Goodput and energy efficiency analysis for wireless relayed communication systems with common destination

Recently, evolution of technology resulted in smaller hardware components to process much more faster. It is now widely possible to use more complex, but better wireless communication systems with these smaller hardware. As a result, proposed high-performance channel coding methods in 90s have been deployed pratically in 2000s. These systems can grant greatly better transfer rates compared to their predecessors and protocol suites like UTRAN, 802.11 have become to serve as an alternative for wired communication systems in case of performance. After 2006, with the help of smartphones, tablets and ultrabook like devices end users requiring wireless communication access has increased rapidly. This has caused wireless communication systems to be more popular and resulted in more researchers to work on this area. Finally the performance improved much more faster that it is now possible to transfer gigabit within a second with recent wireless communication systems such as E-UTRAN (LTE) and 802.11ac. Resources are very limited in wireless communication systems as transmission medium is shared between a great variety of communication systems. This can be concluded to be the main disadvantage of wireless communication systems over wired communication systems. So, utilization of resources as time, frequency and code is crucial in wireless systems. Scheduling of these parameters is a very popular research topic to utilize system performance maximum among users. There is also lots of works done on source and channel coding either to minimize the size of data to be transmitted or reduce error probability during transmission to make it possible transferring more bits with single signal transmission (using higher level of modulation). Source coding is very common in a variety of communication systems (either wireless or wired), greatly reducing the size of raw data either without loss or small amout of loss (which is not recognized generally) such as voice (flac, mp3), image (png, jpeg), video (mpeg). However, channel coding is totally different. It aims to add some extra bits to data bulk to be transmitted. For example; an eight bit data block is represented with ten bit block (8/10 block coding) adding two extra bits for forward error correction. In this example, throughput efficiency is lost by twenty percent; but this loss is greatly covered by forward error correction capability of such a code design making communication systems more immuned against fading, shadowing effects. So, usage of higher leveled modulation schemes becomes possible without increasing transmit power. There are lots of channel coding methods including block, trellis, turbo, ldpc codes; but these are not in the scope of this work. Only a simple precoding method is introduced in this thesis which will be covered in detail later on. The progress mentioned in the first paragraph is closely related with the implementation of MIMO systems. It is shown that transmitting and receiving signals via uncorrelated antenna sets greatly increases the performance of wireless communication systems under fading channels. A lot of work done so far has shown that relevant MIMO systems with special code designs (space – time codes) significantly increases capacity. On the other hand, mobile devices are not big enough to achieve these uncorrelated antenna sets at user side, and they generally become smaller as a market trend whereas their process capacity increases. So it is not always possible to achieve spatial diversity on mobile devices that we can only talk about SIMO, MISO systems in such scenarios. At this point, researchers proposed cooperative communication systems to overcome the physical necessity of multiple antennas in a terminal. Cooperative communication makes it possible users help each other to transmit their signals or dedicated relay systems achieve this functionality. Such a design results in users in the system having virtual antenna arrays that provide MIMO virtually. It is shown that cooperative communication systems are able to grant same kind of diversity gain in wireless communication systems with generic MIMO systems which make this topic very popular recently. Another major point related with capacity of wireless communication systems is the requirement for increase of shared resources as time, frequency, code, etc. with the rise of end users in the system to keep the same service quality. Unfortunately this is not always possible for communication systems that it is a crucial point for the capacity upper bound of wireless communication systems. Besides, just after the cooperative systems, network coding algorithms are proposed to better utilize these shared system resources. Firstly, straight forward network coding which aims to combine different users' data on a common signal is proposed. In these systems, different users transmit their signals traditionally that time, frequency or code is shared between them in the first phase. Later on in the second phase, a dedicated relay combines their data in bit level and transmit combined data with special mapping tables. Finally, users decode the received signal transmitted by relay to obtain other user's data using their own data knowledge. Physical network coding is something similar superceding straight forward network coding that users can transmit simultaneously in the first phase. Straight forward and physical layer network coding methods get lots of attention by researchers and various novel work is proposed on those subjects. These systems are initially designed to address two way relay networks, but later on different protocol sets including common destinated systems are proposed. In this thesis, cooperative communication systems are surveyed with straightforward and physical layer network coding setups in case of goodput and energy efficiency. Six novel protocol schemes are proposed for a generic wireless communication system setup with a common relay and destination shared by two distinct terminals. Protocols are constituted based on two different design criteria focussing each one on reliability and high rate. Later on, they are compared not only between each other but also some other work done recently under fair conditions. High rate protocols aim to transmit maximum source bits using less time slots whereas reliable protocols aim to deliver source bits from two distinct paths; soure – destination and relay – destination. Protocols are grouped with their source transmission schemes: protocol stacks with sources transmitting at the same time are named simultaneous transmission and protocol stacks with sources transmitting separately are named discrete – timed transmission. Simultaneous transmission rely on physical layer network coding and discrete – timed transmission rely on traditional time divided access. Besides in both systems relay acts in a network coded manner. So we can define discrete – timed transmission as a hybrid of time divided and network coded systems. All proposed protocols are designed with ARQ adaptation to prevent propagation of errors. The performance of our six protocols are investigated both by theorical analysis and simulation results in terms of goodput and energy efficiency. These two performance measures are evaluated by outage probability. Outage probability is the probability of transmitting data higher than channel's instaneous capacity. So gooodput is the amount of average data transmitted to destination below channels' capacity, and energy efficiency is the amount of average energy consumed to transmit this data. Based on our results it is shown that simultaneous transmission schemes generally perform better in terms of goodput. On the other hand discrete – timed protocols act better under worse conditions as lower SNR, higher rate. It can also be said that destination oriented ARQ protocols acts better than relay oriented ARQ protocols. In case of energy efficiency generally discrete – timed protocols acts better as these are more prone under extreme conditions. Destination oriented ARQ protocols are more energy efficient than relay oriented ARQ protocols. Last but not least, it is seen that relay based ARQ schemes may outperform other protocols under the path loss effect. It is concluded that relay position optimization is an important design criteria for cooperative systems. Same comment can be generalized for energy efficiency as well.