ECE 4371, Fall, 2013 Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu Han Department of Electrical and Computer Engineering Class 18 Nov. 3 rd, 2014

Outline CDMA(code division multiple access) –Introduction –FHSS(frequency hopping spread spectrum) –DSSS(direct sequence spread spectrum) –Application –Road map

Spread Spectrum Modulation Techniques Definition: –The bandwidth of the transmitted signal is much greater than the bandwidth of the original message. –The bandwidth of the transmitted signal is determined by the message to be transmitted and by an additional signal known as the Spreading Code. Two main Spread Spectrum modulation techniques –Frequency Hopping Spread Spectrum (FHSS) –Direct Sequence Spread Spectrum (DSSS) Two major advantages: –Low power density –Redundancy

Spread Spectrum Transmission A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission: –Spread-spectrum signals are highly resistant to noise and interference. u 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. –Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference.

Spread Spectrum Transmission –Spread-spectrum signals are difficult to intercept. u A spread-spectrum signal may simply appear as an increase in the background noise to a narrowband receiver. u An eavesdropper may have difficulty intercepting a transmission in real time if the pseudorandom sequence is not known. –Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. u The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. u Therefore, bandwidth can be utilized more efficiently.

Outline CDMA(code division multiple access) –Introduction –FHSS(frequency hopping spread spectrum) –DSSS(direct sequence spread spectrum) –Application –Road map

Frequency Hopping Spread Spectrum Definition –A 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 use –Highly resistant to deliberate jamming –Limited protection u The JTIDS/MIDS family, HAVE QUICK and SINCGARS Civilian use –In the unregulated 2.4 GHz band

Frequency Hopping Pattern A transmitter "hops" between available frequencies according to a specified algorithm. The transmitter operates in synchronization with a receiver, which remains tuned to the same center frequency as the transmitter.

System Structure

Pseudo Random Sequence Generator Random sequence – Randomness and noise properties – Provide signal privacy Two properties –Randomness and unpredictability. Pure randomness is hard to achieve. –Pseudorandomness, the sequences produced are long and there is no way of predicting the next number from the sequence.

Pseudo Random Sequence Generator Linear Feedback Shift Registers(LFSR). –LFSR are implemented as a circuit consisting of XOR gates and shift register. –The register is a string of 1-bit storage devices. The m+1th bit is output as the result of an operation on the previous m bits in the register. By using a long enough sequence of bits, the sequence may appear to be random, even though it is actually a long cycle. ECE 4371 Fall 2008

FHSS Resistance of Jamming Narrow band jamming FHSS under broadband jamming FHSS under partial band jamming –Jamming on one frequency affects only a few bits

Multiple User Access Only one user with a large bandwidth is too wasteful. Allows multiple user to be admitted over the same frequency. –This creates the possibility of system data rates that are higher than the Shannon limit for a single channel. Each transmitter is assigned a unique code which allows multiple users to be multiplexed over the same physical channel.

Multiple User Access Well designed PN sequence can prevent user collision. In practice, collision cannot be avoided –Lack of a common synchronization clock –More than L active users access

Outline CDMA(code division multiple access) –Introduction –FHSS(frequency hopping spread spectrum) –DSSS(direct sequence spread spectrum) –Application –Road map

Compare with FHSS FHSS adopts noncoherent detection (FSK) DSSS adopts coherent detection (QAM, PSK, PAM)

Direct Sequence Spread Spectrum Modulates with a continuous string of pseudo-noise code symbols called "chips", each of which has a much shorter duration than an information bit. Spreading code spreads signal across wider frequency band –In proportion to number of bits used –10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code Spreads the bandwidth of the data uniformly for the same transmitted power.

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 One method: –Combine input with spreading code using XOR –Input bit 1 inverts spreading code bit –Input zero bit doesn’t alter spreading code bit –Data rate equal to original spreading code

Multiple User Access 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

Narrowband Interference Suppression Transmitt er Receiver

Wideband Interference Suppression Transmitt er Receiver

Outline CDMA(code division multiple access) –Introduction –FHSS(frequency hopping spread spectrum) –DSSS(direct sequence spread spectrum) –Application –Road map

Application FHSS –Bluetooth –frequency-hopping code division multiple access (FH-CDMA) DSSS –CDMA –GPS –WLAN

Bluetooth Operates in the globally unlicensed (but not unregulated) Industrial, Scientific and Medical (ISM) 2.4 GHz short-range radio frequency band.

CDMA Rake Receiver

Comparison

GPS Why spread spectrum in GPS ? –Signal from satellite can be kept from unauthorized use. –Inherent processing gain of spread spectrum allows reasonable power levels to be user. –Each satellite can use the same frequency band, yet no mutual interference.

WLAN Why spread spectrum in WLAN ? –Operates in the range of 2.4 GHz short-range radio frequency band. –Interference resistance from other wireless device

Outline CDMA(code division multiple access) –Introduction –FHSS(frequency hopping spread spectrum) –DSSS(direct sequence spread spectrum) –Application –Road map

Cellular Telephony Evolution ECE 4371 Fall G Analog voice telephony No data connectivity Example: AMPS 2G Digital voice telephony (9.6 kbps-14.4 kbps) CDMA, TDMA Example: GSM, IS-54, IS-95A, cdmaOne 2.5G Digital voice telephony data connectivity Example: GPRS, IS- 958, IS-136, (E)-GPRS 3G Digital voice telephony broadband and data connectivity Example: WCDMA, CDMA2000 TD-SCDMA 3.9G IP based protocol for voice and data Mobile broadband and data connectivity Example: LTE 4G Built-in self organizing mechanisms Interference mitigation and co- existence Example: LTE- Advanced

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 indoor 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

Outline CDMA(code division multiple access) –Introduction –FHSS(frequency hopping spread spectrum) –DSSS(direct sequence spread spectrum) –Application –Road map Thanks

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