Sung Cheol Chang

Opportunistic downlink interference alignment

We introduce an opportunistic downlink interference alignment (ODIA) for interference-limited cellular downlink, which intelligently combines user scheduling and downlink IA techniques. The proposed ODIA not only efficiently reduces the effect of inter-cell interference from other-cell base stations (BSs) but also eliminates intra-cell interference among spatial streams in the same cell. We show that compared to the existing downlink IA schemes, the minimum number of users required to achieve a target degrees-of-freedom (DoF) can be fundamentally reduced, i.e., the fundamental user scaling law can be improved, by using the ODIA. In addition, we introduce a limited feedback strategy in our ODIA framework, and then analyze the minimum number of feedback bits required to obtain the same performance as that of the ODIA assuming perfect feedback.

Discovering Mobile Applications in Device-to-Device Communications: Hash Function-Based Approach

In this paper, we propose a code-based discovery protocol for device-to-device (D2D) communications. To realize proximity-based services in D2D communications, such as mobile social networks and mobile marketing, each device should first discover nearby devices, which have mobile applications of interest, by using a discovery protocol. The proposed discovery protocol makes use of a hash function-based discovery code that contains compressed information of mobile applications in a device. When a device receives a discovery code broadcasted by the other device, the device can approximately find out the mobile applications in the other device. The proposed protocol is capable of quickly discovering massive number of devices while consuming a relatively small amount of radio resources. By simulation, we show that the proposed protocol greatly outperforms a simple name-based protocol.

Performance analysis of random access in IEEE 802.16m system

IEEE 802.16m, also known as the WirelessMAN-Advanced air interface, system adopts a random access using CDMA (Code Division Multiple Access)-based random access and message-based random access. The message-based random access is intended to reduce access delay when random access load is low. However, CDMA-based random access is used as random access load increases due to failure of message-based random access. In this paper, we analyze the performance of the enhanced random access in the IEEE 802.16m system which supports message-based random access in addition to CDMA-based random access. The analysis and numerical results help understand the performance of random access in IEEE 802.16m with low delay and low collision probability.