, IS95, 2G

– CDMA 2000 and WCDMA, 3G

OFDM– Basics

– IEEE 802.11a/g, WMAX, 4G

Modem– V32

– V90, 56k Modem

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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.

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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

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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.

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Pseudo Random Sequence GeneratorPseudo Random Sequence Generator Pseudorandom sequence

– Randomness and noise properties

– Walsh, M-sequence, Gold, Kasami, Z4

– Provide signal privacy

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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

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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

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Spreading & DespreadingSpreading & Despreading

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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

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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

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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)

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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

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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

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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)

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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

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Orthogonal frequency-division multiplexing Orthogonal frequency-division multiplexing

Special form of Multi-Carrier Transmission.

Multi-Carrier Modulation.– Divide a high bit-rate digital stream into several low bit-rate

schemes and transmit in parallel (using Sub-Carriers)

-6 -4 -2 0 2 4 6

-0.2

0

0.2

0.4

0.6

0.8

Normalized Frequency (fT) --->

Norm

alized A

mplitu

de -

-->

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OFDM bit loadingOFDM bit loading Map the rate with the sub-channel condition

Water-filling

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OFDM Time and Frequency GridOFDM Time and Frequency Grid

Put different users data to different time-frequency slots

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Guard Time and Cyclic Extension...Guard Time and Cyclic Extension...

A Guard time is introduced at the end of each OFDM symbol for protection against multipath.

The Guard time is “cyclically extended” to avoid Inter-Carrier Interference (ICI) - integer number of cycles in the symbol interval.

Guard Time > Multipath Delay Spread, to guarantee zero ISI & ICI.

M ultipath component that does not cause IS I

guard Symbol guard

guard Symbol guard

guard Symbol guard

M ultipath component that causes IS I

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OFDM Transmitter and ReceiverOFDM Transmitter and Receiver

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Pro and ConPro and Con Advantages

– Can easily be adopted to severe channel conditions without complex equalization

– Robust to narrow-band co-channel interference – Robust to inter-symbol interference and fading caused by multipath

propagation – High spectral efficiency – Efficient implementation by FFTs – Low sensitivity to time synchronization errors – Tuned sub-channel receiver filters are not required (unlike in conventional

FDM) – Facilitates Single Frequency Networks, i.e. transmitter macro-diversity.

Disadvantages– Sensitive to Doppler shift. – Sensitive to frequency synchronization problems – Inefficient transmitter power consumption, since linear power amplifier is

required.

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OFDM ApplicationsOFDM Applications ADSL and VDSL broadband access via telephone network copper wires.

IEEE 802.11a and 802.11g Wireless LANs.

The Digital audio broadcasting systems EUREKA 147, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB.

The terrestrial digital TV systems DVB-T, DVB-H, T-DMB and ISDB-T.

The IEEE 802.16 or WiMax Wireless MAN standard.

The IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) standard.

The Flash-OFDM cellular system.

Some Ultra wideband (UWB) systems.

Power line communication (PLC).

Point-to-point (PtP) and point-to-multipoint (PtMP) wireless applications.

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The IEEE 802.11a/g StandardThe IEEE 802.11a/g Standard

Belongs to the IEEE 802.11 system of specifications for wireless LANs.

802.11 covers both MAC and PHY layers.

Five different PHY layers.

802.11a/g belongs to the High Speed WLAN category with peak data rate of 54Mbps

PHY Layer very similar to ETSI’s HIPERLAN Type 2

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Telephone line bandwidth

Traditional telephone lines can carry frequencies between 300 and 3300 Hz, giving them a bandwidth of 3000 Hz.

All this range is used for transmitting voice, where a great deal of interference and distortion can be accepted without loss of intelligibility.

Data signals require a higher degree of accuracy to ensure integrity. For safety’s sake, therefore, the edges of the bandwidth range are not used for data communications.

We can say that the signal bandwidth must be smaller than the cable bandwidth. The effective bandwidth of a bandwidth line being used for data transmission is 2400 Hz, covering the range from 600 to 3000 Hz.

Today some telephone lines are capable of

handling more bandwidth than traditional lines.

A telephone line has a bandwidth of

almost 2400 Hz for data transmission.

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Modulation/demodulation Bandwidth defines a baseband nature, which means we need to modulate if we

want to use this bandwidth for data transmission. Devices that were traditionally used to do so are called modems.

Modem stands for modulator/demodulator Modulator creates a band-pass analog signal from binary data. Demodulator recovers the binary data from the modulated signal. The computer on left sends binary data to the modulator portion of the

modem; the data is sent as an analog signal on the telephones lines. The modem on the right receives the analog signal, demodulates it through its demodulator, and delivers data to the computer on the right. The communication can be bidirectional, which means the computer on the right can also send data to the computer on the left using the same modulation/demodulation processes.

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The V.32 constellation and bandwidth Most popular modems available are base don the V-series standards published

by ITU-T. V.32

– Uses a combined modulation and encoding technique called trelliscoded modulation.

– Trellis is essentially QAM plus a redundant bit.– Data stream is divided into 4-bit sections. Instead of quadbit, however, a

pentabit (5-bit pattern) is transmitted. The value of the extra bit is calculated from the values of the data bits.

– In any QAM system, the receiver compares each received signal point to all valid points in the constellation and selects the closest point as the intended value. A signal distorted by transmission noise can arrive closer in value to an adjacent point than to the intended point, resulting in a misidentification of the point and an error in the received data. The closer the points are in the constellation, the more likely it is that transmission noise can result in a signal’s being misidentified. By adding a redundant bit to each quadbit, trellis-coded modulation increases the amount of information used to identify each bit pattern and thereby reduces the number of possible matches. For this reason, a trellis-encoded signal is much less likely than a plain QAM signal to be misread when distorted by noise.

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V.32bis constellation and bandwidth The V.32 calls for 32-QAM with a baud rate of 2400. Because only 4 bits of each

pentabit represents data, the resulting speed is 4 * 2400 = 9600bps. V.32bits

– First introduced by ITU-T to support 14,400-bps transmission.– V.32bis uses 128-QAM transmission (7 bits/baud with 1 bit for error control) at a

rate of 2400 baud (2400*6 = 14,400 bps)– V.32bis includes an automatic fall-back and fall-forward feature that enables the

modem to adjust its speed upward or downward depending on the quality of the line or signal.

V.34bis: Provides a bit rate of 28,800 with a 960-point constellation to a bit rate of 33,600 with a 1664-point constellation.

V.90: Bit of 56,000bps, called 56K modems.– These modems may be used only if one party is using digital signature.– Supports asymmetric [downloading rate is 56 Kbps; uploading rate is 33.6Kbps].

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Traditional modems After modulation by the modem, an analog signal reaches the telephone

company switching station, where it is sampled and digitized to be passed through the digital network. The quantization noise introduced into the signal at the sampling point limits the data rate according to the Shannon Capacity. This limit is 33.6 Kbps.

56K Modems: – Uploading: Analog signal must still be sampled at the switching station,

which means the data in uploading is limited to 33.6 Kbps.

– Downloading: No sampling involved. Signal is not affected by quantization noise and subject to the Shannon capacity limitation. So, downloading is limited to 56 Kbps.

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56K modems

56 Kbps because telephone companies sample 8000 times per second with 8 bits per sample. One of the bits in each sample is used for control purposes, which means each sample is 7 bits. The rate is therefore 8000 * 7 or 56000 bps or 56 Kbps

V.92– Modems can adjust their speed, and if the noise allows, they can upload

data at the rate of 48 Kbps. The downloading rate is still 56 Kbps.

– Modem can interrupt the Internet connection when there is an incoming call if the line has call-waiting service.

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Questions?Questions?