A Study on Interference Suppresstion to Adjacent Cells/Sectors using window-control based Adaptive Multipath Control Technique

This paper proposes an adaptive multipath control technique using window size controlled wide null creation technique (WNCT) for the adaptive array antenna (AAA) systems in rich scattering channel conditioned broadband wireless communication systems. In the proposed scheme, deep null creation technique (DNCT) and WNCT are employed to create antenna directivity for transmission so that unwanted radiation to the adjacent sectors or cells can be suppressed. Moreover, to efficiently utilize small number of degree of freedom of the AAA under rich scattering channel conditions, we will introduce an window size control technique in the WNCT by which we can create wider null to a direction of interference cluster with strong and large angle spread. Computer simulation confirms that WNCT with the proposed window controlling technique can achieve higher carrier to interference plus noise power ratio (CINR) than the original WNCT in the uplink. Keywords— adaptive multipath control, interference suppression, adaptive array antenna, degree of freedom I. I In the beyond 3G (third generation) systems [1], support of the user rate of 100M -1G bit/s under various environments is the most important requirement because demand for high peak user rate is increasing with the wide-spread of broadband services using cable Internet and assymetric digital subscriber line (ADSL). For this purpose, the most serious requirements are the reduction of tranmit power to prevent electro-magnetic compatibility (EMC) problems and minimization of unwanted radiation to the unnecessary directions to prevent interference with other wireless communication systems. To satisfy these requirements, we have proposed an Adaptive Multipath Control (AMC) technique [2] using adaptive array antenna (AAA) [3] [4] to suppress interference radiation to the nontarget sectors or cells in indoor wireless communication systems. The limitation of the number of antenna degree of freedom against the number of interference waves have been a problem when introducing AAA in rich scattering conditions. Therefore, we have proposed a Deep Null Creation Technique (DNCT) and a Wide Null Creation Technique (WNCT) to effectively direct main beam to the target sector while suppressing unwanted radiation for other sectors using a small number of degree of freedom of the AAA. However with original WNCT we have previously proposed in the AMC, which aims to create equal width of nulls to all the dominant interference radiated directions for all the non-target sectors, radiated interference power are indeed deeply suppressed in specific propagation environment, it lacks flexibile adaptation to more diversed propagation environmnet, because antenna pattern is fully dependent on the angle of departure (AOD) of the dominant interference radiated directions. For example, when most of the dominant interference radiated directions are angle-clustered in a specific direction, it could be more efficient to increase the width of one null rather than to create plural number of nulls to different directions. On the other hand, when angle of the dominant interference radiated directions are uniformly distributed, it is more efficient to create ample number of nulls rather than to increase the width of each null. Thus in this paper, we will propose a window size control technique in the WNCT by which we can create an adjusted wider null to a direction of interference cluster with strong and large angle spread in order to adapt WNCT more diversed propagation environment and use anetnna degree of freedom more effectively. With this window size controlled WNCT, because it aims to give priority to create a wide null to the specific direction where dominant interference waves are densely exist, although in some situations it cannot suppress interference radiation to some directions, it improves the performance of interference suppression in terms of the whole system stand point of view because it use limited number of antenna degree of freedom more effectively by creating an adjusted width of null to the dominant interference radiated directions. Computer simulation confirms that the window size controlled WNCT shows improvement of the Carrier to Interference plus background Noise power Ratio (CINR) performance in the uplink in various propagation environments. II. A P F  AMC In the proposed system, optimum antenna weights are calculated using delay profile measurement based antenna weight controlled technique based on minimum mean square error (MMSE) criterion [5]. In this section we will briefly explain DNCT and WNCT in AMC and theoretically analize relationship between degree of freedom in AAA and antenna pattern characteristic by deriving Wiener-Hopf equation that provides the optimum antenna weights when antenna weights are calculated using MMSE criterion. Here we will make it clear what are the problems in the originally proposed WNCT, when we try to employ AMC to various propagation environment. Then we will explain the window size controlled WNCT by modification of originally proposed WNCT. A. Process of WNCT & DNCT Fig. 1 shows how to generate quasi-received signals to create a wide null with DNCT and WNCT. In Fig. 1, k and l denote the number of sectors and number of delayed paths respectively. S k T (t) denotes the reference signal sequence of k-th sector, and ∑Lk−1 l=0 P k l δ(t − τl ) denotes the estimated delay profile, where Lk is the number of delay paths and Pl , τ k l are the complex amplitude and delay time of each delayed path respectively. The WNCT process is listed as follows. 1) From the estimated delay profile for each sector in the AP ( ∑Lk−1 l=0 P k l δ(t−τl )), select the strongest path (Pkl0 ) and generate virtual delay profile (Pkl0δ(t − τkl0 )). 2) Estimate direction of AOA of the strongest path from each non-target sector, and generate virtual arrived paths in each window, which are centered by the estimated direction of l = 0 Lk-1 P k (t) l k l