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II. Medium Access & Cellular Standards
© Tallal Elshabrawy 3 Multiple Access in Wireless Communications Medium Access Mechanisms to allow many users to simultaneously share a finite amount of wireless communication channels Narrowband Systems The bandwidth of a single communication channel is smaller than the expected coherence bandwidth Wideband Systems The bandwidth of a single communication channel is much larger than the expected coherence bandwidth
© Tallal Elshabrawy 4 Frequency Power BcBc B guard BtBt B t : Total Spectrum Allocation B guard : Guard Band at edge of Allocated Bandwidth B c : Channel Bandwidth Number of Channels in supported in an FDMA System (N) Frequency Division Multiple Access (FDMA)
© Tallal Elshabrawy 5 Efficiency of TDMA: A measure of the percentage of transmitted data that contains information as opposed to providing overhead for the access scheme b OH : Total Number of Overhead Bits per TDMA Frame b T : Total Number of Bits per TDMA Frame Number of Channels in supported in an TDMA System (N) with m Slots per TDMA Frame Time Division Multiple Access (TDMA)
© Tallal Elshabrawy 6 The channel bandwidth in FDMA systems is smaller than that in TDMA systems FDMA systems are less susceptible to frequency selective fading FDMA systems have a larger number of carriers and therefore might suffer from higher costs because of the need for a carrier (i.e., oscillator) per frequency channel TDMA vs FDMA: Channel Bandwidth
© Tallal Elshabrawy 7 FDMA supports continuous transmission TDMA features discontinuous transmission TDMA must use digital communications while FDMA could support analog and digital communications TDMA provides an opportunity to regulate battery consumption by turning off the transmitter when not in use TDMA Enables MAHO to simplify handoffs as mobile units may listen to transmissions from other base stations during idle times The FDMA mobile unit uses duplexers to allow for simultaneous transmission and reception TDMA uses different timeslots for transmission and reception and therefore duplexers need could be avoided TDMA requires synchronization and guard time overhead bits TDMA vs FDMA: Transmission Mode
© Tallal Elshabrawy 8 TDMA opens an avenue for Dynamic capacity allocation by allocating different number of timeslots per frame to different users TDMA vs FDMA: Dynamic Capacity Allocation
© Tallal Elshabrawy 9 Code Division Multiple Access (CDMA): Basic Concepts Signal Spreading: Transmission bandwidth significantly exceeds information bandwidth Each User is assigned a unique spreading Code. Processing Gain: Number of chips per data symbol. Processing gain reflects the ratio between the transmission and information bandwidths. Data Signal Spreading Data Spreading Code Received Signal Spreading Code Transmitted Signal S(f) f f T Symbol T Chip
© Tallal Elshabrawy 10 Signal De-Spreading: Multiplying the received signal by the spreading code De-spreading of the received signal with the same spreading code that was used for spreading restores the original data De-Spread Signal Signal De-Spreading Spreading Code De-Spread Signal Spreading Code at Rx Received Signal S(f) f f T Symbol T Chip Received Signal T Chip Spreading Code at Tx Code Division Multiple Access (CDMA): Basic Concepts
© Tallal Elshabrawy 11 Signal De-Spreading: Multiplying the received signal by the spreading code De-spreading of the received signal with a different spreading code than that was used for spreading does not restore the original data and maintains bandwidth characteristics of spread signal De-Spread Signal Signal De-Spreading Spreading Code De-Spread Signal Spreading Code at Rx Received Signal S(f) f T Symbol T Chip Received Signal Spreading Code at Tx f Code Division Multiple Access (CDMA): Basic Concepts
© Tallal Elshabrawy 12 De-Spread Signal Symbol Detection: De-spreading using the same spreading code that was used for spreading T Symbol 4 -4 -4 4 De-Spread Signal T Symbol 0 0 Symbol Detection: De-spreading using a different spreading code than that used for spreading Code Division Multiple Access (CDMA): Basic Concepts
© Tallal Elshabrawy 13 CDMA Operation m 1 (t) c 1 (t) m 1 (t)c 1 (t) m 2 (t) c 2 (t) m 2 (t)c 2 (t) Transmitter for User 1 Transmitter for User 2 Receiver for User 1 Wireless Channel m 1 (t)c 1 (t)+ m 2 (t)c 2 (t) Receiver for User 2 c 1 (t) c 2 (t) m 1 (t)+ m 2 (t)c 1 (t)c 2 (t) m 2 (t)+ m 1 (t)c 1 (t)c 2 (t) m 1 (t)+e 1 (t) m 2 (t)+e 2 (t) m’ 1 (t) m’ 2 (t) m i (t): Information Message of User i c i (t): Spreading code of user i e i (t): Interference sensed at receiver of user I m’ i (t): Message detected at receiver Important Note: The value of e i (t) depends on the cross correlation properties between c 1 & c 2 e i (t)=0 if c 1 & c 2 are orthogonal
© Tallal Elshabrawy 14 CDMA in Military Applications The CDMA concept has been introduced as early as 1970s in military applications to elude jamming signals frequency Spectral density frequency Spectral density Jamming signal signal De-spreading
© Tallal Elshabrawy 15 CDMA in Wireless Communications
© Tallal Elshabrawy 16 Spreading Code Requirements Good CDMA spreading codes should be characterized by relatively low cross-correlation properties to minimize multiple access interference (MAI). Good CDMA spreading codes should be characterized by low autocorrelation properties to minimize inter-symbol interference due to multi-path channels Ideally it is desirable to have both correlation functions to approach zero
© Tallal Elshabrawy 17 Spreading Codes: Walsh-Hadamard Codes Walsh functions provide orthogonal spreading codes Walsh matrices constructed recursively as follows: c1c1 c2c2 c1c1 c2c2 c3c3 c4c4
© Tallal Elshabrawy 18 Orthogonal Variable Spreading Factor (OVSF) using Walsh Codes Available system bandwidth determines the value of T chip T Symbol =SF x T chip Bit rate is inversely proportional to SF OVSF permits users to be allocated different SF (i.e., bit rates) SF = 1 SF = 2 SF = 4 SF = 8 SF = 16 OVSF TREE c 11 c 21 c 22 c 41 c 42 c 43 c 44
© Tallal Elshabrawy 19 If a user is allocated a certain code, then all codes that branch from such code cannot be allocated to any other user c 21 is orthogonal to c 22, c 43, c 44 c 21 is NOT orthogonal to c 41, c 42 SF = 1 SF = 2 SF = 4 SF = 8 SF = 16 OVSF TREE c 11 c 21 c 22 c 41 c 42 c 43 c 44 Orthogonal Variable Spreading Factor (OVSF) using Walsh Codes
© Tallal Elshabrawy 20 Characteristics of Walsh Codes Walsh codes are orthogonal presuming perfect synchronization Walsh codes suffer from poor auto-correlation properties for time offsets that is greater than zero Walsh codes suffer from poor cross-correlation properties when codes are not perfectly synchronized (i.e., for time offsets greater than zero)
© Tallal Elshabrawy 21 Spreading Codes: Maximal Length Sequences Theoretically A randomly chosen sequence should have good auto-correlation properties For CDMA communications, we need to construct spreading codes that have properties of random sequences and can be generated simply at both transmitter and receiver (Pseudorandom sequences) Feedback shift register with appropriate feedback taps can be used to generate pseudorandom sequence
© Tallal Elshabrawy 22 The registers R 0 R 1 R 2 can assume 2 3 possible states State 0 0 0 will result in all zeros output sequence Maximal length sequence is possible if R 0 R 1 R 2 passes through all 2 3 -1 states before repeating Maximal length sequences are achievable using coefficients of primitive polynomials to determine feedback taps R0R0 R1R1 R2R2 g(x) = x 3 + x 2 + 1 g0g0 g2g2 g3g3 The coefficients of a primitive generator polynomial determine the feedback taps TimeR 0 R 1 R 2 01 0 0 10 1 0 21 0 1 31 1 0 41 1 1 50 1 1 60 0 1 71 0 0 Sequence repeats from here onwards output Spreading Codes: Maximal Length Sequences
© Tallal Elshabrawy 23 Maximal Length Sequence Properties For a generator with m registers the sequence length is 2 m -1 Each maximal length sequence has 2 m-1 ones and 2 m-1 -1 zeros For 2 m -1 initial states of registers, we may construct 2 m -1 sequences that are cyclic shifts of each other. The cross-correlation between maximal length sequences generated by the same generator is 1/(2 m -1) (i.e., they are not perfectly orthogonal) Maximal length sequences have good auto-correlation properties
© Tallal Elshabrawy 24 Generating Spreading Codes from Maximal Length Sequences A Single Maximal Sequence Generator: Assign different shifts of same sequence to different users If transmitters are uncoordinated, they might not know each other’s timing and could reuse the same sequence Multiple Maximal Sequence Generator: Different primitive polynomials to determine the feedback taps of each generator Sequences from different generators of same length do not necessarily have good cross-correlation properties There is a limited number of generators (i.e. primitive polynomials) for each sequence length
© Tallal Elshabrawy 25 Spreading Codes: Gold Codes Sum two maximal–length sequences of the same length but using different generators R0R0 R1R1 R2R2 R3R3 R4R4 R5R5 R6R6 R0R0 R1R1 R2R2 R3R3 R4R4 R5R5 R6R6 Example of Gold Code Generator of length 2 7 -1 Sequence 1 Generator: x 7 +x 3 +1 Sequence 2 Generator: x 7 +x 5 +x 4 +x 3 +x 2 +x+1 Gold Sequence
© Tallal Elshabrawy 26 Gold Sequence Properties For each starting state of the first generator, there are 2 m -1 potential starting states of the second generator Gold was able to show that for particular choices of generator polynomials, Gold sequences could have good cross-correlation properties The auto-correlation of Gold codes is proportional to 2/sqrt(2 m -1)
© Tallal Elshabrawy 27 Diversity in CDMA Systems Multi-Path resistant RAKE Receiver can collect energy spread by the small-scale channel Suitable for bursty applications No need for frequency planning (frequency reuse of one) Soft blocking and soft handoff
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