Mustafa E. Sahin

Handling CCI and ICI in OFDMA femtocell networks through frequency scheduling

Femtocells have a strong potential for increasing the efficiency and coverage of next-generation broadband wireless networks. In this paper, a co-channel framework for the coexistence of orthogonal frequency division multiple access (OFDMA) based macrocell and femtocell wireless networks is proposed. It is based on utilizing the resource blocks of macrocell-associated mobile stations (mMSs) that are far away to a femtocell base station (fBS), therefore avoiding strong interference that may occur between a femtocell and close-by mMSs. An avoidance method that jointly utilizes the spectrum sensing results as well as scheduling information obtained from the macrocell base station (mBS) is introduced. Moreover, the impact of inter-carrier interference (ICI) from the mMSs in the uplink is discussed and evaluated through simulations.

System Design for Cognitive Radio Conmunications

The rising number and capacity requirements of radio systems bring about an increasing demand for frequency spectrum. Cognitive radio offers a tempting solution to this problem by proposing opportunistic usage of frequency bands that are not occupied by their licensed users. Since it is a rather new concept, there is no consensus on the practical implementation of cognitive radio communications. In this paper, a comprehensive system design for cognitive radio is presented. The details of spectrum sensing and dynamic spectrum shaping, which constitute the basics of opportunistic spectrum usage, are given. It is proposed to transmit the spectrum sensing related information via on-off keying (OOK) modulated ultrawideband (UWB). The limits on the range of one-to-one cognitive communications are discussed. A processing gain based approach that leads to an increased range for cognitive networks is proposed. Different signaling options for the real data communications are given, and among these options an impulse radio that employs raised cosine filters is investigated in detail

Opportunity detection for OFDMA-based cognitive radio systems with timing misalignment

Accurate detection of spectrum opportunities within the frequency band of an orthogonal frequency division multiple access (OFDMA) system carries critical importance for OFDMA-based cognitive radios. In this paper, we analyze the opportunity detection performances of energy detection and ESPRIT (estimation of signal parameters by rotational invariance techniques) algorithms in the presence of timing misalignments in uplink (UL) OFDMA. For the energy detector, the statistics of subcarrier power are derived considering timing misalignments, and they are verified through computer simulations. Using these statistics, which take inter-carrier-interference (ICI) effects into account, receiver operating characteristics (ROCs) of the energy detector receiver are obtained. It is shown that energy detection has a considerably better performance than ESPRIT, especially when the subcarrier assignment changes frequently. Moreover, a closed form expression is derived for the UL-OFDMA synchronization point that minimizes the ICI. Finally, it is shown that employing resource allocation blocks with larger sizes in the primary network yields better opportunities for the cognitive radio.

Reception and Measurement of MIMO-OFDM Signals with a Single Receiver

OFDM multi-carrier modulation is expected to be the enabling technology for 4G wireless systems. One of the features that make OFDM the primary choice for 4G is its MIMO compatibility, because MIMO has a very significant potential of enhancing wireless systems for capacity, data rate, and coverage aspects. In this paper, the challenges of MIMO- OFDM measurements are addressed in comparison to SISO, with a special emphasis on WiMAX systems, which employ MIMO- OFDM technology. It is proposed to perform the reception and measurement of MIMO-OFDM signals using a single receiver branch rather than multiple receivers. A complete guide to perform impairment estimation for WiMAX MIMO signals with a single receiver according to the 802.16 standards is also provided.

Interference avoidance in 3GPP femtocell networks using resource partitioning and sensing

Interference problems in cochannel femtocell networks may cause significant performance degradation for certain femtocell/macrocell users. These problems and potential solutions are recently being investigated within the 3GPP. The goal of the present paper is to review the simulation framework for femtocell networks in 3GPP and briefly summarize some of the important inter-cell interference coordination (ICIC) techniques that have been discussed. Then, a new intercell interference avoidance (ICIA) method is proposed for femtocell networks which is based on resource partitioning and sensing. Different than prior art techniques, the proposed approach does not require the use of X2 interface or over-the air signaling, both of which may be infeasible/costly for femtocells with the available releases of the 3GPP.

Opportunity Detection for OFDMA Systems with Timing Misalignment

Accurate detection of spectrum opportunities within the frequency band of an orthogonal frequency division multiple access (OFDMA) system is a critical requirement for the realization of a coexisting cognitive radio. In this paper, we analyze the opportunity detection performances of energy detection and ESPRIT algorithms in the presence of timing misalignments in uplink (UL) OFDMA. For the energy detector, the statistics of subcarrier power are derived, and the accuracy of these derivations is verified through simulations. The feasibility of opportunity detection in an asynchronous UL-OFDMA system is proven. Also, it is shown that energy detection has a considerably better performance than the ESPRIT algorithm especially when the subcarrier assignments change frequently.

The Roles of Ultra Wideband in Cognitive Networks

Cognitive radio is a recent concept that is expected to lead wireless communications to a new era. A proper candidate for the physical layer implementation of cognitive radio networks is ultra wideband (UWB), because UWB is highly competent in satisfying many basic requirements of cognitive radio. Beside this, UWB can also provide various kinds of supportive uses to cognitive radio systems that are realized by means of other wireless technologies. In this paper, a number of possible roles that UWB can take in cognitive networks are discussed in detail.

A narrowband interference identification approach for UWB systems

Ultrawideband (UWB) has been attracting attentions by promising very high data rates along with low cost hardware and low power consumption. However, since UWB occupies a very wide frequency band, it is forced to coexist with numerous powerful licensed communication systems transmitting in the same band. The interference caused by these systems may complicate or even block the UWB communications. In the literature, there is considerable amount of work regarding narrowband interference (NBI) suppression. Most of the proposed methods, which can he classified as NBI avoidance and NBI cancellation techniques, require interference statistics. In this paper, we propose an NBI identification approach, which aims at figuring out some useful interference statistics by assuming some preliminary information about the possible sources of NBI

Iterative Interference Cancellation for Co-Channel Multicarrier and Narrowband Systems

Coexistence of narrowband (NB) and multicarrier technologies will be a major concern in next generation wireless communication systems due to the co-channel interference (CCI) problem. In this paper, an efficient CCI cancellation method is proposed that may be utilized for an improved coexistence. The method treats both co-channel signals as desired signals and enhances them in an iterative manner. Through computer simulations, it is demonstrated that it yields significant gains in the symbol error rate (SER) performance of both the NB and multicarrier systems. Its computational complexity is compared with the complexity of the joint demodulation technique and it is shown to be much lower.

ICI-Minimizing Blind Uplink Time Synchronization for OFDMA-Based Cognitive Radio Systems

Cognitive radio is an enabling technology for efficient utilization of radio spectrum. In this paper, an orthogonal frequency division multiple access (OFDMA) based cognitive (opportunistic) network is considered, which co-exists with a macrocell network through utilizing its unused subcarriers. We particularly consider the uplink opportunity detection problem by the opportunistic network, where accurate synchronization to the macrocell network is crucial for minimizing the interference received from the macrocell users. After demonstrating the impact of synchronization on the inter-carrier-interference (ICI) observed at the opportunistic network, an improved blind first-user synchronization technique is proposed, and its statistics are analyzed. Through computer simulations it is shown that the proposed technique yields less interference and better opportunities for the opportunistic network.