Diversity ppt

1 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath 3. Diversity 2 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath Main story • Communication over a flat fading channel has poor performance due to significant probability that channel is in a deep fade. • Reliability is increased by providing more resolvable signal paths that fade independently. • Diversity can be provided across time, frequency and space. • Name of the game is how to exploit the added diversity in an efficient manner. 3 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath Baseline: AWGN Channel BPSK modulation Error probability decays exponentially with SNR. 4 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath Gaussian Detection 5 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath Rayleigh Flat Fading Channel BPSK: Coherent detection. Conditional on h, Averaged over h, at high SNR. 6 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath Rayleigh vs AWGN 7 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath Conditional on h, When error probability is very small. When error probability is large: Typical error event is due to channel being in deep fade rather than noise being large. Typical Error Event 8 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath BPSK, QPSK and 4-PAM • BPSK uses only the I-phase.The Q-phase is wasted. • QPSK delivers 2 bits per complex symbol. • To deliver the same 2 bits, 4-PAM requires 4 dB more transmit power. • QPSK exploits the available degrees of freedom in the channel better. • A good communication scheme exploits all the available d.o.f. in the channel. 9 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath Time Diversity • Time diversity can be obtained by interleaving and coding over symbols across different coherent time periods. Coding alone is not sufficient! 10 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath Example:GSM • Amount of time diversity limited by delay constraint and how fast channel varies. • In GSM, delay constraint is 40ms (voice). • To get full diversity of 8, needs v > 30 km/hr at f c = 900Mhz. [...]... Tse&Viswanath 18 3: Diversity Antenna Diversity Receive Transmit Fundamentals of Wireless Communication, Tse&Viswanath Both 19 3: Diversity Receive Diversity Same as repetition coding in time diversity, except that there is a further power gain Optimal reception is via match filtering (receive beamforming) Fundamentals of Wireless Communication, Tse&Viswanath 20 3: Diversity Transmit Diversity If transmitter... full diversity be achieved? Fundamentals of Wireless Communication, Tse&Viswanath 28 3: Diversity Reduction to Transmit Diversity Fundamentals of Wireless Communication, Tse&Viswanath 29 3: Diversity MLSD Achieves Full Diversity Space-time code matrix for input sequence Difference matrix for two sequences first differing at is full rank Fundamentals of Wireless Communication, Tse&Viswanath 30 3: Diversity. .. Communication, Tse&Viswanath 25 3: Diversity Frequency Diversity • Resolution of multipaths provides diversity • Full diversity is achieved by sending one symbol every L symbol times • But this is inefficient (like repetition coding) • Sending symbols more frequently may result in intersymbol interference • Challenge is how to mitigate the ISI while extracting the inherent diversity in the frequency-selective... scalar detection problem: Fundamentals of Wireless Communication, Tse&Viswanath 12 3: Diversity Deep Fades Become Rarer Fundamentals of Wireless Communication, Tse&Viswanath 13 3: Diversity Performance Fundamentals of Wireless Communication, Tse&Viswanath 14 3: Diversity Beyond Repetition Coding • Repetition coding gets full diversity, but sends only one symbol every L symbol times • Does not exploit fully... have full rank Coding gain determined by the determinants of Time -diversity codes have diagonal matrices and the determinant reduces to squared product distances Fundamentals of Wireless Communication, Tse&Viswanath 24 3: Diversity Cooperative Diversity • Different users can form a distributed antenna array to help each other in increasing diversity • Distributed versions of space-time codes may be applicable... Wireless Communication, Tse&Viswanath 31 3: Diversity Frequency Diversity via Rake • • Considered a simplified situation (uncoded) Each information bit is spread into two pseudorandom sequences x A and xB (xB= -xA) • Each tap of the match filter is a finger of the Rake Fundamentals of Wireless Communication, Tse&Viswanath 32 3: Diversity ISI vs Frequency Diversity • In narrowband systems, ISI is mitigated... Tse&Viswanath 21 3: Diversity Space-time Codes • Transmitting the same symbol simultaneously at the antennas doesn’t work • Using the antennas one at a time and sending the same symbol over the different antennas is like repetition coding • More generally, can use any time -diversity code by turning on one antenna at a time • Space-time codes are designed specifically for the transmit diversity scenario... Communication, Tse&Viswanath 22 3: Diversity Alamouti Scheme Over two symbol times: Projecting onto the two columns of the H matrix yields: •double the symbol rate of repetition coding •3dB loss of received SNR compared to transmit beamforming Fundamentals of Wireless Communication, Tse&Viswanath 23 3: Diversity Space-time Code Design A space-time code is a set of matrices Full diversity is achieved if all... Tse&Viswanath 15 3: Diversity Example: Rotation code (L=2) x1, x2 are two BPSK symbols before rotation where d1 and d2 are the distances between the codewords in the two directions Fundamentals of Wireless Communication, Tse&Viswanath 16 3: Diversity Rotation vs Repetition Coding Rotation code uses the degrees of freedom better! Fundamentals of Wireless Communication, Tse&Viswanath 17 3: Diversity Product... transform to the time domain Fundamentals of Wireless Communication, Tse&Viswanath 34 3: Diversity OFDM Fundamentals of Wireless Communication, Tse&Viswanath 35 3: Diversity OFDM OFDM transforms the communication problem into the frequency domain: a bunch of non-interfering sub-channels, one for each sub-carrier Can apply time -diversity techniques Fundamentals of Wireless Communication, Tse&Viswanath 36 . Diversity Fundamentals of Wireless Communication, Tse&Viswanath Antenna Diversity Receive Transmit Both 20 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath Receive Diversity . fade independently. • Diversity can be provided across time, frequency and space. • Name of the game is how to exploit the added diversity in an efficient manner. 3 3: Diversity Fundamentals. exploits all the available d.o.f. in the channel. 9 3: Diversity Fundamentals of Wireless Communication, Tse&Viswanath Time Diversity • Time diversity can be obtained by interleaving and coding