Jeongchan Kim

Proportional fair scheduling algorithm for SC-FDMA in LTE uplink

In uplink 3GPP LTE, SC-FDMA has been regarded as an attractive alternative to OFDMA due to lower Peak-to-Average Power Ratio (PAPR) characteristic. However, in order to get the benefit of PAPR, it requires a contiguous assignment of resource blocks to a single user. In this paper, we suggest heuristic algorithm for proportional fair (PF) scheduling which is known to be NP-hard problem. Under the contiguity constraint, conventional heuristic algorithms assign resource block to a user who has the highest metric and keep on doing assignment according to greedy selection but without any examination of prior selected choice. Our work differs from the conventional works in considering user selection based on PF metric difference and comparison of marginal scheduling metric for examination of selected choice. In simulation results, we demonstrated that the proposed scheme shows better fairness in terms of CDF of normalized throughput and Jains fairness index.

A User Selection Algorithm Using Angle between Subspaces for Downlink MU-MIMO Systems

One of major issues in the efficient use of radio resource for multiuser multiple-input/multiple-output (MU-MIMO) systems is the selection of users to achieve the maximum system throughput. The optimal user selection algorithm, which requires exhaustive search, is prohibitive due to its high computational complexity where Block diagonalization (BD) method is applied and known as a suboptimal precoding technique for downlink MU-MIMO systems, which intents to perfectly eliminate inter-user interference. In this paper, we propose efficient, iterative user selection algorithms with low complexity, where the product of eigenvalues of effective channels is utilized as a selection metric by applying the concept of principal angles between subspaces. And, we further examined the applicability of the proposed algorithms to limited feedback systems and proportional fair (PF) scheduling. Through computational complexity analysis, we show that the proposed algorithm has low complexity with a little loss in throughput. Simulation results validate that the proposed algorithm achieves almost the same system throughput by a capacity-based algorithm under high SNR regime with considerable reduction in complexity.

An efficient scheduler for uplink single carrier FDMA system

In uplink 3GPP Long-Term Evolution(LTE), single-carrier frequency division multiple access(SC-FDMA) is considered partly because of the fact that Peak-to-Average Power Ratio(PAPR) for Multi-Carrier Orthogonal Frequency Division Multiplexing(MC-OFDM) is still a hindrance. But, an unique characteristic of SC-FDMA is that subcarriers have to be assigned by contiguous manner for a specific user, which induces a complicated scheduling algorithm to SC-FDMA because a user is not able to exploit frequency selectivity gain fully. To increase system throughput, we propose a new frequency scheduling method using each users channel gain difference and a contiguity assigning method. Focused on maximizing the total system capacity of SC-FDMA uplink, we compare our algorithms with others in uplink 3GPP LTE. Simulation results show that our proposed scheme outperforms over the existing schemes.

Available bandwidth-aware cell selection for expanded regions of small cells adopting ABS

Cell selection in current LTE-Advanced depend on reference signal received power or reference signal received quality which results to low user association probability. Therefore, we propose an aggregate objective cell selection technique based on enhanced inter-cell interference coordination technique developed by 3GPP in Rel-10 known as almost blank subframe. We also adopt the use of cell range expansion technique as means to encourage offloading. Cell range expansion was introduced in LTE-advanced, it encourages user equipment to select a small cell with a weakest signal as thier serving cell by adding a positive bias in cell selection process. Simulation shows that the proposed scheme results to high user association probability, while giving superior performance in average throughput compared to conventional single objective base cell selection for cell range expansion.

An Efficient Prewhitening Scheme for MIMO Cognitive Radio Systems

In this paper, we investigate an interference mitigation scheme under multiple-input multiple-output (MIMO)-based cognitive radio environment, where primary and secondary systems cooperate, sharing the channel state information (CSI) of the interference link. We propose a prewhitening scheme where the precoder at a secondary transmitter uncorrelates the correlated interference in advance using only the direction information on the interference channel reported from a primary system. First, we decompose the precoding matrix of a conventional prewhitening scheme, in which full CSI is required, into direction and magnitude components. Then, using only the direction information on the interference channel, the proposed precoder projects the row-space vectors of the interference channel onto the scaled null-space vectors from the precoder. Through mathematical analysis and simulations, we show that the proposed prewhitening scheme can achieve almost the same performance as the conventional prewhitening scheme. We investigate system performance in terms of achievable rate, average received interference power, and average signal-to-interference ratio. As a result, we can conclude that for prewhitening, the use of the direction information on the interference channel can provide almost the same performance as that of the full CSI.

Uplink spectrum resource allocation in heterogeneous networks (small cell/macrocell)

Small cells are expected to increase network capacity, extend the macrocell coverage and to add edge-based intelligence. These advantages are achieved by overlaying macrocell networks with the small cells, resulting in a two-tier network. However, the average capacity of the network is reduced due to interference generated by the two-tier configuration. Conventional small cells are configured to have either an open or closed access scheme. Small cells with a hybrid access scheme or partially open access scheme are known for their flexibility and improved performance achieved through their interference-mitigating ability and adaptive resource allocation capabilities. In a hybrid access scheme, resource allocation is a vital issue in the design of small cells networks. In this paper, we propose an uplink resource allocation technique to enhance hybrid small cell capacity in orthogonal frequency division multiple access (OFDMA) two-tier cellular networks. In particular, we investigate a new possibility available for developing an optimal scheme for OFDMA hybrid access small cells. We formalize the problem as an optimization problem for an uplink resource allocation that partitions the bandwidth for both subscribed or unsubscribed users. Moreover, we propose a convex optimization model for the gradient of the capacity difference between closed access and hybrid access capacity. We present two schemes, firstly an optimal scheme and secondly a scheme that is the simple version of the optimal scheme. Numerical results show the effectiveness of the proposed bandwidth resource allocation, where the simple solution shows inferior performance than the optimal solution with a small margin while the optimal solution has a more mathematical complexity disadvantage due to the mathematical functions it employs. Numerical results also show the convergence and effectiveness of the proposed uplink bandwidth resource allocation scheme.

Dynamic Subchannelization for Adaptation to Multipath Fading in Mobile WiMAX

In mobile WiMAX based on the IEEE 802.16e wireless MAN technologies using an OFDMA, the system throughput is improved by subchannelization schemes such as Diversity and AMC in multipath fading channel environment. Especially, in high mobility environment, mobile WiMAX systems uses FUSC which is one of the diversity subchannelization schemes, and get frequency diversity under multipath fading channel. However, the FUSC scheme is not flexible for channel variations inherent to users speed. In this paper, we propose dynamic subchannelization scheme under multipath fading channels which is varied with a users speed. Simulation results show that the proposed subchannelization algorithm leads to better the system throughput as compared to conventional subchannelization scheme such as FUSC in various channel environments.

Efficient CQI feedback via network coding for wireless relay networks

For the efficacy of radio resource management in downlink wireless relay networks, the channel quality indicator (CQI) between a mobile station (MS) and a relay station (RS) should be fed back from the RS to its base station (BS), at the cost of additional feedback overhead. In this letter, we propose an efficient feedback scheme based on network coding between a pilot sequence and the CQI of MS-RS link, through which the BS can have both CQIs of MS-RS and RS-BS links without overhead. Numerical results reveal that the proposed CQI reporting scheme has as good feedback performance as conventional one which generally requires additional feedback burden.

A New Hybrid Packet Scheduling Algorithm with QoS Provision in HSDPA

Recently, many channel-dependent scheduling schemes for high speed downlink packet access (HSDPA) system have been introduced to provide high system throughput and quality-of-service (QoS) guarantee. However, no work has considered hybrid automatic repeat request (HARQ), one of special features of Universal Mobile Telecommunications System (UMTS) Release 5 network. In this paper, a new packet scheduling algorithm which suspends HARQ process is proposed. The performance of the proposed scheme is compared with normal HARQ mode with max CIR and PF algorithms in fast fading channels. Results from system-level simulation show that with the proposed algorithm average cell throughput can be increased with no degradation of QoS.

An Efficient Downlink Coordinated Beamforming for Heterogeneous Networks

In heterogeneous networks (HetNet), interference coordination techniques among cells have been widely investigated to reduce co-channel interference (CCI), which causes severe system performance degradation when a same channel is shared at both femtocells and macrocell. In this paper, we examine a coordinated beamforming technique to reduce downlink (from macrocell to femtocells) CCI and processing complexity. Using uplink sounding reference signal (SRS), we propose an efficient beamforming searching function for a macro-BS reducing feedback burden compared to conventional schemes. Simulation results are provided for performance comparison in terms of the spectrum efficiency of femtocells and system throughput.