| 1. |
- Coldrey, Mikael, 1972, et al.
(författare)
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Higher order MIMO outdoor-to-indoor measurements using repeaters
- 2010
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Ingår i: IEEE Vehicular Technology Conference. - 1550-2252. - 9781424425198
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper we present results from a outdoor- to-indoor MIMO measurement campaign where we study the effect of repeaters on singular values, spatial richness, capacity, and delay spread distributions. We present results from different repeater scenarios such as, e.g. different outdoor/indoor repeater deployments with single/dual polarized antennas. The measurements show that repeaters significantly enhance the received SNR while they increase the delay spread and decrease the spatial richness of the MIMO channel. However, the main conclusion is that a significant increase in channel capacity is attained when deploying repeaters mostly thanks to their ability to provide spatial multiplexing over the high-quality eigenmodes they may provide. The number of such eigenmodes is limited by the number of repeaters, deployment, and their antennas. © 2010 IEEE.
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| 2. |
- Ingason, Tryggvi, 1983, et al.
(författare)
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Impact of frequency selective channels on a line-of-sight MIMO microwave radio link
- 2010
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Ingår i: IEEE Vehicular Technology Conference. - 1550-2252. - 9781424425198
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Two line-of-sight (LoS) multiple-input multipleoutput (MIMO) systems for point-to-point microwave radio links are simulated and analyzed. Both 2×2 spatially-separated, singlepolarized and 4×4 spatially-separated, dual-polarized systems are considered in the presence of a frequency selective and time varying channel. Time dependence is introduced by moving the antennas during the simulation and thereby introducing the need for an adaptive space-time-equalizer (STE). Our results indicate that a time-varying frequency selective LoS MIMO channel puts higher requirements on the STE and introduces a higher SNR penalty compared to a single-input single-output (SISO) system. © 2010 IEEE.
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| 3. |
- Kolomvakis, Nikolaos, 1985, et al.
(författare)
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Massive MIMO in Sparse Channels
- 2014
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Ingår i: 2014 IEEE 15th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC). - 2325-3789. - 9781479949038 ; :October, s. 21-25
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Konferensbidrag (refereegranskat)abstract
- Massive multi-user multiple-input multiple-output (MU-MIMO) systems are cellular networks where the base stations (BSs) are equipped with hundreds of antennas, N, and communicate with tens of mobile stations (MSs), K, such that, N >> K >> 1. Contrary to most prior works, in this paper, we consider the uplink of a single-cell massive MIMO system operating in sparse channels with limited scattering. This case is of particular importance in most propagation scenarios, where the prevalent Rayleigh fading assumption becomes idealistic. We derive analytical approximations for the achievable rates of maximum-ratio combining (MRC) and zero-forcing (ZF) receivers. Furthermore, we study the asymptotic behavior of the achievable rates for both MRC and ZF receivers, when N and K go to infinity under the condition that N/K -> c >= 1. Our results indicate that the achievable rate of MRC receivers reaches an asymptotic saturation limit, whereas the achievable rate of ZF receivers grows logarithmically with the number of MSs.
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| 4. |
- Lioliou, Panagiota, 1982, et al.
(författare)
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Efficient Channel Estimation Techniques for Amplify and Forward Relaying Systems
- 2012
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Ingår i: IEEE Transactions on Communications. - 0090-6778 .- 1558-0857. ; 60:11, s. 3150-3155
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present a channel estimation scheme for Amplify-and-Forward (AF) relaying systems, using measurements at the destination only. A Least-Squares (LS) based channel estimation algorithm is developed, that provides the destination with full knowledge of all channel responses involved in the transmission. To investigate the algorithm performance, the Cramer-Rao lower bound (CRB) is analytically computed and compared with the asymptotic covariance of the proposed estimator. Since the existing estimator does not reach the CRB, we also propose and analyze an improved algorithm by taking into account the noise characteristics via weighted LS (WLS). The improved algorithm is asymptotically efficient, since it attains the CRB as the SNR tends to infinity.
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| 5. |
- Lioliou, Panagiota, 1982, et al.
(författare)
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Self-Interference Suppression in Full-Duplex MIMO Relays
- 2010
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Ingår i: 44th Asilomar Conference on Signals, Systems and Computers. - 1058-6393. - 9781424497218 ; , s. 658-662
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Konferensbidrag (refereegranskat)abstract
- Full-duplex relays can provide cost-effective cover-age extension and throughput enhancement. However, the main limiting factor is the resulting self-interference signal which deteriorates the relay performance. In this paper, we propose a novel technique for self-interference suppression in full-duplex Multiple-Input Multiple-Output (MIMO) relays. The relay employs transmit and receive weight filters for suppressing the self-interference signal. Unlike existing techniques that are based on zero forcing of self-interference, we aim at maximizing the ratio between the power of the useful signal to the self-interference power at the relay reception and transmission. Our simulation results show that the proposed algorithm outperforms the existing schemes since it can suppress interference substantially with less impact on the useful signal.
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| 6. |
- Seifi, Nima, 1981, et al.
(författare)
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An Efficient Signaling for Multi-mode Transmission in Multi-user MIMO
- 2010
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Ingår i: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings. - 1520-6149. - 9781424442966 ; , s. 3402-3405
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Konferensbidrag (refereegranskat)abstract
- In this paper the downlink of a multi-user MIMO (MUMIMO)system with multi-mode transmission is considered.We propose a low-complexity algorithm for selecting usersand the corresponding number of data streams to each user,denoted as user transmission mode (UTM). The selectionis only based on the average received signal-to-noise ratio(SNR) from the base station (BS) for each user. This reducesthe overall amount of feedback for scheduling, as opposedto techniques that assume perfect instantaneous channelstate information (CSI) from all users. Analytical averagethroughput approximations are derived for each user at different UTMs. Simulation results demonstrate that the proposed algorithm provides performance close to dirty paper coding (DPC) with considerably reduced feedback.
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| 7. |
- Seifi, Nima, 1981, et al.
(författare)
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Coordinated 3D Beamforming for Interference Management in Cellular Networks
- 2014
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Ingår i: IEEE Transactions on Wireless Communications. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-2248 .- 1536-1276. ; 13:10, s. 5396-5410
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Seifi, Nima, 1981, et al.
(författare)
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Coordinated Single-Cell vs Multi-Cell Transmission with Limited-Capacity Backhaul
- 2010
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Ingår i: IEEE Asilomar Conf. on Signals, Systems, and Computers, Pacific Grove, CA, Nov. 2010. - 1058-6393. - 9781424497218 ; , s. 1217-1221
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Konferensbidrag (refereegranskat)abstract
- Base station coordination is an efficient techniqueto transcend the limits on spectral efficiency imposed by intercellinterference. In this paper, we compare the performance ofdifferent coordination strategies with different amount of channelstate information (CSI) and data sharing among the coordinatingbase stations. We focus on the effect of limited backhaul capacityin a two-cell network. Contrary to the common belief, we showthat coordination strategies with no data and only limited CSIsharing is preferred to those with full data and CSI sharing whenthe backhaul capacity is relatively low and the edge SNR is high.
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| 9. |
- Seifi, Nima, 1981, et al.
(författare)
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Impact of base station antenna tilt on the performance of network MIMO systems
- 2012
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Ingår i: IEEE Vehicular Technology Conference. - 1550-2252. - 9781467309905 ; , s. Art. no. 6239994-
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 10. |
- Seifi, Nima, 1981, et al.
(författare)
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Joint transmission mode and tilt adaptation in coordinated small-cell networks
- 2014
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Ingår i: 2014 IEEE International Conference on Communications Workshops, ICC 2014. - 9781479946402 ; , s. 598-603
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Konferensbidrag (refereegranskat)abstract
- We study downlink transmission in a wireless network consisting of small-sized cells, denoted as a small-cell network (SCN). We consider multi-antenna base stations (BSs) and single-antenna users, BS coordination, 3D antenna patterns, and fair scheduling. We investigate the impact of the elevation angle of the BS antenna pattern, denoted as tilt, on the performance of the considered SCN when employing either a conventional uncoordinated transmission mode or a fully coordinated transmission mode. Using the results of this investigation, we propose a novel hybrid-mode transmission technique that can achieve a performance comparable to that of a fully coordinated transmission but with a significantly lower complexity and signaling requirement. The main idea is to divide the coverage area into two so-called vertical regions and jointly adapt the transmission mode and the tilt at the BSs when serving each region. A fair scheduler is used to share the time-slots between the vertical regions and among the users in each region.
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| 11. |
- Seifi, Nima, 1981, et al.
(författare)
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Multimode Transmission in Network MIMO Downlink with Incomplete CSI
- 2011
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Ingår i: Eurasip Journal on Advances in Signal Processing. - : Springer Science and Business Media LLC. - 1687-6172 .- 1687-6180. ; 2011, s. Art. no. 743916-
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Tidskriftsartikel (refereegranskat)abstract
- We consider a cooperative multi-cell MIMO (a.k.a network MIMO) downlink system with multiantennabase stations (BSs), which are connected to a central unit and communicate with multi-antennausers. In such a network, obtaining perfect channel state information (CSI) of all users at the central unitto exploit opportunistic scheduling requires a substantial amount of feedback and backhaul signaling.In this paper, we propose a scheduling algorithm based only on the knowledge of the average receivedSNR at each user from all the cooperating BSs, denoted as incomplete CSI. Multi-mode transmission isapplied that is able to adaptively adjust the number of data streams transmitted to each user. Utilizingthe results of random matrix theory, an analytical framework is proposed to approximate the ergodicrate of each user with different number of data streams. Using these approximate ergodic rates, a jointuser and mode selection algorithm is proposed, where only the scheduled users need to feed backinstantaneous CSI. Simulation results demonstrate that the developed analytical framework providesa good approximation for a practical number of antennas. While substantially reducing the feedbackoverhead, it is shown that the proposed scheduling algorithm performs closely to the opportunisticscheduling algorithm that requires instantaneous CSI feedback from all users.
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| 12. |
- Seifi, Nima, 1981, et al.
(författare)
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Throughput Optimization for MISO Interference Channels via Coordinated User-Specific Tilting
- 2012
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Ingår i: IEEE Communications Letters. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-2558 .- 1089-7798. ; 16:8, s. 1248-1251
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 13. |
- Seifi, Nima, 1981, et al.
(författare)
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Throughput optimization in MU-MIMO systems via exploiting bs antenna tilt
- 2012
-
Ingår i: 2012 IEEE Globecom Workshops, GC Wkshps 2012. - 9781467349413 ; , s. 653-657
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Konferensbidrag (refereegranskat)abstract
- In this paper, we propose a novel framework to enhance the throughput in a single-cell multiuser multiple-input multiple-output (MU-MIMO) system via adaptive base station (BS) antenna tilting. The key idea is to partition the cell into so-called vertical regions and to apply one out of a finite number of tilt and vertical half-power beamwidth (HPBW) pairs at the BS when serving each vertical region. This strategy results in an increased average received signal-to-noise ratio (SNR) over the coverage area. A scheduler is then employed to schedule the transmission to one of the vertical regions at each time-slot with the objective to maximize a suitable utility function of users' achievable throughput. MU-MIMO zero-forcing beamforming in the horizontal plane is used at the BS to serve the users in each vertical region. Simulation results show that the proposed adaptive BS tilting scheme is a promising approach to provide a noticeable throughput gain compared to a conventional scheme employing a fixed tilt at the BS. The proposed framework also achieves a superior performance-complexity tradeoff compared to a conventional system in which the tilt can be adapted to the location of the served users at each time-slot.
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| 14. |
- Zhang, Xinlin, 1986, et al.
(författare)
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Impact of residual transmit RF impairments on training-based MIMO systems
- 2014
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Ingår i: 2014 IEEE International Conference on Communications, ICC 2014. - : IEEE. - 9781479920037 ; , s. 4741-4746
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Konferensbidrag (refereegranskat)abstract
- Radio-frequency (RF) impairments, that exist intimately in wireless communications systems, can severely degrade the performance of traditional multiple-input multiple-output (MIMO) systems. Although compensation schemes can cancel out part of these RF impairments, there still remains a certain amount of impairments. These residual impairments have fundamental impact on the MIMO system performance. However, most of the previous works have neglected this factor. In this paper, a training-based MIMO system with residual transmit RF impairments (RTRI) is considered. In particular, we derive a new channel estimator for the proposed model, and find that RTRI can create an irreducible estimation error floor. Moreover, we show that, in the presence of RTRI, the optimal training sequence length can be larger than the number of transmit antennas, especially in the low and high signal-to-noise ratio (SNR) regimes. An increase in the proposed approximated achievable rate is also observed by adopting the optimal training sequence length. When the training and data symbol powers are required to be equal, we demonstrate that, at high SNRs, systems with RTRI demand more training, whereas at low SNRs, such demands are nearly the same for all practical levels of RTRI.
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| 15. |
- Zhang, Xinlin, 1986, et al.
(författare)
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On the MIMO Capacity with Residual Transceiver Hardware Impairments
- 2014
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Ingår i: IEEE International Conference on Communications, ICC 2014, Sydney, Australia. - : IEEE. - 9781479920037 ; , s. 5299-5305
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Konferensbidrag (refereegranskat)abstract
- Radio-frequency (RF) impairments in the transceiver hardware of communication systems (e.g., phase noise (PN), high power amplifier (HPA) nonlinearities, or in- phase/quadrature-phase (I/Q) imbalance) can severely degrade the performance of traditional multiple-input multiple-output (MIMO) systems. Although calibration algorithms can partially compensate these impairments, the remaining distortion still has substantial impact. Despite this, most prior works have not analyzed this type of distortion. In this paper, we investigate the impact of residual transceiver hardware impairments on the MIMO system performance. In particular, we consider a transceiver impairment model, which has been experimentally validated, and derive analytical ergodic capacity expressions for both exact and high signal-to-noise ratios (SNRs). We demonstrate that the capacity saturates in the high-SNR regime, thereby creating a finite capacity ceiling. We also present a linear approximation for the ergodic capacity in the low-SNR regime, and show that impairments have only a second-order impact on the capacity. Furthermore, we analyze the effect of transceiver impairments on large-scale MIMO systems; interestingly, we prove that if one increases the number of antennas at one side only, the capacity behaves similar to the finite-dimensional case. On the contrary, if the number of antennas on both sides increases with a fixed ratio, the capacity ceiling vanishes; thus, impairments cause only a bounded offset in the capacity compared to the ideal transceiver hardware case.
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