ProfessorBranka Vucetic

Project Opportunities

Title: Interference Cancellation in Co-working WLANs

Summary of opportunity:

Capacity of cellular networks, such as 3G CDMA, B3G HSPA and LTE, are interference-limited. This means that the maximum number of users in each cell and overall system capacity mainly depend on the signal to interference plus noise (SINR) level. Therefore, how to eliminate the interference is one of most crucial issues to be resolved in future wireless networks. The project aims to reduce interference caused by co-working wireless local area networks (WLANs) and WiMax systems. These WLANs can be either non-cooperative or co-operative. The project will focus on improving Physical (PHY) layer and its interaction with MAC layer through the use of cross-layer design, adaptive modulation, coding, adaptive antennas, co-operation between co-working WLANs and interference cancellation.

Opportunity synopsis:

Capacity of cellular networks, such as 3G CDMA, B3G HSPA and LTE, are interference-limited. This means that the maximum number of users in each cell and overall system capacity mainly depend on the signal to interference plus noise (SINR) level. Therefore, how to eliminate the interference is one of most crucial issues to be resolved in future wireless networks. The project aims to reduce interference caused by co-working wireless local area networks (WLANs) and WiMax systems. These WLANs can be either non-cooperative or co-operative. The project will focus on improving Physical (PHY) layer and its interaction with MAC layer through the use of cross-layer design, adaptive modulation, coding, adaptive antennas, co-operation between co-working WLANs and interference cancellation.

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Project Opportunities

Title: Precoded multiuser MIMO and packet scheduling

Summary of opportunity:

Linear precoding for multiuser Multiple-Input Multiple-Output (MIMO) systems has attracted much attention recently due to the capacity enhancement ability at the system level. In the existing literature, the interactions between packet scheduling and array antenna techniques are studied based on system level simulation models. To our knowledge, theoretical analysis of linearly precoded multiuser Spatial Division Multiplexing (SDM) MIMO systems combined with Frequency Domain (FD) packet scheduling has not been studied so far. In this project, we investigate the distribution of Signal to Interference plus Noise Ratio (SINR) for multiuser spatial multiplexing OFDMA based downlink MIMO systems in combination with the base station based packet scheduler. The packet scheduler is used to exploit the available multiuser diversity in the time, frequency and spatial domains. In 3GPP Long Term Evolution (LTE) (also known as Evolved-UMTS Terrestrial Radio Access (E-UTRA or EUTRA)), Single Carrier (SC) Frequency Division Multiple Access (FDMA) has been adopted for uplink transmission, whereas the Orthogonal FDMA (OFDMA) signalling format has been exploited for the downlink transmission. SC-FDMA technique for uplink transmission has low Peak to Average Power Ratio (PAPR) property compared with competitive OFDMA technique. The SC-FDMA signal can be obtained by using Discrete Fourier Transform (DFT) spread OFDMA, where DFT is applied to convert time domain input data symbols to frequency domain before feeding them into an OFDMA modulator. For wide band wireless transmission systems, e.g., LTE OFDMA downlink and SC-FDMA uplink to simply scheduling task, several consecutive subcarriers are usually grouped together for scheduling. A basic scheduling unit is called a Resource Block (RB). The scheduler in Base Station (BS) may assign single or multiple RBs to a Mobile Station (MS). This project investigates space frequency scheduling for SC-FDMA based uplink multi-user MIMO system. We will also consider channel dependent scheduling algorithms for SC-FDMA based linearly precoded uplink MIMO systems

Opportunity synopsis:

Linear precoding for multiuser Multiple-Input Multiple-Output (MIMO) systems has attracted much attention recently due to the capacity enhancement ability at the system level. In the existing literature, the interactions between packet scheduling and array antenna techniques are studied based on system level simulation models. To our knowledge, theoretical analysis of linearly precoded multiuser Spatial Division Multiplexing (SDM) MIMO systems combined with Frequency Domain (FD) packet scheduling has not been studied so far. In this project, we investigate the distribution of Signal to Interference plus Noise Ratio (SINR) for multiuser spatial multiplexing OFDMA based downlink MIMO systems in combination with the base station based packet scheduler. The packet scheduler is used to exploit the available multiuser diversity in the time, frequency and spatial domains. In 3GPP Long Term Evolution (LTE) (also known as Evolved-UMTS Terrestrial Radio Access (E-UTRA or EUTRA)), Single Carrier (SC) Frequency Division Multiple Access (FDMA) has been adopted for uplink transmission, whereas the Orthogonal FDMA (OFDMA) signalling format has been exploited for the downlink transmission. SC-FDMA technique for uplink transmission has low Peak to Average Power Ratio (PAPR) property compared with competitive OFDMA technique. The SC-FDMA signal can be obtained by using Discrete Fourier Transform (DFT) spread OFDMA, where DFT is applied to convert time domain input data symbols to frequency domain before feeding them into an OFDMA modulator. For wide band wireless transmission systems, e.g., LTE OFDMA downlink and SC-FDMA uplink to simply scheduling task, several consecutive subcarriers are usually grouped together for scheduling. A basic scheduling unit is called a Resource Block (RB). The scheduler in Base Station (BS) may assign single or multiple RBs to a Mobile Station (MS). This project investigates space frequency scheduling for SC-FDMA based uplink multi-user MIMO system. We will also consider channel dependent scheduling algorithms for SC-FDMA based linearly precoded uplink MIMO systems

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Project Opportunities

Title: Cooperative transmission in MIMO relay broadcast channels

Summary of opportunity:

Wireless multiuser communications are suffering from problems of reliability, coverage and spectrum efficiency. Relay broadcast is one of the promising techniques to solve these problems by exploiting the distributed spatial diversity arising from the relay mechanism and the local spatial diversity introduced by the multiple antennas.

The channel capacity of relay broadcast channels has been investigated in literature, which shows a significant improvement over the classical broadcast channels without relay. However, a practical transmission scheme is yet to be invented.

In this project, we design a low-complexity beamforming, relaying and combining scheme for MIMO relay broadcast channels. In the preliminary study, we consider the scenario where one base station (BS) transmits to two mobile stations (MSs), where one of the MSs with strong signal reception and/or processing power can help the other besides receiving its own data. The transmission vector space is partitioned into subspaces to accommodate the transmission from the BS to both MSs and the relaying between MSs. The zero forcing (ZF) criterion is used to avoid interferences among these transmissions. With multiple antennas at both the BS and MSs, and the channel state information (CSI) available (fully or partially) to the relevant parties, the subspace partitioning can be adjusted to optimise the overall system performance. The power allocation between users and relay can also be optimised to improve the performance further. Different levels of CSI availability will be investigated and reduced CSI scheme will be proposed such that the scheme can fit into both TDD and FDD systems. When more than two MSs are involved in the communication scenario, a relay selection algorithm will be developed in such a way the most effective MS is selected to act as the relay.

Opportunity synopsis:

Wireless multiuser communications are suffering from problems of reliability, coverage and spectrum efficiency. Relay broadcast is one of the promising techniques to solve these problems by exploiting the distributed spatial diversity arising from the relay mechanism and the local spatial diversity introduced by the multiple antennas.

The channel capacity of relay broadcast channels has been investigated in literature, which shows a significant improvement over the classical broadcast channels without relay. However, a practical transmission scheme is yet to be invented.

In this project, we design a low-complexity beamforming, relaying and combining scheme for MIMO relay broadcast channels. In the preliminary study, we consider the scenario where one base station (BS) transmits to two mobile stations (MSs), where one of the MSs with strong signal reception and/or processing power can help the other besides receiving its own data. The transmission vector space is partitioned into subspaces to accommodate the transmission from the BS to both MSs and the relaying between MSs. The zero forcing (ZF) criterion is used to avoid interferences among these transmissions. With multiple antennas at both the BS and MSs, and the channel state information (CSI) available (fully or partially) to the relevant parties, the subspace partitioning can be adjusted to optimise the overall system performance. The power allocation between users and relay can also be optimised to improve the performance further. Different levels of CSI availability will be investigated and reduced CSI scheme will be proposed such that the scheme can fit into both TDD and FDD systems. When more than two MSs are involved in the communication scenario, a relay selection algorithm will be developed in such a way the most effective MS is selected to act as the relay.

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Project Opportunities

Title: Iterative channel estimation for high mobility MIMO-OFDM systems

Summary of opportunity:

Multiple-input multiple-output and orthogonal frequency division multiplexing (MIMO-OFDM) techniques have been adopted in 3GPP LTE and WiMAX standards to achieve very high data rates (> 10Mbps). In these standards, high mobility users, moving at speeds higher than 300Km/h, need to be supported. Accurate channel state information (CSI) is required for reliable signal detection at the receiver. Pilot symbols are inserted among sub-carriers before transmission to accurately estimate the wireless channel. In a high mobility environment, the wireless channel is time-variant and frequency-selective causing the symbol transmission to be impaired by the Doppler spread. The Doppler spread destroys the orthogonality and creates inter-carrier interference (ICI) between OFDM sub-carriers. In addition, the channel changes significantly within one OFDM symbol. As a consequence, the Standard channel estimation methods cannot be used in 3GPP LTE and WiMAX to support high mobility users.
We propose a novel pilot-aided iterative receiver, based on pilot symbols and iterative soft-estimate of data symbols. The channel is estimated by time-domain interpolation and least-square (LS) methods. Soft-estimates for data symbols are obtained by a maximum-a-posteriori (MAP) decoder and improved subsequently. The simulation results show that the performance of the proposed iterative receiver outperforms the existing schemes. The performance degradation of the proposed receiver structure when users move at speed of up to 324Km/h compared to the performance of a perfect CSI system with a zero Doppler shift is shown to be very marginal.

Opportunity synopsis:

Multiple-input multiple-output and orthogonal frequency division multiplexing (MIMO-OFDM) techniques have been adopted in 3GPP LTE and WiMAX standards to achieve very high data rates (> 10Mbps). In these standards, high mobility users, moving at speeds higher than 300Km/h, need to be supported. Accurate channel state information (CSI) is required for reliable signal detection at the receiver. Pilot symbols are inserted among sub-carriers before transmission to accurately estimate the wireless channel. In a high mobility environment, the wireless channel is time-variant and frequency-selective causing the symbol transmission to be impaired by the Doppler spread. The Doppler spread destroys the orthogonality and creates inter-carrier interference (ICI) between OFDM sub-carriers. In addition, the channel changes significantly within one OFDM symbol. As a consequence, the Standard channel estimation methods cannot be used in 3GPP LTE and WiMAX to support high mobility users.
We propose a novel pilot-aided iterative receiver, based on pilot symbols and iterative soft-estimate of data symbols. The channel is estimated by time-domain interpolation and least-square (LS) methods. Soft-estimates for data symbols are obtained by a maximum-a-posteriori (MAP) decoder and improved subsequently. The simulation results show that the performance of the proposed iterative receiver outperforms the existing schemes. The performance degradation of the proposed receiver structure when users move at speed of up to 324Km/h compared to the performance of a perfect CSI system with a zero Doppler shift is shown to be very marginal.