The cellular networks are evolving to meet the future requirements of data rate,coverage and capacity. The fourth generation mobile communication system, LTEhas been developed to meet these goals. LTE uses multiple antenna features andlarger bandwidths in order to accomplish this task. These features will furtherextend the requirements of data rate, coverage, latency and flexibility. LTE also utilizes the varying quality of the radio channel and the interferencefrom other transmitters by adapting the data rate to the instantaneous channelquality at all the time. This is typically referred to as Link Adaptation. Thelink adaptation fails from time to time due to the varying channel quality as wellas the interference from other transmitters. In order to counteract these failures,retransmission methods are employed. These methods detect the errors on thereceiver side and signals the transmitter for the retransmission of the erroneousdata. The efficiency of link adaptation increases if combined with a properly designedretransmission scheme at the expense of delays due to retransmissions. This master thesis focuses on the study of the retransmission schemes with fasterfeedback, resulting in a reduction in delay. The feedback is generated by makingan early estimate of the decoding outcome and sending it early to the transmitterresulting in faster retransmission. This is important in certain applications wherethe data transmission is intolerant to delays.The thesis work shows by system performance simulations that fast packet retransmission,precisely called Early HARQ Feedback, significantly affects the systemperformance together with the utilization of the link adaptation. The study alsoshows that the link adaptation, in certain scenarios, can be optimized to improvethe system performance. In that respect, it is also possible to increase the numberof retransmissions within the same resource utilization. That optimization is basicallycalled aggressive link adaptation. Consequently, Early HARQ Feedback incombination with aggressive link adaptation provides a large improvement in thedownlink performance of the studied cases.

Relay terminals are often used in tandem in sensor networks to lessen nodal communication burden. In this light we investigate the problem of power allocation amongst nodes in a relay network in order to maximize the overall achievable rate using link adaptive transmission protocols. We focus on the physical layer characteristics and implementation issues of link adaptation in order to develop a bit-level simulator needed to accurately model the rate performance of such a system. Optimal power allocation values, power adaptation policies, and switching levels for several link adaptive policies over a broad class of Rician fading channels are calculated. Furthermore, the maximum achievable rate for two and three link relay networks using our bit-level simulator and optimal power allocation values for collocated channel distributions is simulated. An overall achievable rate comparison between several link adaptive protocols is also investigated. / Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2008-11-26 14:52:27.65

The ability for base stations to be able to beam-form their signals, directing the signal energy to specific users, is a topic of research that has been heavily studied during the last decades. The beam-forming technique aims to increase the signal-to-interference-and-noise-ratio of the user and, consequently, increase the capacity and coverage of the communication system. One such method is the Differential Beam-Forming technique, that has been developed at Ericsson Research. In this version of beam-forming, the beams can be dynamically sharpened and widened when tracking a specific terminal, to try to optimize the signal energy sent to that terminal. Beam-forming, however, makes the link-adaptation algorithm process substantially harder to perform. The reason for this is that the link-adaptation algorithm now has to take into account not only the changing radio environment, but also the changing transmit signal that is being beam-formed. Fortunately, since the beam-formed signal is known at the point of transmission, there should be a potential to utilize this knowledge to make the link-adaptation more efficient. This thesis, investigates how the link-adaptation algorithm could be changed to perform better in beam-forming setups, as well as what information from the beam-forming algorithm that could be included and utilized in the link-adaptation algorithm. This is done by designing and investigating three new link-adaptation algorithms, in the context of Differential Beam-Forming in an LTE FDD system. The algorithms that has been designed are both of a beam-forming-aware and beam-forming-unaware character, meaning if the beam-forming information is utilized within the algorithm, or not. These algorithms have been simulated for different base station antenna array-sizes. Unfortunately, due to simulator restrictions, the terminals have been simulated in a stationary environment, which has proven to be a limiting factor for the results. However, the results still show that smarter beam-forming-aware link-adaptation could possibly be used to increase the performance of the link-adaptation when using beam-forming.

<p>In order to be competitive with emerging mobile systems and to satisfy the ever growing request for higher data rates, the 3G consortium, 3rd Generation Partnership Project (3GPP), is currently developing concepts for a long term evolution (LTE) of the 3G standard. The LTE-concept at Ericsson is based on Orthogonal Frequency Division Multiplexing (OFDM) as downlink air interface. OFDM enables the use of frequency domain link adaptation to select the most appropriate transmission parameters according to current channel conditions, in order to maximize the throughput and maintain the delay at a desired level. The purpose of this thesis work is to study, implement and evaluate different link adaptation algorithms. The main focus is on modulation adaptation, where the differences in performance between time domain and frequency domain adaptation are investigated. The simulations made in this thesis are made with a simulator developed at Ericsson. Simulations show in general that the cell throughput is enhanced by an average of 3% when using frequency domain modulation adaptation. When using the implemented frequency domain power allocation algorithm, a gain of 23-36% in average is seen in the users 5th percentile throughput. It should be noted that the simulations use a realistic web traffic model, which makes the channel quality estimation (CQE) difficult. The CQE has great impact on the performance of frequency domain adaptation. Throughput improvements are expected when using an improved CQE or interference avoidance schemes. The gains with frequency domain adaptation shown in this thesis work may be too small to motivate the extra signalling overhead required. The complexity of the implemented frequency domain power allocation algorithm is also very high compared to the performance enhancement seen.</p>

In recent years, mobile hotspots have been getting much attention of the researchers. They are implemented on moving platforms. Research interests in mobile hotspots are motivated by the demand of seamless mobility. The IEEE 802.16e or mobile WiMAX opens a new door of possibility of mobile broadband. It provides extended mobility support and larger cell coverage. In this thesis we propose a simple link adaptation (LA) algorithm for the mobile hotspots, which are supported by (mobile) WiMAX network. The role of link adaptation (LA) is very important because it controls the physical layer throughput. Therefore, all the higher layers are affected by LA. The main function of an LA algorithm is to select an appropriate burst profile. We consider downlink scenarios of WiMAX supported mobile hotspot. We formulate a discrete value optimization problem for LA, whose objective is throughput maximization. We choose forward error correction block rate (FBER) as constraint. The proposed LA algorithm comes as solution of the optimization problem. The proposed algorithm adapt with MAC layer performance. We develop a downlink channel estimation technique, propose an intra subchannel power allocation strategy, and propose an adaptive automatic repeat request (ARQ) mechanism as part of LA technique. We estimate SNR using channel estimation and intra subchannel power allocation. Then the estimated SNR is adjusted based on velocity of mobile hotspot. Adjusted SNR is used to select optimum burst profile. The performances of the proposed LA algorithm are evaluated through numerical results obtained from link level simulations. According to numerical results, the proposed LA algorithm is able to maintain a certain level quality of service (QoS).

In order to be competitive with emerging mobile systems and to satisfy the ever growing request for higher data rates, the 3G consortium, 3rd Generation Partnership Project (3GPP), is currently developing concepts for a long term evolution (LTE) of the 3G standard. The LTE-concept at Ericsson is based on Orthogonal Frequency Division Multiplexing (OFDM) as downlink air interface. OFDM enables the use of frequency domain link adaptation to select the most appropriate transmission parameters according to current channel conditions, in order to maximize the throughput and maintain the delay at a desired level. The purpose of this thesis work is to study, implement and evaluate different link adaptation algorithms. The main focus is on modulation adaptation, where the differences in performance between time domain and frequency domain adaptation are investigated. The simulations made in this thesis are made with a simulator developed at Ericsson. Simulations show in general that the cell throughput is enhanced by an average of 3% when using frequency domain modulation adaptation. When using the implemented frequency domain power allocation algorithm, a gain of 23-36% in average is seen in the users 5th percentile throughput. It should be noted that the simulations use a realistic web traffic model, which makes the channel quality estimation (CQE) difficult. The CQE has great impact on the performance of frequency domain adaptation. Throughput improvements are expected when using an improved CQE or interference avoidance schemes. The gains with frequency domain adaptation shown in this thesis work may be too small to motivate the extra signalling overhead required. The complexity of the implemented frequency domain power allocation algorithm is also very high compared to the performance enhancement seen.

In recent years, mobile hotspots have been getting much attention of the researchers. They are implemented on moving platforms. Research interests in mobile hotspots are motivated by the demand of seamless mobility. The IEEE 802.16e or mobile WiMAX opens a new door of possibility of mobile broadband. It provides extended mobility support and larger cell coverage. In this thesis we propose a simple link adaptation (LA) algorithm for the mobile hotspots, which are supported by (mobile) WiMAX network. The role of link adaptation (LA) is very important because it controls the physical layer throughput. Therefore, all the higher layers are affected by LA. The main function of an LA algorithm is to select an appropriate burst profile. We consider downlink scenarios of WiMAX supported mobile hotspot. We formulate a discrete value optimization problem for LA, whose objective is throughput maximization. We choose forward error correction block rate (FBER) as constraint. The proposed LA algorithm comes as solution of the optimization problem. The proposed algorithm adapt with MAC layer performance. We develop a downlink channel estimation technique, propose an intra subchannel power allocation strategy, and propose an adaptive automatic repeat request (ARQ) mechanism as part of LA technique. We estimate SNR using channel estimation and intra subchannel power allocation. Then the estimated SNR is adjusted based on velocity of mobile hotspot. Adjusted SNR is used to select optimum burst profile. The performances of the proposed LA algorithm are evaluated through numerical results obtained from link level simulations. According to numerical results, the proposed LA algorithm is able to maintain a certain level quality of service (QoS).

To enable wireless control of factories, such that sensor measurements can be sent wirelessly to an actuator, the probability to receive data correctly must be very high and the time it takes to the deliver the data from the sensor to the actuator must be very low. Earlier, these requirements have only been met by cables, but in the fifth generation mobile network this is one of the imagined use cases and work is undergoing to create a system capable of wireless control of factories. One of the problems in this scenario is when all data in a packet cannot be sent in one transmission while ensuring the very high probability of reception of the transmission. This thesis studies this problem in detail by proposing methods to cope with the problem and evaluating these methods in a simulator. The thesis shows that splitting the data into multiple segments and transmitting each at an even higher probability of reception is a good candidate, especially when there is time for a retransmission. When there is only one transmission available, a better candidate is to send the same packet twice. Even if the first packet cannot achieve the very high probability of reception, the combination of the first and second packet might be able to. / För att möjliggöra trådlös kontroll av fabriker, till exempel trådlös sändning av data uppmätt av en sensor till ett ställdon som agerar på den emottagna signalen, så måste sannolikheten att ta emot datan korrekt vara väldigt hög och tiden det tar att leverera data från sensorn till ställdonet vara mycket kort. Tidigare har endast kablar klarat av dessa krav men i den femte generationens mobila nätverk är trådlös kontroll av fabriker ett av användningsområdena och arbete pågår för att skapa ett system som klarar av det. Ett av problemen i detta användningsområde är när all data i ett paket inte kan skickas i en sändning och klara av den väldigt höga sannolikheten för mottagning. Denna uppsats studerar detta problem i detalj och föreslår metoder för att hantera problemet samt utvärderar dessa metoder i en simulator. Uppsatsen visar att delning av ett paket i flera segment och sändning av varje segment med en ännu högre sannolikhet för mottagning är en bra kandidat, speciellt när det finns tid för en omsändning. När det endast finns tid för en sändning verkar det bättre att skicka samma paket två gånger. Även om det första paketet inte kan uppnå den höga sannolikheten för mottagning så kan kanske kombinationen av det första och andra paketet göra det.

In Earth-to-Space communications, well-known propagation effects such as path loss and atmospheric loss can lead to fluctuations in the strength of the communications link between a satellite and its ground station. Additionally, a less-often considered effect of shadowing due to the geometry of the satellite and its solar panels can also lead to link degradation. As a result of these anticipated channel impairments, NASA's communication links have been traditionally designed to handle the worst-case impact of these effects through high link margins and static, lower rate, modulation formats. This thesis first characterizes the propagation environment experienced by a software-defined radio on the NASA SCaN Testbed through a full link-budget analysis. Then, the following chapters propose, design, and model a link adaptation algorithm to provide an improved trade-off between data rate and link margin through varying the modulation format as the received signal-to-noise ratio fluctuates. / Master of Science

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