In this chapter, we have studied impacts of the soft handoff in the mixed-size cellular system. We address a “power exhausting problem” and propose a novel LPPA scheme to ease the problem. By taking account of the effects of different cell sizes, simulation results show that LPPA can prevent a microcell base station from wasting too much transmission power in serving handoff users. Consequently, the simulation results show that LPPA can more effectively alleviate the power exhausting problem than others by outstanding the “power balance” characteristic. Also, the LPPA scheme can support higher system capacity than
200
Figure 2.13: Total capacity with and without measurement errors for EPA, QBPA, SSDT, LPPA-RV1 and LPPA-RV2 schemes of (a) the same-size cellular system, (b) the mixed-size cellular system with ρ = 1/2, (c) the mixed-size cellular system with ρ = 1/3.
other schemes in both the same-size and mixed-size cellular systems even with measurement errors. In summary, we find that it is important to design a handoff mechanism from both power efficiency and link reliability perspectives. The concept and the methodology are useful to develop advanced radio resource algorithms for multirate CDMA systems in next chapter.
Chapter 3
A Joint Power And Rate Allocation mechanism for Multirate Soft Handoff in Mixed-Size WCDMA Cellular
Systems
The chapter proposes a joint power and rate assignment (JPRA) algorithm to deal with multirate soft handoffs in WCDMA mixed-size cellular systems.
This JPRA algorithm, containing a link proportional power allocation (LPPA) scheme and an evolutionary computing rate assignment (ECRA) method, can determine an appropriate allocation of power and service rate for multirate soft handoffs, respectively. It can achieve power balance among cells through soft handoffs better than the conventional site-selection diversity transmission (SSDT) scheme with best-effort rate allocation. Simulation results show that the JPRA algorithm can reduce the handoff forced termination probability and improve the total throughput, which means better cell’s service coverage and higher system capacity. Besides, it is shown that JPRA is less sensitive than SSDT to measurement errors occurring in the active set selection.
3.1 Introduction
The tremendous growths of internet services drive multirate transmission becoming neces-sary in the third generation systems such as Universal Mobile Telecommunications Systems (UMTS). Because of abundance multimedia traffic in the downlink (from base station to mobile station), downlink transmission is generally the capacity-limited direction in the multirate wideband code division multiple access (WCDMA) systems. To utilize downlink radio recourses efficiently, many previous studies focus on joint power and rate allocations for all users in the systems [18], [19], whereas the possible combinatorial numbers of the solutions are too large to be tractable for optimal allocations. This problem becomes more complicated when taking into account multi-site transmission mechanisms for soft handoff.
Soft handoff is one of the most important features in WCDMA cellular systems. When mobile users move from one cell to another cell, the soft handoff mechanism can provide seamless connections and better signal qualities for users near the cell boundaries. However, base stations often have to consume more power to serve soft handoff users than that to serve non-handoff users. The fact that the total power resource in each base station is confined and shared among non-handoff and soft handoff users raises the issue of the tradeoffs between coverage and capacity. For example, if a base station fails to serve multirate handoff users near the cell boundaries, the cell’s service coverage is shrunk whereas there are more power applicable to non-handoff users for higher transmission rates. Therefore, joint power and rate allocations of multirate soft handoffs play an important role for downlink radio resource management. Instead of optimal power and rate allocations for all users in the system, the complexity can be greatly reduced by optimal radio resource management for multirate soft handoffs, which makes system implementation feasible.
Furthermore, consider a WCDMA cellular system with mixed-size cells due to non-uniform traffic load distribution, in which all cells utilize the same frequency so that the emitted power of different base stations interferes each other. Generally, congested micro-cells, which are with stringent power budget for maximum total transmission power and maximum link power, may easily exhaust their total transmission power because of
serv-ing soft handoff users in the downlink [8], and then there is no extra power resource to serve other users in the system. When taking into account multirate services, this power exhausting problem becomes more critical. Therefore, the ultimate goal of this chapter is to design an optimal scheme with power balance characteristics for radio resource management of soft handoffs. As long as power balance can be achieved among macrocells and microcells through the optimal scheme of soft handoffs, there are more power resources can be allo-cated for other users in the congested microcells. As a result, the system performance can be improved. References [5]-[7] considered capacity issues in mixed-size cellular systems with mixed-size cells. Both [5] and [6] only focused on the reverse link and only voice service is considered. On the other hand, Kishore, et al, [7] concluded that uplink and downlink direc-tions are equivalent in mixed-size mixed-size cellular systems. However, it does not consider multirate services which are often regarded as highly resource-exhausting traffics and often have more volumes of traffic in the downlink than that in the uplink. Therefore, in this chapter, we specialize in the downlink transmission which is generally the capacity-limited direction in the multimedia WCDMA cellular systems.
Many literatures discussed the topic of joint power and rate assignment for all users in the cellular system in the sense of global optimization problem [18], [19]. However, they focused on the reverse link and did not concern about multirate soft handoffs. Kim [22] dealt with rate-regulated power control in the reverse link without concerning handoff. Reference [23]
discussed radio resource management in multiple-chip-rate direct sequence CDMA systems supporting multiclass services. It arranged handoff in the same subsystem or execute inter-frequency handoff. Kim and Sung [24] proposed a handoff management scheme for multirate services using guard channels and reservation on demand queue control, but a hard handoff scheme was considered. References [20] and [21] proposed joint power and rate allocation algorithms in the downlink WCDMA same-size cellular systems. The former proposed two sub-optimal algorithms based on fairness consideration, and the latter adopted dynamic programming technique to optimize total throughput. However, both considered same-size cellular systems without soft handoff mechanisms. A conventional site selection diversity
transmission (SSDT) scheme was proposed for handoff power allocation in [14]. It provides transmission diversity by dynamically selecting one base station with best link quality in the active set. However, due to the maximum link power constraint, SSDT sometimes could not afford enough power required to multirate soft handoff users. Moreover, since SSDT is a single-site transmission mechanism at one time, it may select the wrong link resulting in wasting more power for handoffs when suffering measurement errors during active set selection. The advantage of the power saving characteristic for SSDT would disappear.
In this chapter, we propose a joint power and rate assignment (JPRA) algorithm for downlink multirate soft handoff users in WCDMA mixed-size cellular systems. The pro-posed JPRA algorithm is a two-phase process, which is compro-posed of LPPA and ECRA. In the first phase, a link proportional power allocation (LPPA) scheme is designed for power allocation of soft handoffs. Unlike the SSDT scheme, LPPA is a multi-site transmission mechanism, which distributes the required power in proportion to the link qualities between a soft handoff user and all base stations in its active set. That is, the base station with better link quality will allocate more power than others with worse link qualities. In the sec-ond phase of JPRA algorithm, an evolutionary computing rate assignment (ECRA) method is proposed to formulate an integer and discrete optimization problem under a predefined total power constraint for soft handoffs in each cell. It is well known that conventional optimization methods can hardly cope with problems with integer and discrete variables, whereas evolutionary computing methods are very efficient for these problems to reduce the searching complexity [40]. In the meantime, a new multi-quality balancing power allocation (MQBPA) algorithm for non-handoff users with multiple service rates is also developed. Pre-vious work for quality balancing power allocation technique were studied only for a single service rate with unique required signal quality [13], [25]. On the other hand, a multirate removal (MRV) algorithm is proposed to pick out a user who consumes system resource most and to reduce its service rate or even block it when the system resource is insufficient.
Several removal algorithms had been proposed in [26]-[28]. Among these, the link-based and received signal-strength based removal algorithms were only suitable for single service [26],
YES Joint power and rate
assignment (JPRA) for handoff users
Multi-quality based power allocation (MQBPA)
for non-handoff users
END System resource sufficient
? NO
START
Multirate removal algorithm (MRV)
Figure 3.1: The system operation of downlink power and rate assignment
[27]. The prioritized removal algorithm in [28], based on predefined service priority, did not consider service rate tuning for users in the reverse link of a multiservice cellular system.
Compared to the conventional SSDT scheme with best-effort rate allocation, simulation results show that the service coverage of a cell and the system capacity can be improved significantly in terms of handoff forced termination probability and total system throughput.
Besides, on the perspective of the users, JPRA can support excellent user satisfaction indexes for voice and data users. Moreover, it is shown that JPRA owns less sensitive than SSDT to the occurrence of the measurement errors during active set selection.
The remaining parts of this chapter are organized as follows. Section 3.2 details the flow of the system operation, and provides the design of the MQBPA and MRV algorithms. In section 3.3, the JPRA algorithm for multirate soft handoffs is proposed, including LPPA al-gorithm and ECRA alal-gorithms. Also, the proof of LPPA convergence is provided. Simulation results are presented and discussed in section 3.4. Finally, section 3.5 provides conclusions of this chapter.