Konstantinos Ntagkounakis

Novel Channel and Polarization Assignment Schemes for 2---11 GHz Fixed---Broadband Wireless Access Networks

The scarcity of available spectrum in the 2–11 GHz frequency range and the continuously increasing number of users that require broadband communication services suggest that emerging fixed-broadband wireless access (F-BWA) networks will be deployed with aggressive frequency re-use to cope with capacity demands. In this context, co-channel interference may arise in high levels compromising the systems capacity and robust operation. Interference is further increased when limited directionality terminal antennas are employed to support non line-of-sight operation and in the case where an adaptive-time division duplex is selected for efficient radio resource management in asymmetric and time-varying traffic conditions. In this paper frequency channel assignment (CA) and antenna polarization assignment (PA) are considered as a means of mitigating interference. Two novel CA schemes that consider the distinct characteristics of F-BWA are proposed; the rotated-interleaved channel assignment and the non-uniform channel assignment. According to statistical interference simulation analysis the proposed schemes are more efficient in suppressing interference, achieving higher capacity compared to existing schemes while incurring no further complexity. In addition, instead of exploiting the performance of CA and PA schemes independently a framework for a joint CA-PA consideration is presented, where for a particular CA scheme an optimized PA pattern is developed. Results show that this approach improves the CA-PA interoperability increasing the overall performance. The efficiency of the proposed schemes is investigated for both FDD and adaptive-TDD schemes and is verified for various sectorization, frequency re-use and terminal antenna directivity configurations to ensure compatibility with different deployment scenarios.

Cost-Efficient WIMAX Network Deployment: The Hybrid Outdoor / Indoor Dual-Layer Coverage Approach

The process of WIMAX radio network design and deployment is greatly affected by the nature of the customer premises equipment and the intended access service, which may be fixed, nomadic or indoor. This paper advances network design methodologies, traditionally used for fixed access, in the scope of hybrid fixed outdoor / indoor-nomadic networks. With the main objective being performance, time-to-market and cost optimisation, a dual layer outdoor / indoor coverage deployment is proposed which allows for co-existence and performance balancing of different customer profiles. By comparing to purely outdoor or indoor deployments, this approach provides benefits, both in terms of equipment reduction and spectrum usage and optimises the deployment costs, especially in the initial phases of the network. This approach is applicable both for OFDM and future OFDMA systems.

A rotated-interleaved channel assignment scheme in adaptive TDD fixed-broadband wireless access

The concept of channel assignment in adaptive TDD (ATDD) fixed wireless networks is addressed in this paper. A novel rotated-interleaved channel assignment (RICA) scheme is proposed that improves throughput, compared to existing schemes, by up to 9% inducing no further costs. Furthermore, the spectral efficiency between FDD and ATDD modes of operation is investigated considering the same network characteristics and RICA. It appears that according to the traffic asymmetry variations as expressed in the moving TDD boundary, the throughput performance of ATDD is from 2% to 14% less than that of FDD.

Influence of code orthogonality on downlink throughput comparison between MC-CDMA and OFDMA in multi-cellular environments

Fourth generation (4G) mobile access is drawing particular attention as it promises to deliver high data rates and reliable coverage for broadband wireless access. In this paper, Orthogonal Frequency Division Multiplexing Access (OFDMA) and Multi-Carrier Code Division Multiplexing Access (MC-CDMA) are investigated to highlight their strengths and weaknesses as candidates for 4G wireless access. Using a multi-cell environment, MC-CDMA and OFDMA coverage is dimensioned based on the value of the Signal to Interference plus Noise Ratio (SINR). The results show that OFDMA system offers cell coverage of 92.5 % and a maximum actual throughput of 24.2 Mbps/cell (fade margin 10 dB). Using the same assumptions for MC-CDMA, the results show better coverage only with complete orthogonal spreading codes which can guarantee 97 % of user coverage and actual throughput of 28.8 Mbps. These measurements decrease as the orthogonality between the spreading codes is reduced. In the case of 94 % orthogonality the MC-CDMA system achieved 89.3 % coverage and 21.9 Mbps throughput while with a 60 % orthogonality 47 % coverage and 5.8 Mbps throughput was achieved.

Adaptive TDD Synchronisation for WIMAX Access Networks

In adaptive time division duplex (ATDD) wireless systems, severe co-channel interference conditions can occur if the movable downlink/uplink (UL) TDD boundary is not synchronised among all frames in base stations. To reduce interference outage and to improve a systems spectral efficiency, a new single frequency cell (SFC) network architecture is proposed, which allows for distributed boundary synchronisation (DBS) via inter-sector signalling. SFC-DBS dynamically synchronises TDD boundaries among neighbouring sectors for each frame, thus avoiding sector-to-sector interference, while preserving the ATDD radio resource assignment efficiency. Analysis shows that SFC-DBS achieves an additional 6–11 dB in the average UL signal-to-interference ratio, compared with existing channel assignment schemes, which corresponds to 25–50 % capacity gain subject to traffic asymmetry in different sectors. More importantly, the proposed SFC scheme does not incur any further cost in the frequency planning, whereas the DBS scheme requires only minor system modifications. Compared with interference cancellation via antenna arrays and beamforming, SFC-DBS achieves similar performance, albeit without the cost for complex radio transceivers and multiple antenna elements.

Distributed DL/UL Asymmetry Synchronization for Adaptive-Tdd Wireless Access Networks

Adaptive time division duplex (ATDD) systems have the ability to efficiently utilize the radio spectrum and achieve higher capacity, when the traffic conditions are time-varying within a cell or diverse among different cells. However, severe co-channel interference conditions can occur when the movable TDD boundary is not synchronized amongst all frames in the base stations, compromising the spectral efficiency and the systems robust operation. To reduce the interference outage probability, a distributed boundary synchronization (DBS) scheme is proposed that dynamically synchronizes the TDD boundaries amongst neighbouring sectors, thus avoiding sector-to-sector interference, whilst preserving the ATDD efficiency. Results show that DBS achieves an 8 dB improvement in the uplink SIR for the same interference outage, and a 27.5-60% increase in terms of capacity, subject to traffic asymmetry conditions. More importantly the proposed DBS scheme requires only simple modifications in the system

Interference mitigation in TDD fixed broadband wireless access using channel assignment and antenna arrays

In time division duplex (TDD) fixed broadband wireless access networks, co-channel interference that arises from the correlation of downlink and uplink signals in different sectors may considerably degrade the spectral efficiency. To overcome the complexity and costs of complete frame-TDD boundary synchronization in the network, a combined channel assignment (CA) and antenna array (AA) approach is proposed to mitigate interference. First a rotated-interleaved CA (RICA) scheme is applied, which exploits sector antenna directivity to reject the majority of interferers by receiving them through the antenna side and back lobes. Combined with RICA, an AA system is used in the base station side to further reduce intersector interference during both transmission and reception phases. For the worst-case interference scenario an unsynchronized TDD scheme, the proposed methodology achieves a 51.5% improvement in the average channel capacity while it considerably improves the uplink availability. An advantage of this approach is that it considers only low-complexity AA system configurations, thereby optimizing the corresponding cost

Design & Optimization Methods for WIMAX Systems

WIMAX is predicted to be a serious competitor to existing wireless systems (WLL, LMDS, GSM, UMTS), offering an advanced air-interface and all-IP architecture, which implies more complex network design and deployment. This tutorial aims to provide a design process and generic methodology that can be applied in different WIMAX deployment scenarios. The presentation of the methodology will be structured in such way to address both industrial and academic audience. The tutorial will present design guidelines and best practice information, based on theory and instructors experience in actual deployments, and outline a mathematical approach of WIMAX network design, which is currently far from optimum given the technological complexity. The tutorial also aims to address ways to improve the design process for current WIMAX networks and also highlight several potential research issues.

Joint channel & polarization assignment for capacity enhancement in non line-of-sight fixed wireless networks

We consider frequency channel assignment (CA) and antenna polarization assignment (PA) schemes jointly in the scope of non line-of-sight fixed wireless networks. First, we introduce a new, more accurate method to characterize CA and PA schemes. Following this approach, we propose a new rotated-interleaved CA scheme that outperforms existing CA schemes and a new interleaved-interleaved PA methodology that is more efficient than basic strip-by-strip PA when applied on top of most CA schemes for re-use factor F/sub R/ = 1. An investigation on the total capacity gains of the CA and PA schemes showed that each CA scheme performs differently when combined with different PA schemes and for different re-use factors and only a joint consideration can highlight the best CA-PA combination. In a further step, instead of simply combining CA, PA schemes, we introduce a methodology to design new PA patterns that are optimized (oPA) on the distinct features of each CA scheme. Our results demonstrate that the oPA strategy achieves higher performance than other PA methodologies for F/sub R/ = 1, 2.