Técnicas de pré-codificação para sistemas multicelulares coordenados

Doutoramento em TelecomunicaçõesCoordenação Multicélula é um tópico de investigação em rápido crescimento e uma solução promissora para controlar a interferência entre células em sistemas celulares, melhorando a equidade do sistema e aumentando a sua capacidade. Esta tecnologia já está em estudo no LTEAdvanced sob o conceito de coordenação multiponto (COMP). Existem várias abordagens sobre coordenação multicélula, dependendo da quantidade e do tipo de informação partilhada pelas estações base, através da rede de suporte (backhaul network), e do local onde essa informação é processada, i.e., numa unidade de processamento central ou de uma forma distribuída em cada estação base. Nesta tese, são propostas técnicas de pré-codificação e alocação de potência considerando várias estratégias: centralizada, todo o processamento é feito na unidade de processamento central; semidistribuída, neste caso apenas parte do processamento é executado na unidade de processamento central, nomeadamente a potência alocada a cada utilizador servido por cada estação base; e distribuída em que o processamento é feito localmente em cada estação base. Os esquemas propostos são projectados em duas fases: primeiro são propostas soluções de pré-codificação para mitigar ou eliminar a interferência entre células, de seguida o sistema é melhorado através do desenvolvimento de vários esquemas de alocação de potência. São propostas três esquemas de alocação de potência centralizada condicionada a cada estação base e com diferentes relações entre desempenho e complexidade. São também derivados esquemas de alocação distribuídos, assumindo que um sistema multicelular pode ser visto como a sobreposição de vários sistemas com uma única célula. Com base neste conceito foi definido uma taxa de erro média virtual para cada um desses sistemas de célula única que compõem o sistema multicelular, permitindo assim projectar esquemas de alocação de potência completamente distribuídos. Todos os esquemas propostos foram avaliados em cenários realistas, bastante próximos dos considerados no LTE. Os resultados mostram que os esquemas propostos são eficientes a remover a interferência entre células e que o desempenho das técnicas de alocação de potência propostas é claramente superior ao caso de não alocação de potência. O desempenho dos sistemas completamente distribuídos é inferior aos baseados num processamento centralizado, mas em contrapartida podem ser usados em sistemas em que a rede de suporte não permita a troca de grandes quantidades de informação.Multicell coordination is a promising solution for cellular wireless systems to mitigate inter-cell interference, improving system fairness and increasing capacity and thus is already under study in LTE-A under the coordinated multipoint (CoMP) concept. There are several coordinated transmission approaches depending on the amount of information shared by the transmitters through the backhaul network and where the processing takes place i.e. in a central processing unit or in a distributed way on each base station. In this thesis, we propose joint precoding and power allocation techniques considering different strategies: Full-centralized, where all the processing takes place at the central unit; Semi-distributed, in this case only some process related with power allocation is done at the central unit; and Fulldistributed, where all the processing is done locally at each base station. The methods are designed in two phases: first the inter-cell interference is removed by applying a set of centralized or distributed precoding vectors; then the system is further optimized by centralized or distributed power allocation schemes. Three centralized power allocation algorithms with per-BS power constraint and different complexity tradeoffs are proposed. Also distributed power allocation schemes are proposed by considering the multicell system as superposition of single cell systems, where we define the average virtual bit error rate (BER) of interference-free single cell system, allowing us to compute the power allocation coefficients in a distributed manner at each BS. All proposed schemes are evaluated in realistic scenarios considering LTE specifications. The numerical evaluations show that the proposed schemes are efficient in removing inter-cell interference and improve system performance comparing to equal power allocation. Furthermore, fulldistributed schemes can be used when the amounts of information to be exchanged over the backhaul is restricted, although system performance is slightly degraded from semi-distributed and full-centralized schemes, but the complexity is considerably lower. Besides that for high degrees of freedom distributed schemes show similar behaviour to centralized ones

A novel distributed power allocation scheme for coordinated multicell systems

Coordination between base stations (BSs) is a promising solution for cellular wireless systems to mitigate intercell interference, improving system fairness, and increasing capacity in the years to come. The aim of this manuscript is to propose a new distributed power allocation scheme for the downlink of distributed precoded multicell MISO-OFDM systems. By treating the multicell system as a superposition of single cell systems we define the average virtual bit error rate (BER) of one single-cell system, allowing us to compute the power allocation in a distributed manner at each BS. The precoders are designed in two phases: first the precoder vectors are computed in a distributed manner at each BS considering two criteria, distributed zero-forcing and virtual signal-to-interference noise ratio; then the system is optimized through distributed power allocation with per-BS power constraint. The proposed power allocation scheme minimizes the average virtual BER over all user terminals and the available subcarriers. Both the precoder vectors and the power allocation are computed by assuming that the BSs have only knowledge of local channel state information. The performance of the proposed scheme is compared against other power allocation schemes that have recently been proposed for precoded multicell systems based on LTE specifications. The results also show that although our power allocation scheme is based on the minimization of the virtual uncoded BER, it also has significant gains in coded systems

Power allocation strategies for distributed precoded multicell based systems

Multicell cooperation is a promising solution for cellular wireless systems to mitigate intercell interference, improve system fairness, and increase capacity. In this article, we propose power allocation techniques for the downlink of distributed, precoded, multicell cellular-based systems. The precoder is designed in two phases: first the intercell interference is removed by applying a set of distributed precoding vectors; then the system is further optimized through power allocation. Three centralized power allocation algorithms with per-BS power constraint and diferente complexity trade-offs are proposed: one optimal in terms of minimization of the instantaneous average bit error rate (BER), and two suboptimal. In this latter approach, the powers are computed in two phases. First, the powers are derived under total power constraint (TPC) and two criterions are considered, namely, minimization of the instantaneous average BER and minimization of the sum of inverse of signal-to-noise ratio. Then, the final powers are computed to satisfy the individual per-BS power constraint. The performance of the proposed schemes is evaluated, considering typical pedestrian scenarios based on LTE specifications. The numerical results show that the proposed suboptimal schemes achieve a performance very close to the optimal but with lower computational complexity. Moreover, the performance of the proposed per-BS precoding schemes is close to the one obtained considering TPC over a supercell.Portuguese CADWIN - PTDC/ EEA TEL/099241/200

Distributed Versus Centralized Zero-Forcing precoding for multicell OFDM systems

In this paper we propose distributed linear multiuser precoding techniques for the downlink of multicell MIMO-OFDM systems. The precoder is designed in two phases: first the intercell interference is removed by applying a set of precoding vectors based on zero-forcing criterion and computed in a distributed manner at each BS; then the system is further optimized through power allocation. Three power allocation algorithms with per-BS power constraint and different complexity tradeoffs are proposed: one optimal to minimize the average BER and two suboptimal. The performance of the proposed schemes is compared to the full centralized multicell based approaches recently proposed, and evaluated considering typical pedestrian scenarios. Numerical results show that the proposed suboptimal schemes achieve a performance very close to the optimal. Also, it is shown that for the same degrees of freedom the performance is close to the one of centralized precoding schemes but with much lower feedback load over the backhaul network.FCT CADWIN (PTDC/EEA TEL/099241/2008)CROWN (PTDC/EEA-TEL/115828/2009 )IT CELCOP project

Performance evaluation of distributed precoding schemes for multicell OFDM systems

In this paper we propose and evaluate distributed multiuser linear precoding techniques for the downlink of multicell MIMO OFDM systems. The precoder is designed in two phases: first the intercell interference is removed by applying a set of precoding vectors computed in a distributed manner at each BS; then the system is further optimized through power allocation. Three centralized power allocation algorithms with per-BS power constraint and different complexity tradeoffs are proposed: one optimal to minimize the average BER and two suboptimal. The performances of the proposed schemes are compared to the centralized multicell based approaches recently proposed, and evaluated considering typical pedestrian scenarios. Numerical results show that the proposed suboptimal schemes achieve a performance very close to the optimal. Also, it is shown that for the same degrees of freedom the performance is closed to the one of centralized precoding schemes but with much lower feedback load over the backhaul network. © 2011 IEEE.Portuguese CADWIN - PTDC/EEA TEL/099241/200

Multiuser precoding techniques for a distributed broadband wireless system

In this paper we propose and asses multiuser linear precoding techniques for the downlink of distributed MIMO OFDM systems. We consider a distributed broadband wireless system where the base stations are transparently linked by optical fiber to a central unit. We further assume that both the distributed base stations and the user terminals are equipped with an antenna array. This architecture provides a high speed backhaul channel allowing an efficient joint multiuser multicell processing. The precoder is designed in two phases: first the intercell interference is removed by applying a block diagonalization algorithm. Then the system is further optimized by using a new power allocation algorithm, based on minimization of the sum of inverse signal-to-noise ratio (SNR−1) on each user terminal over the available subcarriers. The motivation to minimize the sum of SNR−1 instead of bit error rate is the fact that the first criterion achieves a closed-form solution, which is more interesting from practical point of view. The aim is to propose a practical distributed precoding technique to remove the intercell interference and improve the user’s fairness at the cell-edges. The performance of the proposed scheme is evaluated and compared with an iterative precoding scheme designed to minimize the bit error rate, extended to the proposed multiuser distributed scenario, considering typical pedestrian scenarios based on LTE specifications.Portuguese CADWIN - PTDC/EEA-TEL/099241/200

Linear precoding for centralized multicell MIMO networks

The aim of this paper is to propose and evaluate multiuser linear precoding techniques for downlink of multicell MIMO based networks. We consider a high-speed backhaul network where the base stations are transparently linked by optical fiber to a central unit. This architecture allows an efficient joint multicell multiuser processing. The proposed precoder is designed in two phases: first the intercell interference is removed by applying a block diagonalization algorithm. Then the system is further optimized by proposing three power allocation algorithms with per-BS power constraint and different complexity tradeoffs: one optimal to minimize the BER and two suboptimal. The performance of the proposed schemes is evaluated, considering typical pedestrian scenarios based on LTE specifications. Numerical results show that the proposed suboptimal power allocation schemes achieve a performance close to the optimal. © 2011 IEEE.FCT Portuguese CADWIN - PTDC/EEA TEL/099241/200

Distributed precoding with centralized power allocation techniques for multicell OFDM systems

In this paper we propose and compare several distributed linear precoding schemes for the downlink of multicell OFDM based systems. The precoders are designed in two phases: first the precoder vectors are computed in a distributed manner at each BS considering several criteria; then the system is further optimized through power allocation. Two centralized power allocation algorithms with per-BS power constraint and different complexity tradeoffs are proposed: minimization of the instantaneous average BER and minimization of the sum of inverse of signal-to-interference noise ratio. The performance of the proposed schemes is compared considering typical pedestrian scenarios based on LTE specifications. The numerical results show that the performance of the DVSINR and DZF precoders with the proposed power allocation schemes is close. © 2011 National Institute of Inform.Portuguese FCT CADWIN - PTDC/EEA TEL/099241/2008FCT gran

Coordinated precoding techniques for multi-cell MISO-OFDM networks

The aim of this manuscript is to propose and compare several distributed linear precoding schemes for the downlink of multicell MISO-OFDM based systems. The precoder vectors are computed in a distributed manner at each BS, assuming that the BSs have only knowledge of local channel state information and considering several criteria: distributed zero-forcing (DZF), maximum ratio transmission and distributed virtual signal-to-interference noise ratio (DVSINR). Then the system is further optimized through centralized power allocation. Two centralized power allocation algorithms with per-BS power constraint and different complexity tradeoffs are proposed: namely, minimization of the instantaneous average BER and minimization of the sum of inverse of signal-to-interference noise ratio, for which a closed-form power allocation scheme is derived. The performance of the proposed schemes is compared considering typical pedestrian scenarios based on LTE specifications. The numerical results show that increasing the number of degrees of freedom or for high SNR, the proposed DZF approaches tends to the DVSINR based ones

Distributed power allocation schemes for precoded multicell MISO-OFDM systems

In this paper we propose distributed power allocation schemes for the downlink of distributed precoded multicell MISO-OFDM systems. The precoders are designed in two phases: first the precoder vectors are computed in a distributed manner at each BS considering two criteria, distributed zero-forcing and virtual signal-to-interference noise ratio; then the system is optimized through distributed power allocation with per-BS power constraint. The proposed power allocation schemes minimize the average virtual bit error rate over all user terminals and the available subcarriers. Both the precoder vectors and the power allocation are computed by assuming that the BSs have only knowledge of local channel state information. The performance of the proposed schemes are compared against other power allocation schemes that have been recently proposed for multicell systems