| Sažetak (engleski) | Power control is essential in wideband code division multiple access systems (WCDMA) systems to provide satisfactory quality of service (QoS) [1] and to combat several problems: To mitigate the fading effect. To adjust the radiated power of mobile stations in a way that all received signals at the base station have the same signal to interference ratio (SIR) for the same bit rate. This is necessary to prevent the so-called near far problem [2]. To reduce co-channel interference by concurrent users. This has a direct impact on the capacity of cells in the uplink. SIR in the WCDMA system corresponds to the ratio of the received useful signal at the receiver of base station over a frequency range of 5 MHz width compared to interfering cochannel sources. In the uplink, received useful signal at the receiver of base station is signal from the mobile station. Sources of interfering signals are all other mobile stations in the cell, as well as other mobile stations in neighboring WCDMA cells that transmit on the same channel [3]. There are three types of algorithm for power control, which are implemented in WCDMA system: open loop power control, inner loop power control, outer loop power control. All three loops for power control work together during radio connection. Open loop power control is used to calculate the required radiated power by the mobile station for access preamble, the random access channel (RACH) and the initial power for dedicated channels (DCH). Once dedicated channels have been established outer and inner loop power control shall cooperate in order to maintain the necessary block error rate (BLER) for voice or TTE for data calls. This is done in a way that outer loop power control sets and adjusts the SIR target value used by inner loop power control. Outer loop power control for uplink monitors the cyclic redundancy check (CRC) of transport blocks after diversity combining at the control station and change the SIR target value according to the SIR target control algorithms for the uplink. Thus maintains the desired TTE in the uplink, regardless of user's radio conditions, and whether user is stationary or mobile. From the lab investigation it can be concluded that out of two parameters used by uplink outer loop power control, sirMax has dominant impact on uplink received signal strength indicator (UL RSSI) and uplink throughput. Depending on the uplink load of individual cells on the radio base station (RBS) there are two possible scenarios. First scenario is to set high value for sirMax. Benefit from this setting is that maximum uplink throughput for a single user in the cell would be provided. On the other hand, drawback of this setting is that in a multi user environment problem with uplink interference can occur that could lead to problems with call setup success rate. Second scenario is to set low value for sirMax. Benefit from this setting is that in a multi user environment there would be no problem with uplink interference and call setup success rate. On the other hand, in a low loaded cell, users would not be able to exploit entire cell capacity, thus they would not be able to achieve maximum uplink throughputs. UL load on the cell differs not only from different cell to different cell, but also within the cell during the day, depending of number of users and their activity. Thus conclusion can be derived that it would be optimal if uplink outer loop power control would use sirMax that correspond to conditions on the uplink air interface at each cell, instead of using a fixed and same value for every cell whole time. Proposed idea how to achieve optimum uplink outer loop power control algorithm is dynamic changes of sirMax value based on uplink load on the cell. Regarding problem with uplink load on WCDMA cells, many improvements have been developed to improve antennas, RBS processor units, or RBS power amplifiers and receivers. One of the first solutions for uplink interference cancellation was enhanced receivers techniques presented at [14]. Increasing number of receiver antennas was recommended and investigated, like 4RX diversity [15], distributed antenna system (DAS) presented at [16] or directional antennas for wireless indoor solution presented at [17]. Furthermore, measurements in live WCDMA network [18] have been performed to propose solutions for WCDMA uplink air interface capacity increase. Problems with uplink air interface capacity, from live network, can be divided in 3 groups: Uplink air interface overload due to large number of simultaneous users, like concerts, sport events, traffic jams on highway etc. Uplink air interface overload caused from outer source of interference presented at [19]. Sources of outer interference could be malfunctioning radio devices (TV antenna amplifier, DECT systems, FAX devices etc.), mobile operators from neighboring countries that use same frequency spectrum, wrongly installed components (power tappers and splitters) at indoor distributed antenna systems. Uplink air interface overload caused by passive inter modulation (PIM) products created by own system. PIM has become a significant factor in the recent times when the amount of frequency spectrum used by each operator has increased significantly by introducing an increasing number of 5MHz band-carriers (WCDMA), as well as new broadband technologies like long term evolution (LTE). Passive inter-modulation occurs when two or more signals are present in passive device (cable, connector, isolator, switch, splitter, tapper or antenna) and this device exhibits a non-linear response [20]. Simulation has been created and executed in commercial radio planning tool, that use Monte Carlo algorithm, which simulates the way a network allocates resource units to users accessing different services. It is possible to import real network traffic to radio planning tool, obtained by performance management counters on radio network controller (RNC). Worst case scenario has been created that maximum uplink load during day for each cell has been imported to radio planning tool. This is for macro sites very rare case that all cells have at exactly same point in time maximum load. It actually corresponds to overload scenario due to large number of simultaneous users, where all cells do have maximum load at exactly same time. Thus created simulation corresponds to some real case overload scenario during concert or sport event. Two test simulations have been executed on cluster of 9 RBS with 27 cells. From results of simulation it can be concluded that significant improvement has been recorded with unique sirMax implementation per cell. Due to fact that uplink load on the cell differs not only from different cell, to different cell but also within the cell during the day, it is also recommended that sirMax is not only unique per cell, but that is unique within cell during different period of time. Design of self-optimizing uplink outer loop power control is based on fact that RNC is informed by each cell of cell's current UL RSSI. How often RNC is informed about UL RSSI, can be presented through RNC performance management counters that exist for every cell controlled by RNC: pmSumUlRssi is counter that records the value of received total wideband power (RTWP) measured on the cell and sent to the RNC via nodeB application part (NBAP) common measurement reporting, over Iub interface, where RTWP refers to UL RSSI measurement. pmSamplesUlRssi is a counter that records how often sample has been recorded and it is actually captured every 10s. That means that RNC is notified every 10s of UL RSSI measured at each cell on RNC. Design of self-optimizing uplink outer loop power control consists of introduction of new parameter on RNC, which would be used to derive sirMax value. By doing so sirMax would not be any more fixed value, but it would be derived from combination of new parameter and current UL RSSI on the cell. New proposed parameter would define maximum allowed UL RSSI on the cell, thus parameter has been called MaxULRSSI. From results of simulation, it can be concluded that self-optimizing uplink outer loop power control has adapted to the uplink load on the cell. Other important result that can be concluded is that new design would not only limit interference during high load by decreasing sirMax value, but would actually allow higher radiated power from mobile stations during low load on the cell. This would be done by increasing sirMax value, thus allowing higher uplink throughputs. On the other hand, current uplink outer loop power control, regardless of uplink load, is using same sirMax value throughout whole time. Performance management statistic and GPEH measurements have been used to obtain UL RSSI and required SIR target on monitored WCDMA cell. Based on obtained results it has been concluded that improvement in percentage of data connections that will achieve higher uplink throughputs is same for changing sirMax every 15 min or every 10 s. The main difference has been in fact that short periods of uplink overload have been detected and resolved by changing sirMax every 10 s, which has not been detected by changing sirMax every 15 min. High setting of sirMax during short periods of uplink overload could lead to uplink instability of WCDMA cell. Thus it has been proposed, for novel WCDMA uplink outer loop power control algorithm, to change sirMax every 10 s. Load balancing enhances performance of a radio access network by pooling together resources from different parts of the entire network. For high loaded WCDMA cells there are two basic approaches to create balanced condition: Inter-frequency load balancing and Inter- RAT load balancing. Inter-frequency load balancing method refers to traffic load balancing between two overlapping WCDMA cells on different carriers. Inter-RAT load balancing method refers to traffic load balancing between overlapping WCDMA cell and cell from another RAT. There are many works already done regarding traffic load balancing in WCDMA network. Investigation of parameter tuning for load based handover (LBHO) thresholds and impact of CPICH changing on inter-frequency handover (IFHO) performance has been made in [21-22]. WCDMA multi carrier (MC) scenario, load sharing (LS) and load balancing (LB) methods have been investigated in [23-25]. Cell planning, that considers downlink capacity with load balancing, has been investigated in [26]. The large increase in size and complexity experienced by cellular networks in recent years has led to a new paradigm known as heterogeneous networks, or HetNets. In this context, networks with different cell sizes, radio access technologies, or carrier frequencies can be deployed in the same geographic area. Different mechanisms of traffic steering and LB principles in HetNets have been investigated in [27-29]. Innovative approach for load balancing by dynamic tuning of soft handover (SOHO) parameters has been investigated in [30-31]. Furthermore load balancing methods has been enhanced in recent years by, so called, flow of users’ concept. Flow of users’ includes differentiating per user class by different QoS mechanisms [38], or differentiating per terminal equipment class [39]. Even with all hardware and software enhancement available, uplink interference in most critical scenarios still cannot be diminished to some acceptable level. On the other hand all available traffic distribution algorithms are operating based on downlink load, number of users or user QoS class. Thus during uplink overloads, problems with provided quality of service are still present. In this paper, uplink load as threshold for traffic distribution, with dynamic changes of CPICH power, has been investigated. Presented method refers to both load balancing approaches and depending on network topology, inter-frequency or inter-RAT traffic steering would be executed. The proposed solution in cases when all algorithms for uplink interference cancellation are unsuccessful, is to remove users to another cell until uplink overload would be resolved in source cell. Design of algorithm has been based on fact that radio network controller (RNC) is informed by each cell of cell's current UL RSSI. How often RNC is informed about UL RSSI, can be presented through RNC performance management counters that exist for every cell: pmSumUlRssi pmSamplesUlRssi The goal of algorithm is traffic offload from loaded WCDMA cell until it exit overload state. Proposed way how to achieve it is by changing downlink coverage of WCDMA cell. In this way both connected and idle users could be removed from WCDMA cell in state of uplink overload to another WCDMA or GSM (Global System for Mobile communication) cell. Moreover, by changing downlink coverage, users would not be only removed to overlapping cells, but to neighboring cells as well. Proposal is that CPICH0 and MaxPwr could be obtained directly from network by algorithm or both values could be imported by network operator. On the other hand, UL RSSImax, CPICHmin and T values have been introduced as new parameters used by algorithm for dynamic traffic distribution. To verify presented method, algorithm has been created and executed in lab environment. From the obtained results it has been concluded that proposed algorithm could execute standard load balancing between co-sector overlapping cells. On the other hand, simulation results have proven that load balancing between cells belonging to different sectors or sites could be also possible with proposed algorithm. Additionally, possibility of automatically cell lock has been introduced for cases of such uplink overload that satisfactory quality of service could not be achieved. The purpose of proposed algorithm is optimal traffic distribution during high uplink load, while system stability must be maintained. Impact on system stability could be executed by producing unnecessary signaling and processor load on RNC through to many parameter changes. Thus it has been concluded that dynamic traffic distribution based on current uplink load should be performed every 1 min. The results from lab simulations and conclusion for optimal implementation of proposed algorithm have been used to create field trial case on real WCDMA cells. For the purpose of field trial simplified version of proposed algorithm has been created, which has been based on performance management statistic as in lab investigation. The simplification has been done in a way that algorithm has been switching from CPICH0 to CPICHmin when uplink overload has been detected and bringing back CPICH power to CPICH0 when uplink overload has been resolved. As it has been explained UL RSSI measurement can be obtained from performance management statistics in 15 min resolution, which would not correspond to conclusions for optimal implementation. Thus another measure has been used to detect uplink overload, which is number of samples above certain threshold. It is possible to obtain from performance management statistic number of samples of UL RSSI measurement above defined threshold, with no specific UL RSSI measurement per sample. UL RSSI measurements are reported every 10 s from RBS to RNC, which means that in 15 min resolution there are maximum of 90 samples recorded by performance management statistic. UL RSSI measurement result of -90dBm has been defined as uplink overload threshold, while uplink overload detection has been defined as 6 recorded samples above defined threshold in monitored 15 min period. 6 recorded samples correspond to 1 min which has been concluded as optimal implementation of proposed algorithm. The introduction of number of samples above defined threshold as measure for uplink overload detection has enabled possibility to create simplified version of algorithm for field trial. From the obtained results it can be concluded that created version of algorithm for field trial could be used in real network. Despite the fact that uplink overload detection has been influenced by processing delay from performance management statistic, the results from field trial have indicated that created version of algorithm could bring additional QoS improvement during uplink overload. |
