ECE 6332, Spring, 2012 Wireless Communications Zhu Han Department of Electrical and Computer Engineering Class 22 April 23 th, 2012

Outline Chapter 13 CDMA, Spread Spectrum –FHSS –DSSS –Multiuser case

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 Generator Pseudorandom sequence –Randomness and noise properties –Walsh, M-sequence, Gold, Kasami, Z4 –Provide signal privacy

Example: Linear Congruential Generators

Frequency 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.spread- spectrumcarrier channels pseudorandomtransmitterreceiver Military

Frequency Hopping Example

Who is Bluetooth? Harald Blaatand “Bluetooth” II –King of Denmark 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, working group

Adaptive Frequency-hopping spread spectrum 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. u 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: u 10m, can reach up to 100m depending on the power class of the device Bluetooth version 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 Wireless Personal, Local, Metropolitan, and Wide Area Networks 10 Bluetooth radio and baseband parameters TopologyUp to 7 simultaneous links ModulationGaussian filtered FSK RF bandwidth220 kHz (-3 dB), 1 MHz (-20 dB) RF band2.4 GHz ISM frequency band RF carriers79 (23 as reduced option) Carrier spacing1 MHz Access methodFHSS-TDD-TDMA Freq. hop rate1600 hops/s

S 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 GHz GHz Time 1 MHz The signal hops among 79 frequencies (spaced 1 MHz apart) in a pseudo-random fashion MHz

S 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 GHz GHz Time Interference e.g. due to microwave oven => this frequency in the hopping sequence should be avoided. Frequency hopping spread spectrum (2)

S 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 GHz2.48 GHz 79 FHSS frequencies 2.4 GHz2.48 GHz WLAN channel MHz (802.11b) 16.5 MHz (802.11g) Frequency hopping spread spectrum (3)

Direct 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 Diagram 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 & 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

Direct Sequence Spread Spectrum Example

CDMA Example – transmission from two sources Transmitted A+B Signal A Data A Codeword B Data B Codeword A Signal B Signal

CDMA Example – recovering signal A at the receiver A+B Signal received A Codeword at receiver Integrator Output Comparator Output Take the inverse of this to obtain A

CDMA Example – recovering signal B at the receiver A+B Signal received B Codeword at receiver Integrator Output Comparator Output Take the inverse of this to obtain B

CDMA Example – using wrong codeword at the receiver X Noise A+B Signal received Wrong Codeword Used at receiver Integrator Output Comparator Output Wrong codeword will not be able to decode the original data!

Wideband Interference Suppression Transmitt er Receiver

Narrowband Interference Suppression Transmitt er Receiver

CDMA Rake Receiver

Road Map 1XRTT/3XRTT cdma2000 CDMA (IS 95 A) IS 95 B GSM TDMAEDGE UWC-136 GPRSW-CDMA 3X3X No 3X cdmaOneIS-95AcdmaOneIS-95A X1X IS-95BIS-95B 2G2.5G 3G Phase 13G Phase 2

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 2 15 for cell separation Uplink (Mobile to base station): –M-sequence 2 41 for channel and user separation StandardIS-95, ANSI J-STD-008 Multiple AccessCDMA Uplink Frequency MHz Downlink Frequency MHz Channel Separation1.25 MHz Modulation SchemeBPSK/QPSK Number of Channel64 Channel Bit Rate1.25 Mbps (chip rate) Speech Rate8~13 kbps Data RateUp to 14.4 kbps Maximum Tx Power600 mW

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 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 MHz Convolutional Codes Turbo Codes for high data rates

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 Mbps Modulation Scheme –QPSK in downlink –BPSK in uplink

3G: CDMA2000 Spreading Codes Downlink –Variable length orthogonal Walsh sequences for channel separation –M-sequences 3x2 15 for cell separation (different phase shifts) Uplink –Variable length orthogonal Walsh sequences for channel separation –M-sequences 2 41 for user separation (different phase shifts)

3G: W-CDMA (2000) Stands for “wideband” CDMA Channel Bandwidth: –5, 10 or 20 MHz Chip rate at Mbps Modulation Scheme –QPSK in downlink –BPSK in uplink Downlink –Variable length orthogonal sequences for channel separation –Gold sequences 2 18 for cell separation Uplink –Variable length orthogonal sequences for channel separation –Gold sequences 2 41 for user separation

Near/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 1 k