CDMA uses ‘spread spectrum’ techniques.CDMA has been likened to a party: When everyone talks at once, no one can be understood, however, if everyone speaks a different language, then they can be understood. CDMA systems have no channels, but instead encodes each call as a coded sequence across the entire frequency spectrum. Each conversation is modulated, in the digital domain, with a unique code (called a pseudo-noise code) that makes it distinguishable from the other calls in the frequency spectrum. Using a correlation calculation and the code the call was encoded with, the digital audio signal can be extracted from the other signals being broadcast by other phones on the network. Since CDMA offers far greater capacity and variable data rates depending on the audio activity, many more users can be fit into a given frequency spectrum and higher audio quality can be provide. The current CDMA systems boast at least three times the capacity of TDMA systems. CDMA technology also allows lower cell phone power levels (200 miliwatts) since the modulation techniques expect to deal with noise and are well suited to weaker signals. The downside to CDMA is the complexity of deciphering and extracting the received signals.
CDMA is a form of Direct Sequence Spread Spectrum communications. In general, Spread Spectrum communications is distinguished by three key elements: 1. The signal occupies a bandwidth much greater than that which is necessary to send the information. This results in immunity to interference and jamming and multi-user access. 2. The bandwidth is spread by means of a code which is independent of the data. The independence of the code distinguishes this from standard modulation schemes. 3. The receiver synchronizes to the code to recover the data. The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time. In order to protect the signal, the code used is pseudo-random. This pseudo-random code is also called pseudo-noise (PN). What can be gained from apparent waste of spectrum?Immunity from various kinds of noise and multipath distortionCan be used for hiding and encrypting signalsSeveral users can independently use the same higher bandwidth with very little interference
Frequency hopping: The signal is rapidly switched between different frequencies within the hopping bandwidth pseudo-randomly, and the receiver knows before hand where to find the signal at any given time. Time hopping: The signal is transmitted in short bursts pseudo-randomly, and the receiver knows beforehand when to expect the burst. Direct sequence: The digital data is directly coded at a much higher frequency. The code is generated pseudo-randomly, the receiver knows how to generate the same code, and correlates the received signal with that code to extract the data.Frequency hoppingSignal is broadcast over seemingly random series of radio frequenciesA number of channels allocated for the FH signalWidth of each channel corresponds to bandwidth of input signalSignal hops from frequency to frequency at fixed intervalsChannel sequence dictated by spreading codeReceiver, hopping between frequencies in synchronization with transmitter, picks up messageAdvantagesEavesdroppers hear only unintelligible blipsAttempts to jam signal on one frequency succeed only at knocking out a few bitsTransmitter operates in one channel at a timeBits are transmitted using some encoding schemeAt each successive interval, a new carrier frequency is selected
Input is fed into a channel encoder Produces analog signal with narrow bandwidthSignal is further modulated using sequence of digits Spreading code or spreading sequence Generated by pseudonoise, or pseudo-random number generatorEffect of modulation is to increase bandwidth of signal to be transmittedOn receiving end, digit sequence is used to demodulate the spread spectrum signalSignal is fed into a channel decoder to recover dataSignal transmission consists of the following steps: A pseudo-random code is generated, different for each channel and each successive connection. The Information data modulates the pseudo-random code (the Information data is “spread”). The resulting signal modulates a carrier. The modulated carrier is amplified and broadcast. Signal reception consists of the following steps: The carrier is received and amplified. The received signal is mixed with a local carrier to recover the spread digital signal. A pseudo-random code is generated, matching the anticipated signal. The receiver acquires the received code and phase locks its own code to it. The received signal is correlated with the generated code, extracting the Information data.
3G is the latest cellular standard that enhances mobile devices with the ability to receive multimedia services, such as live music, interactive web sessions, or video conferencing.The Global standard for 3G is called IMT-2000, which is published by ITU. There are 2 proposals in the standard. CDMA 2000 and W-CDMA.So far, 3G technology has already been deployed in many countries, such as Japan, Korea and HK.
IMT-2000 specifies that 3G mobile should support excellent voice quality. Data rate requirements are 144 in high mobility environment, 384 kbps in limited mobility and at least 2Mbps for in door applications. The transmission frequencies in the standard is from 1885-2025 MHz. In general, the system uses convolution codes, but turbo codes are required for applications that require high data rates.
Spread Spectrum communications is distinguished by three key elements: Bandwidth Code Synchronous reception
3 ways to spread the bw of the signal: Frequency hopping Time hopping Direct sequence
Each bit in original signal is represented by multiple bits in the transmitted signal Spreading code spreads signal across a wider frequency band ◦ Spread is in direct proportion to number of bits used One technique combines digital information stream with the spreading code bit stream using exclusive-OR
Pseudo-Noise Spreading ◦ Bit rate of PN is much higher. (chip rate)
SS modulation is applied on top of a conventional modulation. One can demonstrate that all other signals not receiving the SS code will stay as they are, unspread.
Secure Communication ◦ The signal can be detected by authorized persons who know the PN code. ◦ The signal power is small due to spreading (hide signal inside the noise) ◦ Difficult to jam since it is wideband Multiple Access ◦ Individual users have independent, uncorrelated spreading codes
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
Increased data rate for internet applications ◦ Up to 115 kbps (8 times that of 2G) Support web browser format language ◦ Wireless Application Protocol (WAP)
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
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
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)
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
High data rates => 100 Mbps Multimedia data up to ~102 Mbps IP-oriented network => Internet MIMO using Multiple Antennas Spreading code ◦ Non-binary sequences Multiple Access Technique ◦ MC-CDMA
Greater capacity TDMA and FDMA have a fixed number of slots Frequencies can be reused in all the cells in CDMA. No hard limit to the number of users. Resistance to multipath fading
GPS (Global Positioning System) ◦ Determine time, location and velocity of a person ◦ Consists of 24 satellites to measure the exact location ◦ Each satellite uses the same frequency band with DS/SS. Military Applications