Rizwan Ghaffar

MU-MIMO in LTE Systems

A relatively recent idea of extending the benefits of MIMO systems to multiuser scenarios seems promising in the context of achieving high data rates envisioned for future cellular standards after 3G (3rd Generation). Although substantial research has been done on the theoretical front, recent focus is on making Multiuser Multiple-Input Multiple-Output (MUMIMO) practically realizable. This paper presents an overview of the different MU-MIMO schemes included/being studied in 3GPP standardization from LTE (long-term evolution) to LTE Advanced. MU-MIMO system concepts and implementation aspects have been studied here. Various low-complexity receiver architectures are investigated, and their performance assessed through link-level simulations. Appealing performance offered by low-complexity interference aware (IA) receivers is notably emphasized. Furthermore, system level simulations for LTE Release 8 are provided. Interestingly, it is shown that MU-MIMO only offers marginal performance gains with respect to single-user MIMO. This arises from the limited MU-MIMO features included in Release 8 and calls for improved schemes for the upcoming releases.

Interference-aware receiver structure for multi- user MIMO and LTE

In this paper, we propose a novel low-complexity interference-aware receiver structure for multi-user MIMO that is based on the exploitation of the structure of residual interference. We show that multi-user MIMO can deliver its promised gains in modern wireless systems in spite of the limited channel state information at the transmitter (CSIT) only if users resort to intelligent interference-aware detection rather than the conventional single-user detection. As an example, we focus on the long term evolution (LTE) system and look at the two important characteristics of the LTE precoders, i.e., their low resolution and their applying equal gain transmission (EGT). We show that EGT is characterized by full diversity in the single-user MIMO transmission but it loses diversity in the case of multi-user MIMO transmission. Reflecting on these results, we propose a LTE codebook design based on two additional feedback bits of CSIT and show that this new codebook significantly outperforms the currently standardized LTE codebooks for multi-user MIMO transmission.

Design and implementation of a single-frequency mesh network using OpenAirInterface

OpenAirInterface is an experimental open-source real-time hardware and software platform for experimentation in wireless communications and signal processing. With the help of OpenAirInterface, researchers can demonstrate novel ideas quickly and verify them in a realistic environment. Its current implementation provides a full open-source software modem comprising physical and link layer functionalities for cellular and mesh network topologies. The physical (PHY) layer of the platform targets fourth generation wireless networks and thus uses orthogonal frequency division multiple access (OFDMA) together with multiple-input multiple-output (MIMO) techniques. The current hardware supports 5 MHz bandwidth and two transmit/receive antennas. The media access (MAC) layer of the platform supports an abundant two-way signaling for enabling collaboration, scheduling protocols, as well as traffic and channel measurements. In this paper, we focus on the mesh topology and show how to implement a single-frequency mesh network with OpenAirInterface. The key ingredients to enable such a network are a dual-stream MIMO receiver structure and a distributed network synchronization algorithm. We show how to implement these two algorithms in real-time on the OpenAirInterface platform. Further more, we provide results from field trials and compare them to the simulation results.

Interference Suppression for Next Generation Wireless Systems

Future wireless communication systems being char- acterized by tight frequency reuse, adaptive modulation and coding schemes and diversified data services will be interfer- ence limited by the interfering signals of diverse rates and strengths. Frequency reuse factor of 1 being proposed for the next generation mobile systems will lead to 1 or 2 dominant interferers. We propose in this paper a low complexity maximum likelihood (ML) demodulator for interference suppression. The proposed demodulator is also applicable to general multi-stream (spatially multiplexed) MIMO systems where it reduces one complex dimension of the system i.e. the complexity of ML detection reduces from O (|χ| n ) to O � |χ| n�1 � where n is number of transmit antennas/spatial streams. This is a fundamental result as space and technology constraints shall be restricting future MIMO systems to low dimensionality. Therefore reduction of one complex dimension in detection shall enable implementation of ML detectors at the receivers. We look at the performance of linear MMSE demodulator and the proposed demodulator in the presence of interferers of diverse strengths and rates and simulation results demonstrate much improved performance of the proposed demodulator for interference suppression.

Making multiuser MIMO work for LTE

Underlining the viability of multiuser (MU) MIMO for future wireless communications as long term evolution (LTE), we propose in this paper a precoding strategy based on the low resolution LTE precoders which necessitates 2 bits feedback from the user equipment (UE). The proposed strategy encompasses geometrical interference alignment at eNodeB (LTE notation for base station) followed by the exploitation of interference structure by the UEs. On one hand, this strategy relegates the interference seen by each UE by a geometric scheduling algorithm while on the other hand, UEs exploit the structure of residual interference in the detection process.

Interference sensitivity for multiuser MIMO in LTE

A recent idea of exploiting the promised gains of multi-user MIMO for future cellular standards as LTE by intelligent interference aware receivers instead of suboptimal singleuser receivers is quite promising. However this detection strategy is based on the exploitation of the structure of interference which necessitates the knowledge of the constellation and precoder of the co-scheduled user equipment (UE). As the downlink control information (DCI) in LTE is devoid of this information, we look in this paper at the sensitivity of LTEs multi-user MIMO mode to the information of co-scheduled UE. We show that exploiting the discrete nature of interference in the absence of the exact knowledge of interference constellation has significant gains as compared to the suboptimal single-user detection. To this end, we propose a blind receiver for LTE which shows significantly improved performance as compared to single-user receivers and exhibits a minor degradation as compared to interference aware receivers.

Low Complexity Metrics for BICM SISO and MIMO Systems

Bit interleaved coded modulation (BICM) because of its improved diversity over fast fading channels is an attractive transmission scheme for future wireless systems. For coded BICM systems, receivers need to employ max log MAP demodulators (demappers) that calculate soft-decision metrics i.e. log-likelihood ratios (LLRs) for the decoder. The complexity of the calculation of these LLRs is exponential in the number of bits per symbol and moreover for systems exploiting spatial dimension (MIMO), the complexity further increases exponentially in the number of transmit antennas. In this paper we propose matched filter (MF) based low complexity max log MAP bit metrics for BICM single input single output (SISO) and low dimensional BICM MIMO systems using Gray encoded

Spatial Interference Cancellation and Pairwise Error Probability Analysis

M

Low complexity BICM demodulation for MIMO transmission

-ary quadrature amplitude modulation (QAM) alphabets. For SISO systems, the maximum likelihood (ML) detector needs computation and comparison of minimum distances between the received symbol and

Near Optimal Linear Precoder for Multiuser MIMO for Discrete Alphabets

M