Andreas Darmawan

Amplify-and-Forward Scheme in Cooperative Spatial Multiplexing

We investigate and propose a new amplify-and-forward cooperative spatial multiplexing scheme in which the transmitter (source), equipped with a single antenna, forms virtual antenna array from a collection of distributed single-antenna wireless terminals, and broadcasts identical signal to those terminals (relays). Each relay amplifies and forwards different portion of the received signal at a reduced data rate to the receiver (destination). The receiver, equipped with multiple antennas, nulls and cancels the interference from different relays in order of the signal-to-noise ratio, and detects the original signal transmitted from the source. The combination of transmitter, relays, and receiver forms a virtual MIMO system in single-antenna wireless terminals environment. Unlike the conventional spatial multiplexing scheme, the proposed scheme does not require multiple antennas at the transmitter side, making it to be particularly useful for uplink (mobile to base-station) data transmissions. It is shown that the performance of the proposed system approaches and, in some cases, surpasses that of the conventional MIMO spatial multiplexing scheme.

Cooperative Diversity using Soft Decision and Distributed Decoding

Cooperative communications techniques exploit the spatial diversity of wireless radio channel by forming antenna arrays in order to achieve Multiple Input Multiple Output (MIMO) performance in multi-user single-antenna environment. The improvements can be in terms of higher data rate, lower transmission power, or increase coverage in the network. In this paper we propose a new cooperative relaying concept using soft information and distributed turbo decoding. The relay regenerates a Log-Likelihood Ratios (LLRs) using a Soft Input Soft Output (SISO) decoder, then forwards this a posteriori information to the destination in its soft form yielding a likelihood for every source transmitted bit. At destination, the forwarded bits serve as a priori information to assist the iterative turbo decoding. Our results show that the gain achievable using the conventional cooperative diversity is obtained through our schemes with less amount of overhead.

Using Shared Beacon Channel for Fast Handoff in IEEE 802.11 Wireless Networks

Handoff process in IEEE 802.11 wireless networks must be accomplished with as little interruption as possible to maintain the required quality of service (QoS). Previous research reported that channel scanning phase accounts for more than 90% of the overall link-layer handoff latency, ranging from 350 to 500 msec. In order to eliminate the time-consuming channel scanning latency we propose a stand-alone, shared beacon channel, where stations (STAs) can update information about neighboring access points (APs) via an extra receiver. Our proposed method contributes to completely eliminate the channel scanning phase, therefore, realizing fast handoff in IEEE 802.11 wireless networks. In this paper, we implement our proposed method on FreeBSD 6.1R kernel with Atheros AR5212-based 802.11 a/b/g chipsets. From experimental results, we find that it takes 6.063 msec delay on average during handoff from 802.11b AP to 802.11a AP while transmitting/receiving 480 byte ICMP frames with an interval of 10 msec.

LLR-based Ordering in Amplify-and-Forward Cooperative Spatial Multiplexing System

The paper investigates and evaluates the application of log-likelihood ratio-based ordering in the decoding stage of amplify-and-forward cooperative spatial multiplexing system. The cooperative spatial multiplexing detects signals transmitted from the source with successive interference cancelation algorithm whose performance varies, depending on the ordering schemes applied. Log-likelihood ratio-based ordering uses and exploits the a posteriori information of the received signal at the destination to get a more accurate final estimation at the decoder output. Based on the evaluation, the authors demonstrate that with marginally extra complexity, log-likelihood ratio based-ordering offers better performance compared to signal-to-noise ratio-based ordering.

Eliminating channel scanning phase in handoff across IEEE 802.11 wireless LANs

The current channel scanning phase in IEEE 802.11 Wireless LAN does not fit fast handoff due to the following two reasons. First, IEEE 802.11 Wireless LAN operates based on CSMA/CA. Unlike cellular system, STAs can not scan other channels while exchanging data frames with currently associated AP, because it would directly cause frame loss. To attack this problem, related works have tried to utilize Power Saving Mode (PSM) or multiple NICs. Second, beacon interval is too far apart for fast handoff (100 msec by default). Since an STA lacks knowledge about which channel its nearby APs are operating on, it should redundantly scan every single channel regardless whether any AP exists on that channel. Though shorter beacon interval or active scan enable an STA to locate its nearby BSSs more quickly, these approaches increase extra traffic to the limited bandwidth. Previous works tried to provide an STA with nearby AP information approximated by APs collaboratively.