Multiple AccessDifferent multiple access methods use different strategies to:Divide the radio spectrum into channels (Time or Freq.)Allocate those channels among users (One channel or more)Identify different users on an RF carrier (Fixed or dynamic)
Frequency Division Multiple Access (FDMA) As AMPS and NMT
Time Division Multiple Access (TDMA) As GSM and IS-136
Why Do we need new access techniques? Band Saturation More BW efficiency Capacity (more than ½ The population of theglobe use mobile phones)
CDMA Spread Spectrum Originally developed for military and navigation purposes NO More necessary As Channel BW get smaller More Capacity
CDMA Spread Spectrum•Researches started even from 1950’s•Cellular spread-spectrum application was suggested by Cooper and Nettleton in 1978•IS-95, the narrow band CDMA mobile network, has been standardized in 1993 andcommercial networks were introduced in 1995•3G wideband CDMA systems, such as CDMA2000 in U.S. and European WCDMAdeveloped from 1990s and still ongoing
Signal Spreading Preview In DS spreading, the incoming digital speech signal is multiplied by a digital pseudo-noise (PN) code through a process called bit stream multiplication. The result of DS spreading is an encoded, spread spectrum, CDMA signal.
Signal Spreading Preview Fast frequency hopping This approach assumes that the two stations have: A common knowledge of the frequency hopping plan. Synchronized clocks
Signal Spreading Preview Time Hopping Time hopping assumes that the two stations in communication have: A common knowledge of the time-hopping "plan." Synchronized clocks
The Strengths of CDMA Increased system capacity. Cell sites can be larger. Enhanced privacy due to digital coding of CDMA signals. Enhanced call quality
Bit Stream Multiplication Concepts Bit stream multiplication is the process where an input bit stream of information (usually a digital speech signal) called b(t) is multiplied by a pseudo-noise (PN) code called c(t) to produce a new composite output signal called y(t).
Bit Stream Multiplication Concepts Consider the difference between spreading and scrambling:
Bit Stream Multiplication Concepts Data 01110101111010111101 Code 10110101001010101011 Code 10110101001010101011 Not Data 00110101001010101001
Spreading and Despreading Remember that spreading occurs when a lower bit rate input signal b(t) is multiplied by a higher chip rate spreading code c(t) as shown. Bit Chip
Spreading and Despreading Cont. despreading takes y(t) and multiplies again by the same spreading PN code c(t) to recover the digital information signal b(t).
Scrambling and Descrambling Scrambling is often used for privacy. When scrambling is used for encryption, then descrambling is used for decryption.
Spreading and Power Spectral Density Spreading does not change total power. Spreading changes how the power is distributed over frequency.The following formulas hold true: G (processing gain) = Fc/Fb SF(Spreading Factor)=Rc/Rb =Tb/Tc
PN Codes A binary sequence with fixed length and has noise-like randomness Low correlation between shifted versions of the sequence Low correlation with other user signals (interference) and noise Good autocorrelation properties with own signal in synchronization
RF Transmission Delay and its Effect Radio signals typically travel at a rate of about five microseconds to the mile. Assuming that a mobile is ten miles away from the base station, a 50 microsecond (µs) delay would be the result. 192 Chip = 260.4 nsec
System PerformanceTraditional Systems Performance is measured by signal- to-noise ratio (S/N). The desired users signal versus noise.CDMA Systems Performance is measured by signal-to- interference ratio (S/I). The desired users signal versus interference from other users signals.