Nima Seifi

Coordinated 3D Beamforming for Interference Management in Cellular Networks

We consider downlink transmission in a cellular network consisting of multi-antenna base stations (BSs), single-antenna mobile users, directional antennas each with a vertically adjustable beam, and control signaling delays in feedback and backhaul links. We propose a novel transmission technique in which intercell interference management is performed via coordinating the beamforming jointly in the horizontal and vertical planes of the wireless channel, denoted as coordinated 3D beamforming. In the horizontal plane, we focus on intercell interference cancelation (ICIC) and investigate its performance when the provided channel state information (CSI) is impaired due to delay/mobility. It is demonstrated that the superiority of ICIC over conventional intracell maximum ratio transmission is highly dependent on the CSI accuracy. In the vertical plane, we consider intercell interference control via coordinatively adapting the elevation angle of the BS antenna pattern, denoted as tilt, to the locations of the scheduled users. It is shown that with perfect CSI interference management should be performed in the horizontal plane using ICIC, while at high delay/mobility it should be done in the vertical plane via coordinated tilt adaptation. For intermediate values of delay/mobility, joint interference management in both planes is required.

15 GHz propagation properties assessed with 5G radio access prototype

This paper presents coverage and penetration loss measurements in an urban environment at 15 GHz to provide insight into the design and deployment of future 5G systems in higher frequency bands. The measurements are performed using a 5G radio access prototype including two transmission points (TPs) and a mobile terminal over a 200 MHz bandwidth. The TPs and the mobile terminal each consists of multiple antennas, enabling spatial multiplexing of multiple data streams. Coverage measurements are performed for both outdoor and outdoor-to-indoor scenarios. Penetration losses are measured for human body, normal and coated windows, a metallic white board, and a concrete pillar. Outdoor microcellular coverage in line-of-sight (LOS) and lightly shadowed areas is shown to be possible with similar antenna directivities as in the existing cellular networks. Transitions into non-line-of-sight (NLOS) bring additional losses in the order of 20 dB, thereby making the NLOS coverage challenging. Outdoor-to-indoor coverage seems to be limited to areas that are in almost LOS with the outdoor TP. Moreover, the penetration loss of indoor blocking objects seems to further restrict the indoor coverage. Potentials of beamforming as a means to improve the coverage are also evaluated via simulations.

Throughput Optimization for MISO Interference Channels via Coordinated User-Specific Tilting

In this letter, we propose a novel framework to enhance the throughput in multiple-input single-output (MISO) mutually interfering channels via selecting the tilt angles at all base stations (BSs) in a coordinated fashion. In the proposed framework, multiple BSs adjust their tilt angles jointly to maximize the sum throughput of the scheduled users, denoted as coordinated user-specific tilting. Assuming the availability of location information of the scheduled users at all BSs, accurate analytical expression for user ergodic rate is provided, which enables a decentralized deployment of the proposed framework at each BS. Simulation results show that the proposed coordinated user-specific tilting scheme outperforms the conventional schemes employing a fixed tilt angle at each BS.

Impact of Base Station Antenna Tilt on the Performance of Network MIMO Systems

We study the downlink of a multicell MIMO system where clusters of multi-antenna base stations jointly serve multiple single-antenna users, commonly referred to as a network MIMO system. Most of the previous studies on network MIMO have only considered the azimuth pattern of the antenna, while ignoring the elevation pattern. In this paper, we consider both the azimuth and the elevation patterns and investigate the impact of the elevation angle tuning parameter, denoted as the antenna tilt, on the performance of such systems. Using system simulations, it is shown that the promised performance gains of network MIMO systems over conventional non-coordinated systems, crucially depend on the choice of the right tilt setting including the tilt type, i.e., mechanical or electrical, and the tilt angle. In particular, for tilt angles smaller than the optimum, network MIMO with intra-site coordination performs almost as well as the conventional system; while for tilt angles larger than the optimum, the performance of network MIMO with intra-site is similar to that of network MIMO with inter-site coordination.

Coordinated single-cell vs multi-cell transmission with limited-capacity backhaul

Base station coordination is an efficient technique to transcend the limits on spectral efficiency imposed by intercell interference. In this paper, we compare the performance of different coordination strategies with different amount of channel state information (CSI) and data sharing among the coordinating base stations. We focus on the effect of limited backhaul capacity in a two-cell network. Contrary to the common belief, we show that coordination strategies with no data and only limited CSI sharing is preferred to those with full data and CSI sharing when the backhaul capacity is relatively low and the edge SNR is high.

Resource allocation and linear precoding for relay assisted multiuser MIMO systems

In this paper we investigate the use of infrastructure half-duplex amplify-and-forward (AF) relays in the downlink of a multi-user (MU) multiple-input multiple-output (MIMO) system, which uses linear precoding based on block-diagonalization. Owing to the half-duplex mode of the relay, the communication is divided into two time-slots. During the first time-slot the relay nodes are treated as an additional mobile station (MS), while during the second time-slot the base-station (BS) and the relays form a distributed, virtual antenna array. In contrast to many relay applications, the purpose of the proposed scheme is not to increase diversity, but rather to increase the system throughput at the borders of a communication cell or in areas with a high user density. The performance of the proposed architecture is investigated by examining the sum-capacity of the system as well as the distribution of the capacity over the cell-area.

Downlink performance and capacity of distributed antenna systems based on realistic channel model

Recent academic studies of distributed antenna system (DAS) using a frequency-flat fading channel model show that DAS outperforms standard systems, where the base-station (BS) antenna elements are centralized at one location. In this paper the performance of a DAS under time-varying frequency-selective fading based on a realistic channel model is investigated. Specifically, we show that if we shift the hexagonal cellular layout in the conventional system and use sectorized antennas instead of omnidirectional antennas at each BS, the performance in terms of outage probability and outage capacity will improve by a large extent without a need for additional BS towers. The results show that with the same total transmit power and bandwidth DAS can reduce the inter-cell interference (ICI) in a multicell environment and improve the outage capacity especially near the cell boundary.

15 GHz Street-Level Blocking Characteristics Assessed with 5G Radio Access Prototype

Knowledge about propagation properties and development of realistic channel models at higher frequencies are crucial for evaluations and design decisions in the upcoming 5G standardizations. One propagation phenomenon that requires special attention at higher frequencies is blocking by objects. In this paper, the propagation characteristics in the presence of street-level blocking objects at 15 GHz are investigated based on measurement with a 5G radio access prototype. It is found that blocking by moving obstacles has similar behavior as that by stationary ones. The results are also used to verify the validity of the blocking model developed in the METIS project at higher frequencies. Blocking loss in the range 3-12 dB is observed, which is not larger than that at lower frequency bands. Moreover, our Doppler analysis reveals that for some objects such as cars and vans propagation happens only around the objects; but for other objects such as trees, propagation happens through the object. Reflection and scattering are also identified to contribute to the limited loss from blocking and increase the channel richness enabling improved spatial multiplexing.

Multi-user diversity with two-step channel state information feedback

This study proposes the performance of a scheduling scheme using two-step quantised channel state information (CSI) feedback. Considering fixed and random access networks, the results are obtained under both short- and long-term power constraints and for both single- and multi-layer transmission techniques. Further, the authors evaluate the asymptotic performance of the proposed scheme and show that, while the users scheduling is based on quantised CSI feedback in the first round, the second round of partial CSI feedback can be provided by either channel quantisation or hybrid automatic repeat request feedback, leading to the same forward channel average rate. In comparison with the case of no CSI feedback, the results indicate substantial average rate increment with limited number of feedback bits. Also, the average rate increases via successive partial CSI feedback when compared with one-step CSI feedback schemes.

Multimode transmission in network MIMO downlink with incomplete CSI

We consider a cooperative multicell MIMO (a.k.a network MIMO) downlink system with multiantenna base stations (BSs), which are connected to a central unit and communicate with multiantenna users. In such a network, obtaining perfect channel state information (CSI) of all users at the central unit to exploit opportunistic scheduling requires a substantial amount of feedback and backhaul signaling. We propose a scheduling algorithm based only on the knowledge of the average SNR at each user from all the cooperating BSs, denoted as incomplete CSI. Multimode transmission is applied that is able to adaptively adjust the number of data streams transmitted to each user. Utilizing the results of random matrix theory, an analytical framework is proposed to approximate the ergodic rate of each user with different number of data streams. Using these ergodic rates, a joint user and mode selection algorithm is proposed, where only the scheduled users need to feed back instantaneous CSI. Simulation results demonstrate that the developed analytical framework provides a good approximation for a practical number of antennas. While substantially reducing the feedback overhead, it is shown that the proposed scheduling algorithm performs closely to the opportunistic scheduling algorithm that requires instantaneous CSI feedback from all users.