CHAPTER 1
Introduction
The increasing demands for the higher speed wireless internet applications impose many new challenges on spectrum and radio resource management in wireless net-works. One of key challenges in supporting the wireless internet services is to handle the traffic asymmetry between the uplink and the downlink. That is, some services may require more radio resources in the downlink transmission, while some services may require more uplink radio resources [1]. Hence, an intelligent radio resource al-location to support asymmetric services becomes an important topic in the future wireless networks.
1.1 Mobile Radio System
Code Division multiple access (CDMA) system is a promising radio access technique for the third-generation mobile communication systems due to its high flexibility and efficiency. In the CDMA systems, there are two different operation modes, namely frequency division duplex (FDD) and time division duplex (TDD). Comparing to the FDD-CDMA system with a pair of separated frequency bands used for downlink and uplink transmissions, the uplink and downlink transmissions in the TDD/CDMA systems multiplex the uplink and downlink time slots on the same frequency band. By exploiting the inherent time division component, time division duplex (TDD) mode is very suitable to provide asymmetric traffic services. [2, 3].
However, to support the asymmetric traffic in the TDD/CDMA system, the different asymmetric traffic conditions among cells may cause heavy cross-slot in-terference, which will seriously degrade the system performance [3–5]. Take the TDD/CDMA systems specified in the Universal Mobile Telecommunications System as an example (UMTS) [6, 7]. A TDD frame has 15 time slots, where the first one is usually used for signaling, and the others can be allocated for either the uplink or the downlink traffic channels as shown in Fig. 1.1. The boundary between the uplink and downlink time slots within a transmission frame is called the switching point. When two neighboring cells have different switching points due to distinct uplink-to-downlink traffic ratios, some time slots may be used for downlink trans-missions in one cell, while being used for uplink transtrans-missions in other cells. The opposite uplink and downlink transmissions in some time slots for two neighboring cells is called the cross-slot interference in this project. Note that in Fig. 1.1, there are two kinds of cross-slot interference: base-to-base cross-slot interference in the up-link and the mobile-to-mobile cross-slot interference in the downup-link. Because the transmission power of a base station is much higher than that of a mobile terminal, the base-to-base cross-slot interference is quite significant. Meanwhile, as a mobile terminal approaches to another mobile of an adjacent cell at the cell boundary, the mobile-to-mobile cross-slot interference can not be ignored. Both types of cross-slot interference will degrade the system performance seriously [8, 9], since it is usually suggested that a time slot should be used for the same transmission direction either uplink or downlink for two neighboring cells. This constraint, however, obviously wastes time slots if traffic asymmetric ratio of two neighboring cells differs signifi-cantly. Apparently, this approach may lose the key advantages of the TDD systems in supporting asymmetric traffic services [3,10]. The key to relax this restriction is to find an effective approach to overcome the cross-slot interference in the TDD/CDMA system.
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Figure 1.1: Frame structure and cross-slot interference in TDD/CDMA system.
In the literature, there are two research directions to avoid the cross-slot in-terference. The first one is to apply the dynamic channel assignment (DCA) tech-niques [11–13]. In [11], the authors proposed an ordered DCA algorithm to reduce the probability to use the time slot that may have a higher chance of experiencing the cross-slot interference. When the traffic load or the traffic asymmetric ratio is high, this method may have difficulty in overcoming the cross-slot interference. The authors in [12] and [13] proposed the region-based and path gain division DCA. In these algorithms, the users close to the home base station are assigned to use the time slots even having the cross-slot interference, whereas the users near the cell boundary are assigned the clean time slots without the cross-slot interference. The performance of this system will highly depend on the way of separating the inner and the outer regions. In the time-varying traffic condition, it is usually hard to accurately separate two regions with one being robust to the cross-slot interference and the other being untolerant to the cross-slot interference.
Another research direction to alleviate the impact of the opposite direction in-terference in TDD/CDMA systems is to apply advanced antenna techniques [14–16].
The authors in [14] proposed to adopt sector antennas combined with time slot allo-cation methods to suppress the opposite direction interference for the TDD/CDMA system and for the TDD/TDMA system, respectively. In [15], Choi and Murch sug-gested to employ a pre-rake transmitter to improve the downlink performance of the TDD/CDMA system and apply spatial diversity to improve the uplink performance.
In [16], a joint space-time detection technique was presented to improve the uplink performance of the TD-SCDMA system. For overcoming the cross-slot interference in TDD/CDMA systems, we suggest combining DCA with antenna techniques to furthermore enhance the system performance.
In chapter 3, we develop an analytical framework to evaluate the interference problem of the TDD-CDMA system characterized by directional antennas and
asym-metric traffics. To our knowledge, in the context of sectorized cellular structures, the interference issues in TDD-CDMA systems with asymmetric traffic have not been fully addressed in the literature. And then we develop a resource allocation algorithm as applied to the “virtual cell” for a TDD-CDMA system. In [17], the concept of vir-tual cell was introduced, where a virvir-tual cell consists of three neighboring sectors from three neighboring cells (e.g. sector S1 of cell A, sector S2 of cell C, and S3 of cell D in Fig. 3.11). The similar concept was also proposed in [18] to reduce interfer-ence in FDD-CDMA systems. However, in the context of the TDD-CDMA system with asymmetric services, a resource allocation algorithm to explore the advantage of direction separation in the trisector cellular architecture has not been found in the literature. In this chapter, we extend the work in [17, 18] to develop such a resource allocation algorithm to resolve the cross-slot interference for the TDD-CDMA sys-tem. The basic idea of the proposed algorithm can be described in two folds. On the one hand, by employing simple sector antennas at base stations (as shown in Figure 3.11), we restrict the cross-slot interference within a cell coverage area. Applying the developed interference analysis framework, we will show that the interference between virtual cells can be suppressed due to the directivity of directional antennas. On the other hand, the cross-slot interference within a virtual cell can be resolved by a simple time slot allocation method. Hence, with trisector cellular architecture, the switching points of neighboring virtual cells can differ arbitrarily. In brief, taking advantage of this additional orthogonality from the direction separation of sector antennas, we propose a virtual cell-based interference-resolving algorithm to support asymmetric traffic services in TDD-CDMA systems.
In chapter 4, we focus on the uplink performance of TDD/CDMA systems.
Compared with other categories of smart antenna technology, beamforming is known for its capability of suppressing strong interference [19, 20]. In addition, beamform-ing can easily exploit the reciprocity of TDD channels to leverage the benefit of
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Figure 1.2: A trisector cellular system with the virtual cell.
joint downlink and uplink beamforming. Thus, beamforming is a promising technol-ogy in resolving the opposite direction interference of TDD/CDMA systems. The application of beamforming technique in FDD/CDMA systems has been studied ex-tensively [21–23]. To our knowledge, in the context of the TDD/CDMA system with consideration of asymmetric traffic, the performance improvements by adopting antenna beamforming techniques have not been studied fully in the literature yet.
The goal of this chapter is, from a system perspective of the TDD/CDMA cellular network, to investigate how to effectively apply antenna beamforming techniques to suppress the opposite direction interference. To this end, we evaluate two types of antenna beamformers: the conventional beam-steering technique and the minimum variance distortionless response (MVDR) beamformer. We will derive the received bit energy-to-interference density ratio of TDD/CDMA signals in the presence of opposite direction interference, and evaluate how these two antenna beamforming techniques can improve the performance. In addition, we exploit the channel reciprocity of TDD systems and propose to incorporate downlink transmitting beamforming at base sta-tions. Although downlink transmitting beamforming can significantly enhance the downlink capacity of a cellular system [23, 24], it is still a challenging task to imple-ment the optimal downlink beamforming solution. Specifically, the optimal downlink beamforming solution requires sophisticated calculations for the beamformer weights of all users and the transmission power levels of all base stations in the entire net-work [23, 24]. In order to get the insight of how to leverage the synergy of combining transmitting and receiving beamforming, we adopt a simpler downlink beam-steering technique in this work. We believe that the concept of simultaneously using trans-mitting and receiving beamformers is new in the TDD/CDMA system because the synergy of combining the downlink transmitting and uplink receiving beamforming has not been fully investigated from a system perspective, i.e., from the angle of suppressing the opposite direction interference.
Aiming to alleviate the cross-slot interference, we propose a link-proportional dynamic channel assignment scheme (LP-DCA) with sectorized antennas in Chap.
5. With the assistant of directional antennas, we utilize the concept of virtual cell composed from three neighboring sectors with the same coverage area of a cell [17]. By taking the advantages of virtual cell, we propose a effective DCA algorithm to flexibly alleviate the co-channel interference, especially for the cross-slot interference. The key idea of LP-DCA scheme is to classify the cross-slot interference and allocate the radio resource according to the users’ received signal quality. The total users of a sector are sorted based on their received signal strength. We partition these sorted users into some different groups and allocate the time slots with the consideration of alleviating the cross-clot interference. Specifically, the sector with the largest downlink traffic load will allocate both the downlink and uplink groups in a ascending order from the left side of available time slots. The sector with largest uplink traffic load will allocate both the downlink and uplink groups in a ascending order from the right side of the available time slots. The sector with similar uplink and downlink traffic load will allocate the downlink groups in ascending order from left side of the available time slots and uplink groups in ascending order from right side of the available time slots. By properly allocating users, LP-DCA outperforms other DCA algorithms with high ability to alleviate the co-channel interference. Nevertheless, we find that most DCA algorithms including the LP-DCA can not effectively alleviate the base-to-base cross-slot interference [11].
To furthermore alleviate the cross-slot interference, we propose a cross-slot interference-based dynamic channel assignment scheme combined with the MVDR beamformer in chap. 6. In the proposed scheme, the DCA is focused on reducing the mobile-to-mobile cross-slot interference, while the MVDR beamformer is aiming to suppress the base-to-base cross-slot interference. To alleviate the mobile-to-mobile cross-slot interference, the basic idea of the cross-slot interference-based DCA is to
allocate time slots to users in a specific order. Specifically, for reducing the base-to-base cross-slot interference, both receiving and transmitting beamforming weights are designed according to the MVDR beamformer criterion and the fourier beamformer criterion, respectively. According to our numerical results, the cross-slot interference-based DCA can improve system performance in both downlink and uplink, while providing asymmetric traffic services with a great deal of flexibility.
Finally, we present a new hierarchical cellular system with an underlaid TDD/CDMA microcell and overlaying FDD/CDMA macrocells. With an objective to exploit the underutilized FDD uplink capacity due to traffic asymmetry, the TDD/CDMA mir-cocell is operated within the uplink frequency band of the overlaying FDD/CDMA macrocells. By jointly applying the proposed antenna arrays at the cell site and a new power ratio adjustment technique, we evaluate the outage probabilities of the macrocell and the microcell. From the simulation results, we demonstrate that the full capacity of the TDD/CDMA mircocell can be obtained without degrading the performance of FDD/CDMA marcocells.