Performance Evaluation of WiMAX Broadband from High Altitude Platform Cellular System and Terrestrial Coexistence Capability
© Z. Yang et al 2008
Received: 1 November 2007
Accepted: 14 August 2008
Published: 25 September 2008
The performance obtained from providing worldwide interoperability for microwave access (WiMAX) from high altitude platforms (HAPs) with multiple antenna payloads is investigated, and the coexistence capability with multiple-operator terrestrial WiMAX deployments is examined. A scenario composed of a single HAP and coexisting multiple terrestrial WiMAX base stations deployed inside the HAP coverage area (with radius of 30 km) to provide services to fixed users with the antenna mounted on the roof with a directive antenna to receive signals from HAPs is proposed. A HAP cellular configuration with different possible reuse patterns is established. The coexistence performance is assessed in terms of HAP downlink and uplink performance, interfered by terrestrial WiMAX deployment. Simulation results show that it is effective to deliver WiMAX via HAPs and share the spectrum with terrestrial systems.
Delivering worldwide interoperability for microwave access (WiMAX) services in the 3.5 GHz band from HAPs is an effective way to provide wireless broadband communications. HAPs, recently proposed novel aerial platforms operating at an altitude of 17–22 km, have been suggested by the International Telecommunication Union (ITU) for providing communications in mm-wave broadband wireless access (BWA) and the third-generation (3G) communication frequency bands [1–3]. Investigations on HAPs have therefore been mainly concentrated in mm-wave band and code division multiple access (CDMA) schemes delivered from HAPs. HAP systems have many characteristics including high receiver elevation angle, line of sight (LOS) transmission, large coverage area, and mobile deployment. These characteristics make HAPs to be competitive to conventional terrestrial and satellite systems, and furthermore contribute to a better overall system performance, greater system capacity and cost-effective deployment.
Many countries have made significant efforts in the research of HAPs and potential applications. Some well-known projects are: (1) the HeliNet and CAPANINA projects funded by the European Union (EU) , (2) the SkyNet project in Japan , (3) a HAP project started by ETRI and KARI in Korea , (4) a series of research and demonstrations of HAP practical applications carried out in the U.S. by Sanswire Technologies Inc. (Fort Lauderdale, USA), and Angel Technologies  (St. Louis, USA). These projects mainly focus on international mobile telecommunications 2000 (IMT-2000) services, IEEE802.1x services and fixed broadband wireless access (FBWA) in different frequency bands.
WiMAX is a standard-based wireless technology for providing high-speed, last-mile BWA up to 50 km for fixed stations and 5–15 km for mobile stations in frequency bands ranging from 2 to 66 GHz . In contrast, the wireless fidelity (WiFi/802.11) wireless local area network (WLAN) standards are limited in most cases to only 100–300 feet (30–100 m). WiMAX has been regarded as one of the most promising standards for delivering broadband services in the next few years and a strong competitor to the 3G system. Its standards based on IEEE 802.16a offer the potential to deliver a significantly enhanced nonline of sight (NLOS) coverage area from HAPs in the frequency bands below 11 GHz, which leads to a more favorable propagation path due to its unique position compared with traditional base stations located on mountains or tall buildings. Providing WiMAX from HAPs is a novel approach, and some preliminary research has been done to show its effectiveness [3, 8, 9]. In this paper, we focus on the application scenario for delivering WiMAX IEEE802.16a from HAPs. In our scenarios, we assume fixed users with the directive antenna mounted on the roof to receive signals from HAPs. It is anticipated that providing WiMAX from HAPs is a competitive approach with a low deployment complexity of broadband services.
Terrestrial cellular architectures described by Lee in  are based on a division of the coverage area into a number of cells which are assigned to different channels with respect to adjacent cells, in order to manage cochannel interference and achieve frequency reuse. Conventionally, cells are grouped into clusters of three, four, seven, or nine cells, with all the available frequency bands allocated between them. A cluster with a larger number of cells has a greater reuse distance but fewer number of channels per cell. Previous works  have initially examined the fundamental performance achievable from a single-HAP and a single-terrestrial base station without considering cellular deployment for both systems. In a HAP cellular system with multiple antennas, interference is mainly caused by antennas serving cells on the same channel employing the terrestrial frequency reuse schedule .
This paper focuses on the system performance in different cellular reuse schemes and investigates coexistence performance with terrestrial WiMAX deployments. The paper is organized as follows. In Section 2, the description of the HAP WiMAX cellular system model, the signal path loss, and antenna models considered for HAPs and that of the terrestrial deployments is described. In Section 3, the downlink and uplink HAP WiMAX cellular system performance is evaluated in terms of criteria such as carrier to interference ratio (CIR) and carrier to interference plus noise ratio (CINR). In Section 4, a coexistence scenario is proposed to investigate the coexistence capability of HAP and terrestrial systems. In Section 5, an approach to improve HAP system performance by increasing the frequency reuse factor is shown. Conclusions are given in Section 6.
2. Hap Wimax System
Most of the research on HAPs considers the stratospheric platform equipped with a multiantenna payload projecting a number of spot beams within its coverage area. HAPs are employed as a group of base stations in terrestrial communication systems. A spot beam antenna architecture is able to rapidly provide a high system capacity to a number of users with a narrow beamwidth . Consequently, we assume that the HAP is fitted with WiMAX base stations onboard.
2.1. Hap Cellular System
The receiver shown in Figure 1, which we refer to as a "user", is assumed to be located on the ground on a regular grid with one kilometer separation distance. This allows the performance of the coverage plot to be evaluated. After the performance is evaluated at one point, the user is moved to the next point and the same simulation test is carried out again. At anytime only one user is considered to be involved in the simulation, so interference between multiple users is not taken into account. The user is located on the grid points, spaced of one kilometer in the horizontal and vertical direction. The reason of choosing the one kilometer spacing distance is that CNR or CINR does not change significantly over the distances of less than one kilometer, and also to ensure that the computation burden is not heavy especially when the coverage area is extended further.
2.2. Hap Antenna and User Antenna Patterns
An antenna radiation pattern is an important and critical design factor in determining the performance of radio communication systems. Ideally in cellular system, the antenna pattern would radiate uniform power across its serving cell and no power should fall outside. In practice, there is unavoidably power spilling outside the coverage area, which can cause interference to other cells.
2.3. Hap Cellular Cochannel Interference Evaluation
2.4. Hap Cellular Reuse Pattern
The WiMAX performance from a HAP cellular system is evaluated by assuming that the user inside the HAP coverage area communicates with the HAP and is interfered by the cochannel HAP antennas. In practice, a precise hexagonal pattern cannot be generated, due to topographical limitations, local signal propagation conditions, and practical limitations on sitting antennas .
2.5. Path Loss Calculations and Simulation Parameters
Important system simulation parameters.
30 km (R H )
7 km (R T )
17 km (H H )
30 m (H T )
40 dBm (P H )
40 dBm (P T )
User roll-off rate
58 (n U )
User boresight gain
18 dB (G U )
−30 dB (s f )
7 MHz/1.75 MHz (DL/UL)
3. Hap Cellular System Performance
3.1. Hap Downlink System Performance Evaluation
P H is the transmission power of the HAP transmitter;
P Hi is the transmission power of the interfering HAP antennas;
A H and A U are the antenna gains of the HAP and the user, and they depend on the angular deviation from the boresight;
PLHU is the propagation pathloss from HAP to user;
N H is the total number of cochannel cells in the HAP system;
N F is the noise power (-100.5 dBm).
3.2. Hap Uplink System Performance Evaluation
P U is the transmission power of user in the target cell (30?dBm);
PLUH is the propagation pathloss from user to HAP;
N F is the noise power (-106.5 dBm).
4. Performance of A Hap Coexistence Scenario
Providing WiMAX from HAPs is a novel means to deliver broadband services. Thus, it is vital to consider its coexistence capability with current terrestrial WiMAX system in order to get an assessment of the performance. In this paper, we mainly focus on evaluating interference from terrestrial WiMAX to the HAP system.
4.1. Hap Coexistence Scenario
4.2. Hap Downlink Coexistence Scenario
P T is the transmission power of the interfering terrestrial base station;
PLTU is the pathloss from the terrestrial base station to user;
A UT is the user antenna gain at an angle away from its boresight.
4.3. Hap Uplink Coexistence Evaluation
PLUT is the propagation path loss from the user to the terrestrial base station.
5. Hap Wimax System Improvement
A number of approaches have been used to improve the cellular system performance, for example, adding new channels, cell splitting, cell sectoring. Increasing D, the minimum distance between cochannel cells, is an effective approach, since it can keep the cochannel cells further away from each other and therefore decrease the interference without requiring more spectrum.
In this paper, we have shown the performance of both downlink and uplink WiMAX broadband standard transmitted from a HAP cellular system in the 3.5 GHz band across a coverage area of 30 km radius, while operating in the same frequency band with terrestrial WiMAX deployments based on a proposed coexistence scenario. A cellular configuration has been proposed for the HAP WiMAX system based on the typical WiMAX terrestrial system. The HAP coverage area was divided into 19 individual cells served by multiantenna payload. WiMAX broadband system performance of individual HAP system was evaluated both separately and when taking into account the cochannel interference from the antennas operating in the same frequency band. Coexistence capability was investigated based on a proposed coexistence scenario and examined by considering interference from the nearest terrestrial base station to HAP system. Simulation results clearly demonstrate that the internal cochannel interference was dominant when delivering WiMAX via HAPs, and HAP system can effectively share the spectrum with terrestrial WiMAX systems.
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