Abstract The research of early ad hoc-like networks, namely multi-hop packet radio networks, was mainly concentrated on lower layers (below network layer). Interestingly, the research of modern ad hoc networks has been mainly restricted to routing protocols. This is understandable, since routing is very challenging in such dynamic networks, but the drawback is that the lower layer models used in the studies are often highly simplified, giving inaccurate or even incorrect results. In addition, modern ad hoc network solutions are usually suboptimal because lower layers, not designed especially for ad hoc networking, are used. Thus, ad hoc networking performance, in general, can be notably enhanced by considering also the design of lower layers. The simple deployment and robustness make wireless ad hoc networks attractive for several applications (e.g., military, public authority, peer-to-peer civilian, and sensor networking), but the performance of the current solutions is typically not adequate. The focus of this work is on the effects of lower layer functionalities on the performance of ad hoc networks, while also taking into account the effects of upper layers (e.g., the effect of application traffic models). A CDMA (Code Division Multiple Access) based dual channel flat ad hoc network solution, incorporating cross-layering between all three lowest layers, is proposed and analyzed. The main element of this is the Bi-Code Channel Access (BCCA) method, in which a common code channel is used for broadcast information (e.g., route discovery), while a receiver-specific code channel is used for all directed transmissions. In addition, a new MAC (Medium Access Control) solution designed for BCCA is presented. Moreover, a novel network layer spreading code distribution (NSCD) method is presented. The advantage of these methods is that they are designed especially to be used in ad hoc networks. With an extensive set of case studies, it is shown that the presented methods outperform the typically used ad hoc network solutions (based on IEEE 802.11) in different kind of scenarios, environments, modeling concepts, and with different parameters. Detailed simulations are carried out in order to analyze the effects of different features at the lower layers, finding also interesting phenomena and dependencies between different layers. It is also shown that close attention should be paid to lower layer modeling even though the overall network performance would be in focus. In addition, various interesting features and behavior models regarding ad hoc networking are brought up. / Tiivistelmä Ensimmäiset tutkimukset rakenteettomista (ad hoc) verkoista esiintyivät nimellä monihyppypakettiradioverkot, ja ne koskivat pääasiassa verkkokerroksen alapuolella olevia tietoliikennekerroksia, mutta nykyiset tutkimukset ovat kuitenkin keskittyneet pääasiassa reititysprotokolliin. Tämä on sikäli ymmärrettävää, että reititys on hyvin haasteellista tämän tyyppisissä dynaamisissa verkoissa, mutta ongelma on, että käytetyt alempien kerrosten mallit ovat usein hyvinkin yksinkertaistettuja, mikä voi johtaa epätarkkoihin tai jopa vääriin tuloksiin. Tämän lisäksi nykyiset ehdotetut rakenteettomien verkkojen ratkaisut ovat usein tehottomia, sillä käytettyjä alempien kerrosten ratkaisuja ei ole tarkoitettu tällaisiin verkkoihin. Niinpä rakenteettomien verkkojen suorituskykyä voidaan parantaa huomattavasti kiinnittämällä huomiota alempien kerrosten suunnitteluun. Verkkojen rakenteettomuus on ajatuksena houkutteleva useissa käyttökohteissa (esimerkiksi sotilasympäristössä, viranomaiskäytössä, käyttäjien välisissä suorissa yhteyksissä ja sensoriverkoissa), mutta suorituskyky ei useinkaan ole riittävällä tasolla käytännön sovelluksiin. Työssä tutkitaan pääasiassa alempien kerrosten toiminnallisuuden vaikutusta rakenteettomien verkkojen suorituskykyyn ottaen huomioon myös ylemmät kerrokset, kuten sovellustason mallit. Työssä esitellään ja analysoidaan koodijakomonikäyttöön (CDMA, Code Division Multiple Access) perustuva kaksikanavainen tasaisen rakenteettoman verkon ratkaisu, jossa hyödynnetään kaikkien kolmen alimman kerroksen välistä keskinäistä viestintää. Ratkaisun ydin on BCCA-menetelmä (Bi-Code Channel Access), jossa käytetään kahta kanavaa tiedonsiirtoon. Yksi kanava on tarkoitettu kaikille yhteiseksi kontrollikanavaksi (esimerkiksi reitinmuodostus voi käyttää tätä kanavaa), kun taas toinen kanava on käyttäjäkohtainen kanava, jota käytetään suoraan viestittämiseen kyseiselle käyttäjälle (varsinainen data yms.). Tämän lisäksi esitellään myös BCCA-menetelmää varten suunniteltu kanavakontrollimenetelmä sekä verkkotasolla toimiva hajotuskooditiedon jakamiseen tarkoitettu menetelmä. Näiden uusien menetelmien etu on se, että ne on suunniteltu nimenomaan rakenteettomiin verkkoihin. Kattavan testivalikoiman avulla osoitetaan, että esitetty uusi ratkaisu peittoaa tyypilliset IEEE 802.11 -standardiin pohjautuvat rakenteettomien verkkojen ratkaisut. Testeissä käytetään erityyppisiä verkkorakenteita, ympäristöjä, mallinnusmenetelmiä ja parametreja. Yksityiskohtaisissa simuloinneissa ajetaan eri testitapauksia ja selvitetään, miten alempien kerrosten eri menetelmät missäkin tapauksessa vaikuttavat suorituskykyyn. Alempien kerrosten mallinnuksessa on syytä olla tarkkana, sillä työssä käy ilmi, että mallinnusvirheillä voi olla suurikin vaikutus myös ylempien kerrosten suorituskykyyn. Työ myös paljastaa useita mielenkiintoisia ilmiöitä ja vuorovaikutussuhteita, jotka liittyvät tutkittujen menetelmien ja yleisesti rakenteettomien verkkojen toimintaan.

The present explosion in digital communications and multi-user wireless cellular networks has urged a demand for more effective modulation methods, utilizing the available frequency spectrum more efficiently. To accommodate a large number of users sharing the same available frequency band, one requirement is the availability of large families of spreading sequences with excellent AC and CC properties. Another requirement is the availability of sets of orthogonal basis functions to extend capacity by exploiting all available degrees of freedom (e.g., temporal, frequency and spatial dimesions), or by employing orthogonal multi-code operation in parallel, such as used in the latest 3GPP and 3GPP2 Wide-band Code Division Multiple Access (WCDMA) modulation standards by employing sets of orthogonal Walsh codes to improve the overall data throughput capacity. The generic Direct Sequence Spread Spectrum (DSSS) transmitter developed in this dissertation has originally been designed and implemented to investigate the practicality and usefulness of complex spreading sequences, and secondly, to verify the concept of non-linearly interpolated root-of-unity (NLI-RU) filtering. It was found that both concepts have a large potential for application in point-to-point, and particularly micro-cellular Wireless Local Area Networks (WLANs) and Wireless-Local-Loop (WLL) environments. Since then, several novel concepts and subsystems have been added to the original system, some of which have been patented both locally and abroad, and are outlined below. Consequently, the ultimate goal of this research project was to apply the principles of the generic DSSS transmitter and receiver developed in this study in the implementation of a WLL radio-frequency (RF)-link, and particularly towards the establishment of affordable wireless multimedia services in rural areas. The extended coverage at exceptionally low power emission levels offered by the new design will be particularly useful in rural applications. The proposed WLL concept can for example also be utilized to add a unique mobility feature to for example existing Private Automatic Branch Exchanges (PABXs). The proposed system will in addition offer superior teletraffic capacity compared to existing micro-cellular technologies, e.g., the Digital European Cordless Telephony (DECT) system, which has been consider by Telkom for employment in rural areas. The latter is a rather outdated interim standard offering much lower spectral efficiency and capacity than competitive CDMA-solutions, such as the concept analyzed in this dissertation, which is based on the use of unique large families of spectrally well confined (i.e., band-limited) constant envelope (CE) complex spreading sequences (CSS) with superior correlation properties. The CE characteristic of the new spreading sequences furthermore facilitates the design of systems with superior power efficiency and exceptionally robust performance characteristics (much less spectral re-growth) compared to existing 2G and 3G modulation standards, in the presence of non-linear power amplification. This feature allows for a system with larger coverage for a given performance level and limited peak power, or alternatively, longer battery life for a given maximum communication distance and performance level, within a specified fixed spreading bandwidth. In addition, the possibility to extend the concept to orthogonal multi-code operation provides for comparable capacity to present 3G modulation standards, while still preserving superior power efficiency characteristics in non-linear power amplification. Conventional spread spectrum communication systems employ binary spreading sequences, such as Gold or Kasami sequences. The practical implementation of such a system is relatively simple. The design and implementation of a spread-spectrum communication system employing complex spreading sequences is however considerable more complex and has not been previously presented, nor been implemented in hardware. The design of appropriate code lock loops for CSS has led to a unique design with 3dB performance advantage compared to similar loops designed for binary spreading sequences. The theoretical analysis and simulation of such a system will be presented, with the primary focus on an efficient hardware implementation of all new concepts proposed, in the form of a WLL RF-link demonstrator. / Dissertation (MEng (Electronic Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / unrestricted

The objective of this research is to investigate different propagation models to determine if specified models accurately predict received signal levels for short path 900 MHz spread spectrum radio systems. The City of Denton, Texas provided data and physical facilities used in the course of this study. The literature review indicates that propagation models have not been studied specifically for short path spread spectrum radio systems. This work should provide guidelines and be a useful example for planning and implementing such radio systems. The propagation model involves the following considerations: analysis of intervening terrain, path length, and fixed system gains and losses.

The need for multiple access strategies arises whenever a number of users have to share a communication resource, since it is usually either cost prohibitive or impractical to dedicate a communication channel to a particular user. A need for such algorithms arises in many instances, particularly in applications utilizing wireless systems where all users access a common channel or medium. Such random access techniques as ALOHA and slotted ALOHA have been successfully implemented in a number of wireless applications. One of the major drawbacks of these algorithms is the necessity of a return path from the central station to each system user, which makes their use both inefficient and expensive for applications where one-way communication would suffice. For such applications, a need remained for a random access algorithm which can maximize the probability of successful message transmission in a one-way communication environment. A random access technique that addresses the above-mentioned need is developed. With this technique, each user sends an original message of predetermined length to a central receiver. The user then retransmits the message a specified number of times in a predetermined interval reserved for the retransmission process. The time interval between each successive retransmission of a given message is random. Assuming total annihilation of all colliding messages, the expression for the probability of successful transmission of a given message in terms of the major channel parameters is theoretically formulated. This technique offers a significant improvement, compared to a single transmission, in ensuring that a message is successfully received. The actual message collision dynamics in this system are experimentally studied using two different types of direct-sequence spread spectrum receivers, one employing a sliding correlator and the other using a matched filter. The spreading code in such systems offers extra protection for messages against possible interferers. The results indicate that it is often possible to properly receive a given message in the presence of co-channel interferers, thus significantly improving the overall system performance. These results are subsequently incorporated with the propagation data for several different types of microcells to arrive at a more precise theory of the link. / Ph. D.

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An adaptive MAC protocol is developed and analyzed that offers a "best case" scenario by allowing the MAC to control medium parameters thereby fully exploiting the channel of an ad hoc wireless LAN. This new, opportunistic medium access control protocol is called CATER (Code Adapts To Enhance Reliability) and is based on the proposed MAC standard for wireless local area networks (WLAN)-IEEE 802.11 [IEE96]. As currently proposed, IEEE 802.11 uses a fixed pseudo-noise (PN) code for spreading the information signal, implying a fixed process gain at the receiver. When the channel degrades, IEEE 802.11 offers only retransmissions at the MAC layer to combat a corrupt medium. However, CATER allows communicating stations to reconfigure their transceivers to use a longer PN code after a prescribed number of failed retransmissions. This longer code increases the process gain of the receiver and reduces the error rate. After the two stations are reconfigured, the source station sends the frame in question. Immediately after that frame is acknowledged, the source station may send additional frames during the reconfigured period. Simulation and emulation are used to demonstrate and validate the adaptive protocol's capabilities. Results show that this new protocol offers substantial improvement in system throughput when the channel degrades to a point that reliable transmission of frames is not feasible in a standard IEEE 802.11 WLAN. Specifically, CATER continues to function, permitting up to 14 percent normalized aggregate throughput at times when IEEE 802.11 permits no frames to pass through the WLAN. In addition, throughput experiences only a small decrease due to protocol overhead during periods when stations experience a good channel with few bit errors. Moreover, CATER does not adversely affect the predominate transport layer protocol (i.e., TCP), and provides equitable service to all stations within the network. / Ph. D.