Improved scheme of Open loop Transmit Diversity (STTD) in the DS- - PowerPoint PPT Presentation

Improved scheme of Open loop Transmit Diversity (STTD) in the DS- CDMA case S. MAYRARGUE , CEA-LETI T. CLESSIENNE, France Telecom R&D IWCT’05 6-10-th June 2005 1

Outline • STTD (Spatio-temporal transmit diversity or « Alamouti scheme ») in the flat fading case • STTD: extension to frequency selective channels in UMTS • Problems with UMTS approach in terms of intra-cell interference • Conditions that spreading codes should fulfill in order to obtain full benefit of STTD for frequency selective channels in a CDMA system • Simulation results • Conclusion IWCT’O5 6-10-th June 2005 2

STTD- flat fading case • Spatio-temporal Transmit diversity original scheme Time : n , n+1 h 1 h 1 s n, s n+1 s n, s n+1 y n, y n+1 y n, y n+1 h 2 h 2 -s n+1 * , s n * -s n+1 * , s n * y h h s b − � � � � � � � � • y n = h 1 s n - h 2 s n+1 * + b n n 1 2 n n = + � � � � � � � � y * h * h * s * b * � � � � � � � � • y n+1 = h 1 s n+1 + h 2 s n * + b n+1 n 1 2 1 n 1 n 1 + + + After matched filtering : 2 2 � � � * � y h + h 0 s � * � b R h h � � � � h h � � � � n 1 2 n n n 1 2 1 2 � � = = + � � � � � � � � � � � � R * * * * y � 2 2 � s * b h h h h − − � � � � 0 h h � � + � � � � � � � � � � n + 1 n + 1 � � n + 1 n + 1 2 1 2 1 1 2 IWCT’O5 6-10-th June 2005 3

STTD: extension to frequency selective case, CDMA • STTD introduced in UMTS standard « open loop- diversity » downlink together with the Rake receiver h 2 data h j h 1 Spreading * h 1 h L despreader despreader + * h j delays despreader + detection • each branch of the Rake receiver corresponds to a flat fading case • STTD flat fading is applied on each branch IWCT’O5 6-10-th June 2005 4

Problem with the previous approach • CDMA is a MULTI CODE transmission system • Simultaneously transmitted codes are orthogonal • Multipath propagation induces orthogonality loss Rake receiver is impaired by MAI (Multiple Access Interference) STTD implemented as above creates additional MAI caused between codes transmitted during different time instants of the STTD symbol pair. IWCT’O5 6-10-th June 2005 5

Example with 2 codes d n , d n+1 Walsh (l) Channel 1 scrambling Walsh (k) s n s n+1 Walsh (l) - d n+1 * , d n * scrambling Channel 2 Walsh (k) y n , y n+1 * * -s n+1 s n spreading Spreading codes are time dependent after scrambling k = diag ( spreading seq. (n))* Walsh (k) C n l = diag ( spreading seq. (n))* Walsh (l) C n IWCT’O5 6-10-th June 2005 6

Example with 2 codes- Time domain d n l C n h 1 (t) s n k C n * - d n+1 l C n y n h 2 (t) * -s n+1 C n k Y n ( τ ) = (s n h 1 (t) -s n+1 k (t) + (d n h 1 (t) - d n+1 l (t) * h 2 (t) ) * C n * h 2 (t) ) * C n Y n+1 ( τ ) = (s n+1 h 1 (t) + s n * h 2 (t)) * C n+1 k (t) + (d n+1 h 1 (t) - d n * h 2 (t)) * C n+1 l (t) IWCT’O5 6-10-th June 2005 7

Examples with 2 codes Frequency domain • For notational simplicity, equations are considered in the frequency domain, which is equivalent to assuming a cyclic prefix, and to assimilate the Rake receiver to a matched filtering. k (f)+ (H 1 (f) d n – d n+1 *H 2 (f)) C n l (f) • Y n (f) = (H 1 (f) s n – s n+1 * H 2 (f)) C n, k (f) + (H 1 (f) d n+1 - d n *H 2 (f)) C n+1 l (f) • Y n+1 (f) = (H 1 (f) s n+1 + s n * H 2 (f)) C n+1 Y ( f ) � � s H ( f ) C k ( f ) H ( f ) C k ( f ) � � − � � n n 1 n 2 n = + � � � � � � Y * ( f ) s * * k * * k * H ( f ) C ( f ) H ( f ) C ( f ) � � � � � � � � n 1 n 1 + + 2 n + 1 1 n + 1 d � l l � H ( f ) C ( f ) H ( f ) C ( f ) � � − n 1 n 2 n � � � � * d H * ( f ) C l * ( f ) H * ( f ) C l * ( f ) � � � � � � n + 1 2 n 1 1 n 1 + + s d � � � � ( f ) ( f ) n n = + H H � � � � s * d * int � � � � n 1 n 1 + + IWCT’O5 6-10-th June 2005 8

Example with 2 codes Matched filtering • Matched filtering the previous equation gives Y ( f ) s � d � R � � � � � � � � * * * � � � � ( f ) n � ( f ) � ( f ) n � ( f ) � � ( f ) n � n = df = df + df � � � � � � � � � � R * * * Y ( f ) s d int � � B � � B � � B � � � � n 1 + n 1 n 1 n 1 + + + • In order to avoid s n+1 (resp. s n ) interfere with estimate of s n (resp. s n+1 ), we have to have � * � ( f ) ( f ) df // identity matrix � B • « usual » MAI is created by dn (resp d n+1 ) interfering on s n (resp. s n+1 ). « extra » MAI would be created by d n+1 (resp. d n ) interfering on s n (resp. s n+1 ). Avoiding this « extra » MAI, we should have � � * ( f ) ( f ) df diagonal matrix = � int B IWCT’O5 6-10-th June 2005 9

Dependencies on codes � 2 2 2 2 2 2 � k k * k k H C + H C H H ( C − C ) � * ( f ) ( f ) � � � 1 n 2 n + 1 1 2 n + 1 n � � df df = � � 2 2 2 2 2 2 B H * H ( C k C k ) H C k H C k − + � � � � 1 2 n 1 n 1 n 1 2 n + + 2 2 k k C = C Clearly, this matrix is proportional to Identity iff n n 1 + 2 � � 2 k * l k * l * * k l * k * l H C C ( f ) H C C ( f ) H H ( C C C C ) + − * � n n 2 1 2 n 1 n n � 1 n + 1 n + 1 + n + 1 ( f ) ( f ) � � � � df df = � � int 2 B 2 * k l * k * l k * l k * l * H H ( C C C C ) H C C ( f ) H C C ( f ) − + � � � 1 2 n + 1 n + 1 n n 1 n + 1 n + 1 2 n n � This matrix is diagonal if C n+1 k (f) = C n k * (f) for all codes NB This second condition implies the first one IWCT’O5 6-10-th June 2005 10

What does it mean? k (f) = C n k * (f) in the frequency domain means • C n+1 k (t) = C n k * (T-t) in the time domain C n+1 Codes on each symbol of the STTD pair should be time reversed and conjugate from each other. Both orthogonal codes and scrambling sequence should have this property. Otherwise, additional MAI arises. This is the case in UMTS!! IWCT’O5 6-10-th June 2005 11

Simulation results for UMTS • Comparison of «usual » STTD with STTD with modified codes • SF =32 • Mobile speed = 3km/h • Channel 3 of 3GPP • Ior/Ioc = 10 dB, 20 dB (ratio between intra cell interference caused by other codes, and extra cell interference, modelled by gaussian noise) • Channel estimation using CPICH • 2 sets of simulations: with and without coding. Coding is a turbo coding with rate = 1/3 IWCT’O5 6-10-th June 2005 12

Simulation results 1/3 IWCT’O5 6-10-th June 2005 13

Simulation results 2/3 IWCT’O5 6-10-th June 2005 14

Simulation results 3/3 IWCT’O5 6-10-th June 2005 15

Conclusion • A simple modification in the spreading code allows a 1dB improvement on STTD performances, compared to the usual UMTS implementation IWCT’O5 6-10-th June 2005 16