Optimizing Downlink Coexistence Performance of WiMAX Services in HAP and Terrestrial Deployments

in Shared Frequency Bands

Zhe Yang, Abbas Mohammed, and Tommy Hult Department of Signal Processing

Blekinge Institute of Technology, BTH SE-372 25 Ronneby, Sweden Email: {zya,amo,thu}@bth.se

David Grace Department of Electronics, University of York, UOY

YO10 5DD York, UK Email: dg@ohm.york.ac.uk

Abstract—In this paper, we investigate techniques for optimizing the coexistence performance of providing WiMAX (IEEE802.16a) from High Altitude Platforms (HAPs) and terrestrial deployments in shared 3.5 GHz frequency bands are presented. The paper will show that it is effective to provide WiMAX services from HAPs with optimized performance by appropriate choice of parameters, including varying the HAP deployment spacing radius and directive antenna beamwidth based on the adopted antenna models for HAPs and receivers. Illustrations and comparisons of changing the antenna pointing offset, narrowing the transmitting and receiving antenna beamwidth as well as keeping the accepted CINR performance across the HAP coverage area, will demonstrate that that efficiently utilizing these techniques are able to enhance the HAP system performance while effectively coexisting with the terrestrial WiMAX systems.

I. INTRODUCTION

High Altitude Platforms (HAPs) are either quasi-stationary airships or aircrafts operating in the stratosphere, 17-22 km (72,000ft) above the ground and have been suggested as a way of providing 3G and mm-wave broadband wireless access (BWA) [1-3]. Recently a HAP trial held by EU CAPANINA project has successfully tested the usage of a HAP at 24 km altitude, operating in mm-wave band to send data via Wi-Fi (IEEE802.11b) to a coverage area 60 km in diameter [4]. Worldwide Interoperability for Microwave Access (WiMAX) is a standards-based wireless technology for providing high-speed, last-mile broadband connectivity to homes and businesses and for mobile wireless networks ranging from 2 to 66 GHz in frequency.

Providing WiMAX from HAPs using IEEE802.16a, which is designed for transmission below 11 GHz, is an innovative way of providing broadband communication services. WiMAX (IEEE802.16a) is now widely accepted as a future air-interface broadband standard capable of delivering several megabits of shared data throughput for

fixed, portable and mobile operators [5]. Related researches [6, 7] have been carried out to examine the WiMAX downlink performance in the coexisting environment composed of a single HAP and a single/multiple terrestrial base stations. It has been shown that WiMAX is effectively deployed from HAPs with accepted downlink communication quality at the receivers' side.

The purpose of this paper is to propose new methods to optimize the downlink coexistence performance of providing WiMAX from HAPs and terrestrially. It is organized as follows: section II will give a description of coexistence system model, signal pathloss and antenna models in simulations for HAPs and terrestrial deployments, as well as important system parameters. Criteria to measure the interference and system performance e.g. downlink Carrier to Noise Ratio (CNR) and downlink Carrier to Interference plus Noise Ratio (CINR), are also defined. In section III improved system performance and analysis is shown under varying the spacing distance of a single HAP deployment, testing different antenna beamwidth and roll off factors. Conclusions are given in section IV.

II. SYSTEM SIMULATION MODEL AND PARAMETERS

The basic system model to simulate the coexistence environment is shown in Fig. 1. It is composed of a HAP base station (H-BS), a terrestrial base station (T-BS) and a test user. The HAP base station is assumed to be located at an altitude of 17 km above the ground with a radius of coverage area equal to 30 km. The terrestrial base station is deployed on the ground with an appropriate separation distance 50 km away from the Sub Platform Point (SPP) of the HAP on the ground.

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Undesired signal

Boresight of HAP antenna

Desired signal

R T-BS

θU

ϕH

HAP coverage area

RHAP

ϕ,θ Angle from the boresight

R Radius coverage area

One base station cell

User (x,y,z)

Separation distance

Separation distance

HAP

T-BS

Fig. 1 Coexistence environment model of providing WiMAX from

HAP base station (H-BS) and terrestrial base stations (T-BS); RT-BS=7 km and RHAP=30 km

A. HAPs to Terrestrial Interferece The gain of the antennas of H-BS AH (ϕ ) at an angle

ϕ with respect to its boresight and the ground receiver antenna AU (θ ) at an angle θ away from its boresight are approximated by a cosine function raised to a power roll-off factor n, with a flat side lobe level. They are represented in (1) and (2), respectively.

]),)(max[cos()( fn

HH sGA Hϕϕ = (1)

]),)(max[cos()( fn

UU sGA Uθθ = (2)

where GH and GU state the boresight gain of the H-BS antenna and receive user antenna respectively. nH and nU control the rate of power roll-off of the main lobe individually. sf in dB is a notional flat side lobe floor. The boresight of the H-BS antenna points at the centre of its coverage area. A circular symmetric radiation pattern in [8] is used and a 10-dB roll-off beamwidth is selected for the H-BS in this paper to better decrease the interference from the HAP to terrestrial WiMAX deployment.

Fig. 2. HAP Antenna Performance across the coverage area with

different beamwidth (BW)

The propagation model used for H-BS is the Free Space Path Loss (FSPL) PLH shown in (3). The height of the HAP will result in a high minimum elevation angle at the edge of

coverage area (EOC), so diffraction and shadowing are not explicitly considered in this paper.

2)4(λπdPLH =

(3)

The propagation pathloss model PLT is shown in (4) for terrestrial signal propagation model as presented in [9]. This model corrects the Hata-Okumura model to account for limitations in communication with lower base station antenna heights and higher frequencies.

hfmT PLPLPLPL Δ+Δ+= (4)

where PLT is composed of a median path loss PLm, receiver antenna height correction term ΔPLh and frequency correction term ΔPLf. The two-correction terms ΔPLh and ΔPLf are defined to make PLT more accurate by accounting for the antenna heights and frequencies used by IEEE802.16a. In this paper parameters in suburban environment (category C [9]) are used for simulation of T-BS deployment environment.

TABLE 1 IMPORTANT SYSTEM SIMULATION PARAMETERS

Parameters H-BS T-BS Coverage Radius

Transmitter Height Transmitter Power

30 km (RH) 17 km (HH)

40 dBm (PH)

7 km (RT) 30 m (HT)

40 dBm (PT) Antenna Efficiency 80% User Roll-off Rate

User Boresight Gain User Antenna Height

Side lobe Level

58 (nU) 18 dBi (GU) 6.5 m (HU) -30 dB (sf )

Bandwidth Frequency

Noise Power

7 MHz 3.5 GHz

-100.5 dBm (NF)

B. Interference Analysis Based on the previous coexistence environment, we

proposed the interference analysis scenario to evaluate H-BS WiMAX system performance. The test user is assumed to communicate with the HAP and receive interference from the terrestrial base station. The system performance could be determined by CNR in (5) and CINR in (6) respectively:

F

HUHHH N

PLAAPNCCNR ==

(5)

TUTTF

HUHHH PLAAPN

PLAAPIN

CCINR+

=+

= (6)

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Here PH and PT are the H-BS and interfering T-BS transmission powers. NF is the thermal noise floor. AH and AT is the transmission gain of H-BS and T-BS antenna. AU is the receiver gain of the user antenna receiving signals from the H-BS and T-BS.

III. OPTIMIZATION OF SYSTEM PEROFRMANCE

The coexistence system model was presented in section II. Based on this model, optimization techniques for decreasing interference from HAPs to terrestrial WiMAX system will be investigated in this section.

A. Varying HAP Spacing Radius In the previous investigations, we assume that SPP of

HAP is in the center of the HAP coverage area, which has illustrated a good system performance. Since a directive antenna is used on the HAP, it could allow HAPs to be deployed in the different parts of sky while the boresight of its antenna is still pointing at the desired coverage area. Furthermore in practice it is quite hard to keep HAPs stationary absolutely above the center of the coverage area and antenna array will be adopted to serve multi cells to increase the system capacity and efficiency of spectrum utilization. So we now consider the system performance under the changeable HAP spacing distance, which means that the SPP of HAP is now always overlapping the center of its serving area in. The location of the T-BS is fixed at 50 km away from the center of the HAP coverage area.

HAP coverage area

User (x,y,z)

One base station cell

……..

0 km-10 km-20 km-50 km

……..

T-BS

H-BS

Spacing Distance

Antenna boresightDesired SignalUndesired Signal

Fig. 3. Illustration of changing of HAP spacing radius while keeping

the antenna pointing offset at the center of serving area

As the HAP antenna is not pointing at the SPP of HAP due to the variable HAP spacing distance, the HAP antenna gain across the HAP coverage area will change accordingly.

From Fig. 4 we could see the HAP antenna gain with different spacing distances. It shows that curves fall more rapidly to the side lobe level with the wider spacing distance on the left side of the coverage area, e.g. when the spacing radius is equal to 20 km, the antenna will enter into its side lobe level just at the left edge of its coverage. In this case, if the T-BS is deployed on the left side of the HAP coverage area as shown in Fig. 4, outside the HAP coverage area will received an interfering signal coming from the side lobe of the HAP antenna, not the main lobe. Interfering signals

coming from terrestrial base stations are also suppressed since they are received by the side lobe of the antenna. On the right side of the HAP coverage area, the HAP antenna curve falls more slowly compared with the zero spacing distance case, which will provide higher gain to the users using HAP services. Interfering signals coming from terrestrial base stations are also suppressed since they undergo a longer distance to the HAP antenna with a higher attenuation.

Considering the efficient utilization of limited payload of HAP, this technique could be used in the multiple HAP deployment to serve multiple cells from HAPs by suppressing interfering signals into side lobe of the HAP antenna.

Fig. 4. HAP antenna gain with different spacing distance (0 km,-10 km,

-20 km) and different beamwidth roll off (-3 dB, -10 dB, -30 dB)

B. Varying HAP Antenna Beamwidth The antenna beamwidth is another important parameter

affecting system performance. It determines the directivity of the antenna and hence controls the footprint on the ground. As shown in Fig. 2, we can see the narrow beamwidth can bring a high peak gain and rapid falling rate of antenna gain corresponding to the angle extended from the boresight. At the edge of the HAP coverage area, the antenna gain will decrease to the appropriate level to create an acceptable coexistence environment to terrestrial WiMAX communication deployment.

Different antenna beamwidths are investigated in Fig. 5 to show the improvement that can be achieved by decreasing the HAP antenna beamwidth. When the beamwidth is narrowed to 43 degrees, less than 90% coverage area will achieve a CINR of 35 dB and less than 10% area achieve a CINR of 10 dB at the edge of the coverage area. Compared with the 43 degree beamwidth performance, a 72 degree beamwidth antenna, which is adopted for simulation in HAP's side, will both give 50% area inside the HAP coverage a higher CINR at 25 dB and a higher CINR at the edge of coverage area. The 72 degree beamwidth will also

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provide the potential to extend the HAP coverage area of offering better link budges at the edge of coverage.

Fig. 5. CINRH performance under different HAP antenna beamwidth

C. Varying the User Antenna Beamwidth Similar to changing the HAP antenna beamwidth,

varying the user antenna beamwidth is also an effective means to improve the system performance as shown in Fig. 6. We can see that with the narrower antenna beamwidth of the receiver the CINR performance will be improved gradually. For example the 17 degrees beamwidth is selected in the simulation and achieves a mean CINR inside the HAP coverage area around 23 dB, when we specify that it is equal to its half power beam width (roll off at -3 dB).

If we consider the movements of HAPs and receivers, a narrower beamwidth of user antenna will require a higher antenna pointing accuracy and is not suitable for mobile communication applications.

Fig. 6. CIRH performance against increased user antenna beamwidth

IV. CONCLUSIONS

In this paper, we presented techniques for optimizing the downlink coexistence performance of providing WiMAX services in HAP and terrestrial deployments in the shared frequency band of 3.5 GHz. These techniques included varying the HAP spacing radius, HAP antenna beamwidth and the user antenna beamwidth. Simulation results have shown that efficiently utilizing these parameters can achieve a better HAP system performance, while at the same time can efficiently coexist with the terrestrial WiMAX system.

REFERENCES

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[2] D. Grace, J. Thornton, G. Chen, G. P. White, and T. C. Tozer, "Improving the System Capacity of Broadband Services Using Multiple High Altitude Platforms," IEEE Transactions on Wireless Communications, vol. 4, pp. 700-9, 2005.

[3] G. M. Djuknic, J. Freidenfelds, and Y. Okunev, "Establishing Wireless Communications Services via High-Altitude Aeronautical Platforms: A Concept Whose Time Has Come?," IEEE Commun. Mag., vol. 35, pp. 128-135, 1997.

[4] BBC, "Broadband net goes stratospheric," http://news.bbc.co.uk/1/hi/technology/4354446.stm, 2005.

[5] IEEE, "IEEE Standard 802.16-2004," 2004. [6] Z. Yang, D. Grace, and P. D. Mitchell, "Coexistence Performance of

WiMAX in HAP and Multiple-Operator Terrestrial Deployments in Shared Frequency Bands," presented at Wireless Personal Multimedia Communications (WPMC), Aalborg, Denmark, 2005.

[7] Z. Yang, D. Grace, and P. D. Mitchell, "Downlink Performance of WiMAX Broadband from High Altitude Platform and Terrestrial Deployments Sharing a Common 3.5GHz Band," presented at IST Mobile Communications Summit, Dresden, Germany, 2005.

[8] J. Thornton, D. Grace, M. H. Capstick, and T. C. Tozer, "Optimising an Array of Antennas for Cellular Coverage from a High Altitude Platform," IEEE Transactions on Wireless Communications, vol. 2, No. 3, pp. 484-92, 2003.

[9] "Modifications and Additional Physical layer Specifications for 2-11GHz," IEEE Standard 802.16a-2003, 2003.

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