Adaptive yielding scheme for link scheduling in OFDM-based synchronous device-to-device (D2D) communication system
© Kang et al.; licensee Springer. 2014
Received: 10 March 2014
Accepted: 9 September 2014
Published: 27 September 2014
Compared to asynchronous contention-based random access, e.g., carrier sensing multiple access, synchronous and distributed link scheduling for orthogonal frequency-division multiplexing (OFDM) systems is a viable solution to improve system throughput for device-to-device (D2D) ad hoc network. In particular, spatial spectral efficiency can be improved by scheduling as many concurrent D2D links necessary to satisfy individual signal-to-interference ratio (SIR) requirements. In this paper, we propose an adaptive yielding mechanism that can further improve the spatial spectral efficiency by allowing for more concurrent D2D links whenever more interference can be accepted, e.g., when the instantaneous bandwidth efficiency requirement is less than the current link capacity. Even if the system throughput varies with the link density, it is shown that the average system throughput can be significantly improved by the proposed yielding mechanism.
As opposed to the cellular systems which have been designed to support the mobile station throughout of the rather wide-area coverage, there have been various types of schemes or systems, including Wi-Fi Direct or Bluetooth, which establish links between nearby devices without resorting to an associated access point[1–3]. In the recent advance in cellular systems, such a device-to-device (D2D) communication scheme has been considered as a means of device-centric mobile social network service (SNS), which discovers the proximity devices and then connect them over the direct link. In general, it can be implemented either as inband or outband D2D. In the inband D2D type, the cellular spectrum is used for both cellular and D2D links. As the licensed spectrum can be fully controllable, quality of service (QoS) provision is ensured in the inband D2D type. Inband D2D communication can be further divided into underlay and overlay scenarios. In this paper, we consider the overlay scenario, in which the D2D links are allocated to the dedicated cellular resources, thus incurring no co-channel interference between the cellular and D2D links. The obvious advantage of D2D communication in this scenario is that radio resources can be reused by supporting multiple D2D communication pairs at the same time if any, without resorting to macro-cell links[6–12]. In order to improve the spectral efficiency in the overlay D2D scenario, the direct communication can be either assisted by the base station (BS), which involves scheduling the resource for D2D link. However, D2D communication can be autonomous for the outband overlay scenario, in which D2D links can be scheduled without any centralized assistance by the base station. Once a dedicated spectrum is set aside for D2D communication in the overlay scenario, the idea is that D2D communication in the cellular system would turn it into a peer-to-peer ad hoc network. Then, we need to develop a distributed link scheduling protocol that can improve the spectral efficiency while spatially reusing the radio resource among the concurrent D2D links. One particular example in this scenario is the Qualcomm FlashLinQ system, which will be focused in this paper. A distributed type of D2D communication will be useful by connecting the handsets with each other in the cellular system when a base station is lost or the handsets are out of coverage, e.g., during an emergency situation. For example, the IEEE 802.16n and IEEE 802.16.1 standards have been specified for public safety or Public Protection and Disaster Relief (PPDR) applications[14, 15]. Recently, the same design objective has been considered by employing a new concept of information-theoretic independent sets (ITIS), which may achieve a theoretical upper bound on the D2D capacity. However, the underlying assumption behind the performance in is that every transmitter knows the link scheduling status of all other links a priori, which cannot be implemented in practice.
On the other hand, new application services, including mobile advertisement, mobile social network service (SNS), mobile content sharing, and group data communication, can be developed if one mobile device discovers other nearby devices and D2D links are established. In particular, an obvious advantage of D2D communication in the orthogonal frequency-division multiplexing (OFDM)-based cellular system is extending coverage, such as that in D2D communication between handsets in the different cells, by concentrating the carrier power into a subset of the subcarrier, which facilitates different application services. This might not be possible in the existing local area networks (LANs) or personal area networks (PANs), simply due to the limited coverage. However, a distributed nature in their collision-avoidance type of access control mechanism is still similar to that in the peer-to-peer ad hoc networks.
A D2D communication system must deal with two different functionalities: discovery and communication. Consider multiple D2D mobile devices randomly distributed in a system. From a communication perspective in this system, the overall system throughput is one of the most important performance factors, as in all other networks. Link scheduling is required to establish the links for the multiple D2D pairs at the same time under QoS constraints while maximizing the overall system throughput. In particular, an OFDM-based D2D ad hoc network employs a synchronous and distributed medium access control protocol which controls the admissible interference with respect to signal-to-interference ratio (SIR) and outperforms the conventional carrier sensing multiple access scheme. In order to guarantee the received SIR in all links, it executes the yielding procedures with the fixed SIR thresholds in both the transmitter and receiver. In this paper, we propose a new type of link scheduling scheme that can eliminate the inefficiency associated with distributed scheduling in a synchronous D2D ad hoc network and demonstrate its performance gain over the existing scheme. It allows for reusing other links spatially within the permissible range while consuming the link capacity only as much as enough to satisfy the required bandwidth efficiency for each link. Furthermore, it allows for a data rate fall-back when the traffic load drops, e.g., a transmission buffer becomes less congestive, which can maximize the spatial reuse efficiency adaptively with the traffic load and link distribution. In addition to improvement in the system throughput, these features enhance the fairness among the links, attributing to improving the average throughput of the low-priority access users, as opposed to the FlashLinQ system in which some users may always suffer from the lower access priority in each frame.
In Section 2, we first describe a basic model and the existing yielding scheme for link scheduling in the OFDM-based D2D communication system. A concept of the proposed adaptive yielding scheme is presented in Section 3. The specific procedure to implement the proposed concept is given in Section 4. Its performance is evaluated by system-level simulation in Section 5. Finally, Section 6 presents the concluding remarks.
2 Yielding mechanism for OFDM-based synchronous D2D communication
As our proposed scheme is strictly based on the existing system, e.g., FlashLinQ, we describe a common system model for link scheduling in the OFDM-based D2D communication system. In fact, the frame and signaling structures in the specification of FlashLinQ are provided to serve as a common system model for our discussion. Furthermore, since our scheme is an improved version of the link-scheduling mechanism in FlashLinQ, the procedure of the existing one is detailed in this section.
2.1 System model
2.2 Yielding procedure for connection scheduling
where η RX is referred to as an Rx yielding threshold. If (1) is satisfied, then Rx node D does not transmit a CTS in response to an RTS, which indicates Rx yielding.
where η TX is referred to as a Tx yielding threshold. In order to check the Tx yielding condition in (2), Tx node C must be able to estimate the value of P A |h AB |2/|h BC |2. Toward this end, Rx node B of the high-priority link transmits an inverse power echo, which corresponds to the inverse of the received power from Tx node A. Then, Tx node C receives a power of r p = |h BC |2/(P A |h AB |2), which includes the channel gain h BC between Tx node C and Rx node B. After Rx node C receives inverse power echo, the left-hand side of (2) can be estimated by 1/(r p · P C ), which allows for the determination of whether Tx node C must yield or not.
3 Adaptive yielding mechanism: overview
The key idea of improving the throughput over the existing link scheduling scheme in this paper is to allow the low-priority link to be scheduled within a range of the allowable interference by minimizing the unnecessary transmission opportunities of the high-priority links. A notion of unnecessary transmission is viewed from the two different points. One point is that a link can reduce its own transmission rate by accepting additional interference when a packet waiting in the transmit buffer is too short to fill up the current slot in the course of its link scheduling. In other words, a link capacity is set to the minimum data rate that is required to meet its own demand in the current slot while accepting more interference so as to allow the low-priority links to be scheduled simultaneously. As additional interference is allowed only as long as the minimum capacity required for the high-priority link is guaranteed, the current sharing process is referred to as conservative yielding. That is, a conservative yielding mechanism of the high-priority link allows for the low-priority link to be scheduled opportunistically, only whenever the high-priority link can reduce its own data rate for the packet to transmit in the current slot. In spite of conservative yielding, however, a receiver of the low-priority links may still have to yield its transmission (i.e., Rx yielding) due to interference caused by the high-priority links. For the conservative yielding mechanism to be effective from the first viewpoint, therefore, a transmitter of the high-priority link must give up its transmission or reduce its transmit power for the low-priority link not to perform Rx yielding. Toward this end, the second viewpoint of minimizing the unnecessary transmission of the high-priority link is to reduce its transmission power as long as the total size of the packets waiting in the buffer of the high-priority link is below the given threshold. As the high-priority link reduces its own transmit power, it may be faced with Rx yielding. Since this particular yielding process involves with sacrificing the high-priority link when its capacity is not immediately required, e.g., in terms of its buffer status, it is referred to as generous yielding. Note that the aforementioned conservative yielding becomes effective only when it is combined with generous yielding. It is due to the fact that the low-priority link can be scheduled only when it is not subject to both Rx yielding and Tx yielding at the same time. In other words, even if the high-priority link is not subject to Tx yielding by conservative yielding of the high-priority link, its receiver still may have to yield due to interference from the links of the higher priority, making the conservative yielding mechanism useless. Therefore, our proposed design aims at combining both conservative and generous yielding mechanisms so tightly that the system throughput performance can be enhanced by minimizing unnecessary transmission of the high-priority links.
3.2 Throughput enhancement with Tx fallback-based adaptive yielding mechanism
The D2D link in Figure3 can still be referred to in order to investigate the throughput enhancement effect of the proposed adaptive yielding mechanism. Let the slot length and packet length be T (seconds) and L (bytes), respectively. Furthermore, let Q denote the number of bits for the awaiting packets in the buffer. Without loss of generality, channel bandwidth is normalized as W = 1 Hz for simplicity of presentation. Let μ A be the capacity required to handle the packet waiting in the buffer at Tx node A, which is given as μ A = Q/T (bits/s). Meanwhile, assume that the low-priority link C-D yields to the high-priority links by taking their interference into account, as illustrated in Figure3. Then, the received SIR over the link A-B is given by, which corresponds to the channel capacity of log(1 + SIR A ) (bits/s).
4 Implementation of adaptive yielding
4.1 CTS power control for conservative yielding
4.2 RTS power control for generous yielding
The low-priority links that can be accepted by conservative yielding may still be subject to Rx yielding, which makes conservative yielding useless in practice. It is due to the situation that the received RTS signal of the low-priority link is weaker than the RTS interference signal of the high-priority link, which forces the low-priority link into Rx yielding even if it is exempted for Tx yielding by the conservative yielding mechanism in the high-priority links. In order to handle this situation, generous yielding by the high-priority links must be implemented.
The number of bits that are buffered in the transmitter of the high-priority link can be a basis to determine whether it yields or not by setting up a buffer threshold. In the case that the number of bits in the buffer is below the threshold, the transmit power can be reduced so that the low-priority links that are accepted by conservative yielding may not yield to the high-priority link. However, as it cannot be immediately known how generous it must be, we consider a step-by-step power control with a step size of Δ (dB). More specifically, its RTS transmit power is reduced by Δ for conservative yielding if the number of bits in the buffer exceeds the buffer threshold, or increased by Δ otherwise.
Of course, in the case that the number of bits exceeds the buffer threshold, the transmit node A increases its power by a factor of 1/β such that generous yielding is given up.
5 Performance analysis
In this section, we provide system-level simulation (SLS) results to analyze the performance gain of the proposed adaptive yielding scheme. We consider the FlashLinQ specification by Qualcomm Inc. as a baseline system model for our simulation, which is described in Section 2. We first describe a simulation scenario to evaluate the performance of the proposed adaptive yielding scheme and then present the simulation results to provide its performance gain over the conventional yielding scheme in FlashLinQ.
5.1 Simulation scenario
System model and simulation parameters
System bandwidth (W)
Slot duration (T)
Maximum MS power (Pmax)
Minimum MS power (Pmin)
Power control step (Δ)
Buffer threshold (τ B )
Simulation area (S)
500 × 500 m
Link distance (d)
Uniform [1,100] m
Arrival rate (λ)
4 packets/slot duration
Packet length (L)
Path-loss exponent (n)
5.2 Simulation results
In this subsection, we consider four different yielding schemes for performance analysis. The first scheme is the one with the fixed yielding thresholds, η TX and η RX , which correspond to the existing scheme (denoted as ‘fixed threshold’). Two other schemes are the adaptive ones with CTS power control for conservative yielding only (denoted as ‘adaptive yielding: CTS power control’) and RTS power control for generous yielding only (denoted as ‘adaptive yielding: RTS power control’). The last scheme is an adaptive the proposed scheme that takes both conservative and generous yielding into account (denoted as ‘the proposed adaptive yielding’).
In order to maximize the spatial resource reuse efficiency in the OFDM-based synchronous device-to-device (D2D) communication system, the number of devices that can maintain acceptable signal-to-interference ratio (SIR) in the receiver must be maximized. In this paper, we have introduced a notion of conservative yielding that allows for the low-priority link to be scheduled without overprovisioning the link quality of its proximate high-priority links with respect to the required bandwidth efficiency as a means of improving the throughput performance over the existing link scheduling scheme. We also have proposed an idea of generous yielding that allows for the low-priority link to be scheduled by falling back the data rate or delaying the transmission for a high-priority transmitter without too much data waiting in the buffer. We have tried to maximize the spatial reuse efficiency for the D2D link by integrating these concepts into the combined yielding scheme that adapts to the traffic load and link distribution. It has been demonstrated by system-level simulation that the proposed adaptive yielding scheme can improve the average throughput performance by more than 20% over the existing scheme with a fixed yielding threshold when the devices are uniformly distributed. As the actual performance gain mainly depends on the distribution of D2D links, its gain can be much higher under some other situations. In conclusion, the proposed link scheduling scheme can achieve more spatial reuse gain than the existing scheme, which has already packed the D2D links spatially as much as possible while providing fairness among the users by sharing the resources dynamically based on traffic demand. It is conjectured that the various types of yielding schemes considered in this paper can be applicable to different situations in which the bandwidth efficiency must be maximized by managing interference in actual traffic demand. For example, a similar concept can be applicable to operating wireless backhaul links in a wireless relay system in which the relay links must be reused while managing the inter-link interference in adaptation with the access traffic load in each relay.
This work was supported in part by Communications Research Team, DMC R&D Center, Samsung Electronics Co., Ltd. and also in part of the project titled Research on Fundamental Core Technology for Ubiquitous Shipping and Logistics funded by the Ministry of Oceans and Fisheries, Korea.
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