Modification to the Generic QOSTBCs with Rotated Constellations

Abstract

In the recent time, the need of high data rates intended for mobile communication has been

increasing considerably. To persuade these huge demands of communications exploration of

new methods are going which exploits the restricted resources like bandwidth and power in

the best efficient way possible. MIMO (Multiple input and Multiple Output) scheme is one of

the techniques having multiple antenna elements on both the link ends which mark an

efficient solution for upcoming field of wireless communications systems because it provides

higher data rates by utilizing the space domain beneath the various constraints of limited

bandwidth as well as transmit power. Space-Time Block Coding (STBC) is one of the MIMO

transmit approach that will exploit the transmit diversity. STBCs can be classified into two

major categories, named as, Orthogonal Space-Time Block Codes (OSTBCs) and Non- Orthogonal Space-Time Block Codes (NO-STBCs). The Quasi-Orthogonal Space-Time

Block Codes (QO-STBCs) comes under the category of non-orthogonal-STBCs and is

currently an intensive area of research. The Orthogonal-STBCs are able to achieve full

diversity along with lower decoding complexity, although at the price of loss in some data

rate. Achieving full data rate along with full diversity in case of two antennas is possible.

Taking more than two transmit antennas and achieving full data rate in case of QSTBCs is

not possible; there would be some loss in the diversity gain.

The foremost aim of this work is to grant an integrated theory of QSTBCs considering four

transmit antennas and one receive antenna. The dissertation majorly consists of two parts: In

the former part detailed analysis is done for various QOSTBC’s with a technique employed

of removing interference and also assuming that there is no knowledge of channel on the

transmitter front. And in the later part rotation mechanism is applied on a specific code to

achieve full rate, diversity as well as pairwise decoding.

In the first part of this dissertation a description of QSTBC considering four transmit

antennas is shown. It is also revealed that by applying linear transformations on various

QOSTBC’s and other conventional codes transformation into one another can be done. The

major transformation of converting (4 × 1) MIMO channel into its equivalent virtual (4 x 4)

MIMO highly structured channel is shown for the same quasi orthogonal space time block

codes. The arrangement and the composition of the equivalent channel are of key significance

for measuring the performance of these QSTBCs. The off-diagonal elements of this virtual

channel matrix are accountable for self-interference at the receiver from other signals.

The closer these off-diagonal elements of the virtual channel matrix are to zero, the closer is

the code to an orthogonal one. After that, outcome of decoding technique is illustrated upon

the performance of the code. And finally, a new QOSTBC is anticipated whose decoding

complexity is much less in comparison to the conventional QOSTBC without any loss in its

performance.

In the second part of this dissertation rotation mechanism is employed upon the existing

QOSTBC. . In order to achieve full transmit diversity a novel criteria of rotated constellations for

different transmitted symbols is explored. This scheme outperforms transmit antenna shuffling (TAS)

and also removes its disadvantage of dedicated feedback channel for CSI. The final code thus

achieved is very influential as it grants code rate one, full diversity as well as pairwise decoding.

Finally, simulations show that bit error rate performance is significantly improved by gain of 3dB in

comparison to TAS scheme