Wireless technology is experiencing spectacular developments, due to the emergence of interactive
and digital multimedia applications as well as rapid advances in the highly integrated
systems. For the next-generation mobile communication systems, one can expect
wireless connectivity between any devices at any time and anywhere with a range of multimedia
contents. A key requirement in such systems is the availability of high-speed and
robust communication links. Unfortunately, communications over wireless channels inherently
suffer from a number of fundamental physical limitations, such as multipath fading,
scarce radio spectrum, and limited battery power supply for mobile devices.
Cooperative diversity (CD) technology is a promising solution for future wireless communication
systems to achieve broader coverage and to mitigate wireless channels’ impairments
without the need to use high power at the transmitter. In general, cooperative relaying systems
have a source node multicasting a message to a number of cooperative relays, which
in turn resend a processed version message to an intended destination node. The destination
node combines the signal received from the relays, and takes into account the source’s original
signal to decode the message. The CD communication systems exploit two fundamental
features of the wireless medium: its broadcast nature and its ability to achieve diversity
through independent channels.
A variety of relaying protocols have been considered and utilized in cooperative wireless
networks. Amplify and forward (AAF) and decode and forward (DAF) are two popular
protocols, frequently used in the cooperative systems. In the AAF mode, the relay amplifies
the received signal prior to retransmission. In the DAF mode, the relay fully decodes the
received signal, re-encodes and forwards it to the destination. Due to the retransmission
without decoding, AAF has the shortcoming that noise accumulated in the received signal
is amplified at the transmission. DAF suffers from decoding errors that can lead to severe error propagation. To further enhance the quality of service (QoS) of CD communication
systems, hybrid Automatic Repeat-reQuest (HARQ) protocols have been proposed. Thus, if
the destination requires an ARQ retransmission, it could come from one of relays rather than
the source node.
This thesis proposes an improved HARQ scheme with an adaptive relaying protocol (ARP).
Focusing on the HARQ as a central theme, we start by introducing the concept of ARP.
Then we use it as the basis for designing three types of HARQ schemes, denoted by HARQ
I-ARP, HARQ II-ARP and HARQ III-ARP. We describe the relaying protocols, (both AAF
and DAF), and their operations, including channel access between the source and relay, the
feedback scheme, and the combining methods at the receivers.
To investigate the benefits of the proposed HARQ scheme, we analyze its frame error rate
(FER) and throughput performance over a quasi-static fading channel. We can compare
these with the reference methods, HARQ with AAF (HARQ-AAF) and HARQ with perfect
distributed turbo codes (DTC), for which correct decoding is always assumed at the relay
(HARQ-perfect DTC). It is shown that the proposed HARQ-ARP scheme can always performs
better than the HARQ-AAF scheme. As the signal-to-noise ratio (SNR) of the channel
between the source and relay increases, the performance of the proposed HARQ-ARP
scheme approaches that of the HARQ-perfect DTC scheme.