Multi-layer heterogeneous network (HetNet) deployments including small cell base stations (BSs) are considered to be the key to further enhancements of the spectral efficiency achieved in mobile communication networks [1]. Besides the capacity enhancement due to frequency reuse, a limiting factor in HetNets has been identified as inter-cell interference. The 3rd Generation Partnership Project (3GPP) discussed inter-cell interference coordination (ICIC) mechanisms in long term evolution (LTE) Release 8/9 [2]. LTE Release 8/9 ICIC techniques were introduced to primarily save cell-edge user equipments (UEs). They are based on limited frequency domain interference information exchange via the X2 interface, whereby ICIC related X2 messages are defined in the 3GPP standard [3]. In LTE Release 8/9 ICIC, a BS provides information about set of frequency resources in which it is likely to schedule DL transmissions to cell-edge UEs, for the benefit of a neighboring BS. The neighboring BS in turn avoids scheduling its UEs on these frequency resources.

With the growing demand for data services and the introduction of HetNets it has become increasingly difficult to meet a UE's quality of service (QoS) requirements with these mechanisms. To cope with the QoS requirements and growing demand for data services, enhanced ICIC (e-ICIC) solutions have been proposed in LTE Release 10 and further e-ICIC (Fe-ICIC) solutions to reduce cell reference signal (CRS) interference in e-ICIC techniques are discussed in LTE Release 11 [4].

In LTE Release 10 e-ICIC techniques, the focus is on time- and frequency-domain techniques and power-control techniques. While in time-domain techniques, the transmissions of the victim UEs are coordinated in time-domain resources, in frequency-domain techniques, e-ICIC is mainly achieved by frequency-domain orthogonalization. The power-control techniques have been intensively discussed in 3GPP. Hereby, power control is performed by the aggressor cell to reduce inter-cell interference to victim UEs. In 3GPP studies, e-ICIC mechanisms with adaptive resource partitioning, cell range expansion (CRE), and interference coordination/cancellation take a central stage [5].

In the following, the inter-cell interference problem in HetNets is introduced and time- and frequency-domain e-ICIC techniques are discussed based on 3GPP specifications. In addition, single- and multi-flow transmission techniques for e-ICIC and system capacity improvement are described.

Inter-cell interference in HetNets

One of the major features extensively studied for LTE Release 10, also known as LTE-Advanced, is the HetNet coverage and capacity optimization, e.g., through the use of cell-range expansion (CRE) techniques.

Introduction

Compared with current cellular networks, next generation mobile networks are expected to encompass more sophisticated features, including the support of higher data transmission rates and user equipment (UE) mobility, location management, diversified service levels, etc. In order to accommodate these requirements, the 3rd Generation Partnership Project (3GPP) is devoted to the standardization of Long Term Evolution (LTE) and LTE-Advanced systems, which have been recognized as major candidates for the fourth-generation (4G) mobile networks. In LTE/LTE-Advanced systems, the network structure will be heterogeneous. How to maintain and improve mobility, handover (HO), and location management, while avoiding user experience deterioration, is a challenging task. In this chapter, we will study the mobility management challenge and illustrate advanced mobility management schemes.

In LTE/LTE-Advanced systems, the factors that make mobility, HO, and location management a challenging task are as follows

• The rapid evolution of cellular networks results in the coexistence of multiple radio access technologies (RATs), e.g., Global System for Mobile Communications (GSM), Universal Mobile Telecommunication System (UMTS) and LTE/System Architecture Evolution (SAE). This demands optimized cooperation among multiple RATs to enable UEs to roam from one RAT to another.

• The introduction of low-power nodes (LPNs) largely increases the total number of base stations (BSs), making the network structure and interference conditions more intricate. Thus, traditional mobility load balancing (MLB) and mobility management schemes need to be revisited to suit the new heterogeneous cellular network (HCN) architecture.

• The complexity of LTE/LTE-Advanced systems leads to a large number of network parameters. Therefore, efforts need to be made in defining proper key performance indicators and developing optimization techniques for mobility management in various scenarios.

As discussed in Chapter 1, heterogeneous cellular networks (HCNs) with low-power nodes (LPNs) are important for improving the capacity and coverage of next generation broadband wireless communication systems. However, interference problems in HCNs pose an important challenge, and thus efficient interference management techniques are required to fully benefit from their deployments. The main contribution of this chapter is to review interference problems and interference management techniques for HCNs. A general notion of macrocell base stations (MBSs) and LPNs is adopted, but the simulations are based on Long Term Evolution (LTE) scenarios with macro eNBs and pico eNBs or femto HeNBs. More specifically, cell-selection and interference coordination methods are discussed, including mechanisms recently proposed in the 3rd Generation Partnership Project (3GPP) LTE, and their performances are evaluated through system-level simulations. In such simulations, LTE-specific notation is used, and macrocell user equipment (MUE) and picocell user equipment (PUE) denotes UEs served by macro eNBs and pico eNBs, respectively.

This chapter is organized as follows. First, Section 7.1 reviews the main reasons for excessive intercell interference in HCNs. In Section 7.2, due to its significance, range expansion (RE) for HCNs is treated in more detail. Some example simulation results that demonstrate the downlink (DL)/uplink (UL) coverage imbalance in heterogeneous deployments are also provided. Section 7.3 gives a high-level overview of intercell interference coordination (ICIC) methods that are applicable to HCNs, and the next three sections are dedicated to specific ICIC approaches: frequency-domain, power-based, and time-domain ICIC techniques are discussed in Section 7.4, Section 7.5, and Section 7.6, respectively.

Introduction and prior art

Frequency allocation for heterogeneous networks

In heterogeneous networks, frequency resources can be allocated to different tiers in a co-channel (shared-spectrum) or dedicated channel (split-spectrum) manner, or through a hybrid technique, which is a combination of the two approaches. In the co-channel approach shown in Figure 15.1(a), while the spectrum resources are fully reused in different tiers, cross-tier interference may cause crucial setbacks to the system. For example, macrocell users in the vicinity of closed subscriber group (CSG) femtocells are not allowed to connect to the femtocells, even if their link quality with these femtocells is good. Therefore, such macrocell users receive strong downlink interference from CSG femtocells and may fall into outage.

The split spectrum approach shown in Figure 15.1(b), on the other hand, partitions the allocated spectrum between multiple tiers. Each tier can use its own segment of resource and therefore there is no cross-tier interference [1]. However, the amount of bandwidth available to each tier is reduced. Hybrid methods as shown in Figure 15.1(c) use a mixture of co-channel and dedicated channel methods, and aim to reuse the spectrum resources whenever feasible. For example, in [2], the macrocell users are dedicated a component carrier (CC), referred as the “escape carrier,” which is not used by the femtocell network. Any macrocell mobile station (MMS) that is close by to a femtocell is scheduled within this escape carrier, if the interference observed from the femtocell network is above a threshold.

Driven by a new generation of wireless user equipment and the proliferation of bandwidth-intensive applications, user data traffic and the corresponding network load are increasing in an exponential manner. Most of this new data traffic is being generated indoors, which requires increased link budget and coverage extension to provide satisfactory user experience. As a result, current cellular networks are reaching their breaking point and conventional cellular architectures, which are devised to cater to large coverage areas and optimized for homogeneous traffic, are facing unprecedented challenges to meet these user demands.

In this context, there has been an increasing interest to deploy small cellular access points in residential homes, subways, and offices. These network architectures, which may be either operator deployed and/or consumer installed and are comprised of a mix of low-power cells underlying the macrocell network, are commonly referred to as small cell networks. By deploying additional network nodes within the local area range and bringing the network closer to end users, spatial reuse and coverage can be significantly improved, thus allowing future cellular systems to achieve higher data rates, while retaining the seamless connectivity and mobility of cellular networks.

Inspired by the attractive features and potential advantages of small cell networks, their development and deployment are gaining momentum in the wireless industry and research communities during the last few years. It has also attracted the attention of standardization bodies such as the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE)-Advanced (see Chapter 14) and the IEEE 802.16 Wireless Metropolitan Area Networks.

Even though there is no universally accepted definition, short-range wireless communications typically refers to a wide variety of technologies with communication ranges from a few centimeters to several hundreds of meters. While the last three decades of the wireless industry have been mostly dominated by cellular systems, short-range wireless devices have gradually become a more integrated part of our everyday lives over the last decade. The Wireless World Research Forum (WWRF) envisions that this trend will accelerate in the upcoming years: by the year 2017, it is expected that seven billion people in the world will be using seven trillion wireless devices [1]. The majority of these devices will be short-range wireless devices that interconnect people with each other and their environments.

While the reliability of wireless communication systems has been studied in detail in the past, a comprehensive study of different factors affecting reliability for short-range wireless systems and how they can be handled is not available in the literature, to date. The present book intends to fill this gap by covering important reliability problems for short-range wireless communication systems. The scope of the contributions in the book is mostly within the domain of wireless personal area networks (WPANs) and wireless sensor networks (WSNs), and issues related to wireless local area networks (WLANs) are not specifically treated.

Due to the differences in application scenarios, quality of service (QoS) requirements, signaling models, and different error sources and mitigation approaches, the high-rate and low-rate systems will be addressed in separate parts of the book.

In this chapter, technologies and standards for low data rate communication systems for wireless personal area networks (WPANs) and wireless sensor networks (WSNs) are discussed. First, ZigBee technology based on the IEEE 802.15.4 standard, and then low-rate UWB technology based on the IEEE 802.15.4a standard are reviewed. Finally, some of the related standards that are being developed by IEEE 802.15 working groups (WGs) are summarized.

Overview and application examples

Together with the recent advances in radio frequency (RF) and MEMS integrated circuit technologies, wireless sensors are becoming cheaper, smaller, and more capable. Through WSNs, a wealth of new applications are becoming possible, including surveillance, building control, factory automation, and in-vehicle sensing [1]. In the near future, we will observe that buildings, furniture, cars, streets, highways, etc. will all comprise WSNs. The Wireless World Research Forum (WWRF) envisions that by the year 2017 about 7 billion people in the world are expected to be using 7 trillion wireless devices, and the majority of these devices will be short-range wireless devices including small-size, low-power, low-complexity WSNs [2]. In order to provide a better picture of potential WSN applications, recent example applications in the literature are listed in Table 6.8 towards the end of the chapter.

WSNs may be typically deployed in large numbers and the network may need to operate for an extensive duration on the same battery. Therefore, key requirements for WSN transceivers include low-cost sensor nodes, small form factors, and low energy consumption.