Users In Adjacent Cells Research Articles

A Smart Pilot Assignment in Multi-Cell Massive MIMO Systems Using Virtual Modeling of Assigning Cost

In this paper, a smart pilot sequence assignment method is proposed to minimize inter-cell interference generated in a massive multi-input multi-output (MIMO) system due to pilot contamination in uplink TDD (Time Division Duplex) mode. The proposed method employs a zero-one integer linear programming method as the assignment algorithm. The amount of intercell interference imposed on the target cell users is considered as assigning cost. The introduced assigning cost is composed of the steering vectors of the desired users in the target cell, and the sum of the channel correlation matrices of interference users in adjacent cells. By exploiting the virtual channel modeling technique, we develop a simple alternative approach to calculate the assigning cost, which has low complexity. We prove that the new assigning cost, called virtual assigning cost, is equal to the assigning cost defined based on the physical channel model. Moreover, we develop a token-based protocol to manage the coupling between cells in the whole multicellular network based on a distributed manner without the need for data exchange between the BSs. Simulation results demonstrate that the proposed low complex smart pilot sequence assignment method achieves a good performance, which is better than those of some other related works in multicellular design regarding normalized mean square error (NMSE) and achievable rate criteria.

Pilot Decontamination in Noncooperative Massive MIMO Cellular Networks Based on Spatial Filtering

Pilot contamination has been known as one of the most challenging issues in massive multiple-input multiple-output (MIMO) systems. Every user will experience interferences from users in adjacent cells who employ the same pilot sequence. For cell-edge users, pilot contamination is particularly detrimental, because their signals might be overwhelmed by the interference. In this paper, we propose a pilot decontamination method based on a spatial filter, which exploits the spatial sparsity of massive MIMO channels. In massive MIMO systems, the communication protocols are generally divided into four phases: pilot transmission, processing, uplink data transmission, and downlink data transmission. In the first phase, the base station (BS) receives both the desired signal and the pilot contaminated signal. In the second phase, all users in the target cell stay silent for one symbol period, and the BS only receives interference from adjacent cells. The fast Fourier transform can then be employed to analyze the spatial spectrums of the received signals. The spatial sparsity of the massive MIMO channels makes it possible to identify the pilot contamination components by comparing the two spectrums on different spatial signatures (or angles of arrival). A spatial filter can then be constructed to eliminate pilot contamination. Both the theoretical analysis and simulation results demonstrate the effectiveness of the proposed method, whose complexity is comparable to that of the traditional matched filter-based channel estimator.

Proposed Model for Interference Estimation in Code Division Multiple Access

Cellular CDMA systems are usually affected by interference experienced by users in adjacent cells that decrease the Quality of Services in wireless communication network. Hence, interference is the limiting factor of capacity in CDMA cellular and it is one of the problems fighting against the high efficiency of any mobile network. In this paper, a mathematical model to estimate the average number of users contributing in inter-cell interference at the busy hours of CDMA network is proposed. As the power exponent value has significant affect on interferer signal attenuation and hence other-cells interference, measurements were carried through a drive test to determine the received power level at various distance from CDMA base stations at Baghdad. The results obtained show that the power exponent was 2.71. This value was applied in dual-slop path loss model to determine the expected interference factor, and the number of users that can be hold at each cell. Simulations showed that users at a boundary cell generate more interference than those close to the base station. Furthermore, it was denoted that greater number of users caused to increase the interference factor, and greater power exponent value result in interference factor reduction.

Dynamic interference coordination with analytical near-optimal power allocation toward high user fairness

To mitigate interference on cell-edge users and improve fairness of the whole system, dynamic inter-cell interference coordination (ICIC) is one of the promising solutions. However, traditional dynamic ICIC is considered as an NP-hard problem and power variability further adds another dimension to this joint optimization issue, making it even more difficult to quickly reach a near-optimal solution. Therefore, we theoretically obtain the closed-form expression of the near-optimal power allocation ratio for users in adjacent cells paired in the same resource block and interfere each other, so that the total utility corresponding to α-fairness is maximized. Dynamic ICIC using this closed-form solution could improve user fairness without causing an increment of the computational complexity. Numerical results show that, compared with the schemes using identical power for different users, our method does not obviously degrade the system's average spectral efficiency.

Pilot distribution optimization in multi-cellular large scale MIMO systems

Salient characteristics of a wireless communication system deploying a great number of antennas at the base station (BS), namely a massive MIMO system, are investigated in this work. The asymptotic performance of the linear zero-forcing precoding scheme is found, both in terms of signal-to-interference-plus-noise ratio (SINR) and bit-error rate (BER), and shown to be equivalent to the matched-filter beamforming performance. Furthermore, analysis of the massive MIMO system downlink is carried out from the viewpoint of uncoded BER performance, including some realistic adverse effects, such as interference from neighbouring cells, channel estimation errors due to background thermal noise, and pilot contamination. The latter has been shown to be the only impairment that remains in the MIMO multicell system with infinite number of BS antennas. For such scenario, we derive expressions for the asymptotic BER, i.e. in the limit of infinite number of antennas at BS. A quite simple and efficient method for optimizing the massive MIMO system performance under different optimization metrics is proposed, which consists of simply distributing the pilot sequences among the users of the cell in an efficient manner. As a result, a user rate gain of six times regarding the random strategy has been achieved for the downlink with unitary reuse factor, while the user rate increases twice for reuse factor of three. These benefits are achieved by only knowing the powers and the long-term fading coefficients of users in adjacent cells, for each pilot sequence.

Soft Pilot Reuse and Multicell Block Diagonalization Precoding for Massive MIMO Systems

The users at cell edge of a massive multiple-input–multiple-output (MIMO) system suffer from severe pilot contamination (PC), which leads to poor quality of service (QoS). To enhance the QoS for these edge users, soft pilot reuse (SPR) combined with multicell block diagonalization (MBD) precoding is proposed. Specifically, the users are divided into two groups according to their large-scale fading coefficients, which are referred to as the center users, who only suffer from modest PC, and the edge users, who suffer from severe PC. Based on this distinction, the SPR scheme is proposed for improving the QoS for the edge users, whereby a cell-center pilot group is reused for all cell-center users in all cells, whereas a cell-edge pilot group is applied for the edge users in the adjacent cells. By extending the classical block diagonalization precoding to a multicell scenario, the MBD precoding scheme projects the downlink transmit signal onto the null space of the subspace spanned by the intercell channels of the edge users in adjacent cells. Thus, the intercell interference contaminating the edge users' signals in the adjacent cells can be efficiently mitigated, and hence, the QoS of these edge users can be further enhanced. Our theoretical analysis and simulation results demonstrate that both the uplink and downlink rates of the edge users are significantly improved, albeit at the cost of the slightly decreased rate of center users.

An Overview of Massive MIMO: Benefits and Challenges

Massive multiple-input multiple-output (MIMO) wireless communications refers to the idea equipping cellular base stations (BSs) with a very large number of antennas, and has been shown to potentially allow for orders of magnitude improvement in spectral and energy efficiency using relatively simple (linear) processing. In this paper, we present a comprehensive overview of state-of-the-art research on the topic, which has recently attracted considerable attention. We begin with an information theoretic analysis to illustrate the conjectured advantages of massive MIMO, and then we address implementation issues related to channel estimation, detection and precoding schemes. We particularly focus on the potential impact of pilot contamination caused by the use of non-orthogonal pilot sequences by users in adjacent cells. We also analyze the energy efficiency achieved by massive MIMO systems, and demonstrate how the degrees of freedom provided by massive MIMO systems enable efficient single-carrier transmission. Finally, the challenges and opportunities associated with implementing massive MIMO in future wireless communications systems are discussed.

A Multi-Cell Coordinated Scheduling Algorithm Based on Delta-CQI for CoMP Network Systems

In the coordinated scheduling/beamforming of coordinated multi-point (CoMP) systems, a multi-cell coordinated scheduling algorithm is proposed for inter-cell interference coordination and cell-edge users’ throughput enhancement. The concept of Delta-CQI is introduced to indicate the interference level experienced by users in one cell when the users in adjacent cells of the cooperation region using different precoding vectors. Adjacent base stations exchange the information of Delta-CQI which is fed back from the users in each cell and calculate the weighted interference factor, then the user scheduling is performed by the multiple cells in the cooperative cell set along with the priority factor given by the proportional fairness scheduling algorithm used in each cell. The proposed multi-cell coordinated scheduling algorithm considers the user interference among different cells to improve the data rate of users in cell edge, which gives a good tradeoff between system fairness and spectrum efficiency. Beside that, the overhead of the control information exchange among coordinated cells are reduced by the Delta-CQI feedback and exchange.

Downlink Noncoherent Cooperation without Transmitter Phase Alignment

Multicell joint processing can mitigate inter-cell interference and thereby increase the spectral efficiency of cellular systems. Most previous work has assumed phase-aligned (coherent) transmissions from different base transceiver stations (BTSs) so that the signals superpose coherently at each receiver, which is difficult to achieve in practice. In this work, a noncoherent cooperative transmission scheme for the downlink is studied, which does not require phase alignment. The focus is on jointly serving two users in adjacent cells sharing the same resource block. The two BTSs partially share their messages through a backhaul link, and each BTS can transmit a superposition of two codewords, one for each receiver. Each receiver decodes its own message, and treats the signals for the other receiver as background noise. With narrowband transmissions the achievable rate region and maximum achievable weighted sum rate are characterized by optimizing the power allocation (and the beamforming vectors in the case of multiple transmit antennas) at each BTS between its two codewords. For a wideband (multicarrier) system, a dual formulation of the optimal power allocation problem across sub-carriers is presented, which can be efficiently solved by numerical methods. Results show that the proposed cooperation scheme can improve the sum rate substantially in the low to moderate signal-to-noise ratio (SNR) range.

Multicell Interference Mitigation with Joint Beamforming and Common Message Decoding

Conventional wireless cellular systems treat out-of-cell interference as noise. This paper proposes methods and examines the benefit of designing decodable interference signals, whereby a transmitter may split its message into a common and a private part, and the common message may be decoded and subtracted by users in adjacent cells. This paper considers a downlink scenario, where the base-stations are equipped with multiple antennas, the mobile users are equipped with a single antenna, and multiple users are active simultaneously via spatial multiplexing. The network optimization problem consists of jointly determining the appropriate users in adjacent cells for rate splitting, the optimal transmit beamformers for common and private messages, and the optimal common-private rates to maximize the minimum achievable rate across the users. This paper shows that for fixed user selection and fixed common-private rate splitting, the optimization of transmit beamformers can be solved using a semidefinite programming (SDP) relaxation approach. Further, it is shown that for the case where the network consists of two message-splitting pairs, SDP relaxation is tight, i.e., beamforming is optimal. Finally, this paper proposes a heuristic user-selection and rate splitting strategy to characterize the performance improvement for cell-edge users due to common-message decoding.

Multiuser MISO Transmitter Optimization for Intercell Interference Mitigation

The transmitter optimization (i.e., steering vectors and power allocation) for a MISO Broadcast Channel (MISO-BC) subject to general linear constraints is considered. Such constraints include, as special cases, the sum power, the per-antenna or per-group-of-antennas power, and "forbidden interference direction" constraints. We consider both the optimal dirty-paper coding and the simple suboptimal linear zero-forcing beamforming strategies, and provide numerically efficient algorithms that solve the problem in its most general form. As an application, we consider a multi-cell scenario with partial cell cooperation, where each cell optimizes its precoder by taking into account interference constraints on specific users in adjacent cells. The effectiveness of the proposed methods is evaluated in a simple system scenario including two adjacent cells, under different fairness criteria that emphasize the bottleneck role of users near the cell "boundary". Our results show that "active" Inter-Cell Interference (ICI) mitigation outperforms the conventional "static" ICI mitigation based on fractional frequency reuse.

Efficient Architecture and Handoff Strategy used for VoIP Sessions in SIP Based Wireless Networks

In the near future, the Internet is likely to become an All-IP network that provides various multimedia services over wireless networks. Although the earliest VoIP applications did not consider the end-node mobility, researchers have attempted to support mobility in current VoIP protocols, such as Session Initial Protocol (SIP)-based mobility. The SIP-based mobility is considered because it can readily support mobility. However, calling disruptions may occur in traditional SIP mid-call terminal mobility because handoff procedure may be required, depending on the implementation and the real network deployment considerations. In any case, issues in the combined SIP/RSVP for guaranteeing QoS of VoIP service under mobile environment are also considered to be crucial. Therefore, this study describes the solutions by devising novel hierarchy network architecture. Also, the mechanisms including help with neighboring users in adjacent cells and the third party call control to overcome those issues are included. The simulation results indicate that the proposed technique is practical and better executive than conventional schemes.

Modeling Dynamic Channel-Allocation Algorithms in Multi-BS TDD Wireless Networks With Internet-Based Traffic

Future time-division-duplex (TDD) systems operating over small wireless networks will utilize intelligent base station (BS)-coordinated dynamic channel-allocation algorithms in order to support high-bandwidth asymmetric traffic in adjacent cells. In this paper, we use extensive measurements of wireless Internet traffic from a large 802.11b network to create two random traffic models. One model, called binomial, is memoryless and the other, called dynamic, is based on an event-driven Markov state model with bidirectional flows and deterministic residence times. We then develop a two-BS two-zone wireless TDD interference model that describes the spatial features of interference between cochannel mobile stations (MSs) in adjacent BSs. This is a simplified precursor to more sophisticated models for multiple BSs and/or multisector BSs. We present a set of candidate TDD channel-allocation algorithms, which vary in their level of time-slot coordination and intelligent allocation between BSs. Lastly, we combine the three components (i.e., traffic models, interference models, and channel-allocation algorithms) to demonstrate the capacity for evaluating dynamic channel-allocation algorithms in realistic interference and Internet traffic scenarios. The results show that, for active MSs, the dynamic traffic model has a higher number of packet requests per time frame than the binomial traffic model, given the same mobile activity factor. Additionally, fixed channel-allocation algorithms generally perform much worse than pseudorandom and intelligent BS-coordinated algorithms, especially for asymmetric BSs. The pseudorandom algorithm performs well at low traffic, but suffers from severe interference blocking at high traffic. The intelligent BS-coordinated algorithm performs best, as it avoids MS-to-MS interference blocking from nearby users in adjacent cells and maximizes the overall throughput by attempting to allocate up- and downlink packet requests in corresponding time slots matched to the incoming uplink-downlink traffic demand for each time frame.

Opportunistic beamforming using dumb antennas

Multiuser diversity is a form of diversity inherent in a wireless network, provided by independent time-varying channels across the different users. The diversity benefit is exploited by tracking the channel fluctuations of the users and scheduling transmissions to users when their instantaneous channel quality is near the peak. The diversity gain increases with the dynamic range of the fluctuations and is thus limited in environments with little scattering and/or slow fading. In such environments, we propose the use of multiple transmit antennas to induce large and fast channel fluctuations so that multiuser diversity can still be exploited. The scheme can be interpreted as opportunistic beamforming and we show that true beamforming gains can be achieved when there are sufficient users, even though very limited channel feedback is needed. Furthermore, in a cellular system, the scheme plays an additional role of opportunistic nulling of the interference created on users of adjacent cells. We discuss the design implications of implementing. this scheme in a complete wireless system.

A Time Reuse Capture Access (TRCA) Protocol for PCS and Wireless ATM Applications

The Time Reuse Capture Access (TRCA) protocol isa medium access control protocol appropriate forpersonal communications and wireless AsynchronousTransfer Mode (ATM) applications. It is based on time reuse rather than frequency reuse. Frequencychannels are reused in every cell (frequency reuse one).Each FDMA channel has a TDMA frame structure. The numberof slots in the frame is equal to the time reuse factor and each slot is assigned to aspecific cell in the reuse cluster. Each mobile usertransmits in its assigned frequency channel and TDMAtime slot which corresponds to the cell it currently belongs. The protocol also exploits the powercapture phenomenon in which simultaneous transmissionsof users in adjacent cells may be successfully received.Two possible applications of the proposed TRCA protocol are considered in this paper. In thefirst application, TRCA is used for random packet accessin wireless personal communication. Users transmit theirpackets using the assigned and non-assigned time slots to take advantage of the effect ofcapture and non-uniform traffic loads. The portion oftransmissions that takes place in each time slot iscontrolled so that the overall throughput is maximized. In the second application, TRCA is used forreliable transmission of ATM cell over a wirelesschannel. The proposed protocol allows the transmissionof continuous bit rate (CBR) ATM cells over a wireless link so that bit error rate (BER) and delayrequirements are met. Transmissions take place in theassigned time slots but the non-assigned time slots canbe used for re-transmission if the previous transmission was unsuccessful.