real networks that not only support multiple applications but also recognises the business models of the existing networks and aims to provide new revenue generation possibilities [7].
Hybrid systems integrate different technologies in order to meet the user needs and environments and provide the right product key for the network space. Such hybrid systems have many advantages such as:
• Providing a range of services and high bandwidth
applications for subscribers
• Cover a wider area including rural and remote regions
• Make use of low cost equipments and devices
• Challenges to a convergence towards IP
• Flexible communications and infrastructure
• Using communication resources more effectively
• Gathering benefits of all systems and eliminates any drawbacks
• More reliability networking
• Reduce power consumption
• Less use of antennas and base stations and gateways
III. THE HYBRID SATELLITE-HAP SYSTEMS
In an integrated HAPs-Satellite communication architecture, HAPs can be used for the direct provision of mobile services and as support for the provision of IP broadband fixed services through satellites. This function of HAPs can be implemented in two configurations as shown below. In both scenarios, the HAP provides the return link of satellite-based internet services [2] [10].
1. Symmetric Configuration
As shown in Figure 1, in this configuration, both forward and return channels (from the end user) go through the HAP. There are two possible cases in this type of configuration. In the first case, as shown in Figure 1a, no HAP is considered in the feeder link (between satellite and terrestrial gateway). The feeder link is usually characterised by a lower error rate as compared to the user terminal link. In the second case shown in Figure 1b, the network comprises a HAP in the feeder link.
Figure 1: Symmetric HAP-SAT Architectures: (a) Feeder Link via
Satellite; (b) Feeder Link via HAP
The main advantages of this architecture is the use of a TCP splitter at the HAP to split the earth-satellite link into two parts, the Earth-HAP link where most of the link impairments are concentrated, and the HAP-Satellite link where the attenuation is mainly due to free space losses. In
the HAP-Satellite link adaptive modulation and coding techniques and link layer error recovery mechanisms (like ARQ) can be effectively exploited more than in a pure satellite link. While the latency for the Earth-HAP link is assumed to be 15 ms, the latency for the Earth-Satellite link is 250 ms. In addition, the link between the HAP and the satellite is almost error free link but with a high propagation delay [1].
2. Asymmetric Configuration
In several satellite systems like those using DVB-S/S2, a separate return link either via the fixed terrestrial infrastructure (PSTN) or even via the GSM network is used for the return direction. As shown in Figure 2, in this configuration while the forward channel is the transmission link provided by the satellite, the return channel goes through the HAP.
Figure 2: Asymmetric HAP-SAT Architecture
The main advantage of this architecture is that the return channel does not suffer from high congestion and low bandwidth, as it does for a PSTN return channel. Therefore, it is reasonable to expect an improvement of the TCP/IP throughput.
3. HAP-Satellite Hybrid Architecture Advantages
The telecommunication union has defined the hybrid networking as a solution for providing broadband services to users by merging different network platforms in a complementary way making the system more efficient. The HAP-Satellite hybrid network can be used to provide IP-based broadband services to customer premises in remote areas. With the use of mobile WiMAX, such a hybrid system can also provide IP services to fast moving vehicles like trains and coaches. Figure 3 illustrates the general network architecture of an IP-based HAPs-Satellite hybrid system. Such a system offers the following advantages:
• The improved link budget could reduce the transmit
power giving better carrier-to-interference ratio (CIR) between the systems and thereby significantly
reducing the cost and size of the user terminals.
• The integration can bring an obvious improvement in
the throughput of TCP/IP connections.
• Reducing PPP satellite capacity requirements and
cost, in reducing the protocol overhead for the
satellite links.
• Integrating HAPs with satellites can be used to
provide efficient data management, traffic control
and data-relay systems [1] [2].
• The cost of services and applications could be shared
among these systems and other networks giving rise
to new business models.
Figure 3: A Powerful hybrid IP-Based Architecture
IV. WIMAX PAYLOAD ON HAPS
A typical HAP design should seek high reliability, low power consumption, high communication data rates and light payload, thus leading to an architecture that places most of the system complexity on the ground segment.
Worldwide Interoperability for Microwave Access, IEEE 802.16 (WiMAX) provides an efficient technology for the communication between the flying HAP and the user on the ground. IEEE 806.16 standard focuses mainly on how to provide broadband connection at link layer and physical layer independent on the upper layers [4] [5]. The primary reason for the selection of IEEE 802.16 was the ability to cope with the HAPs data rates of up-to 120 Mbps and the forecast requirement for the link symmetry in future broadband applications. The main issue faced by this choice is the complexity of handling mobility. While at this stage, the WiMAX standard is mainly intended for fixed access, extensions for mobility management are being currently developed by IEEE 802.16e working group [4]. WiMAX standard was designed primarily for point-to-multipoint (PMP) communication scenario to support mesh architectures. The HAPs-Satellite hybrid architecture also considers PMP scenarios with the subscriber terminals communicating with the base station placed on the HAP.
Figure 4: An OSI Model for a Single HAP
The MAC layer of IEEE 802.16 standard on the HAP comprises of three sub layers as shown in Figure 4. The
Service Specific Convergence Sub layer (CS), transforms the data received from upper layers through the CS service access point to the MAC Common Part Sub-layer (CPS) [5], thus making the choice of wireless access standard transparent to the communication technologies that are implemented on the upper layers. The transformation includes the mapping of data from upper layer to MAC connection identified by a Connection Identification (CID). The CPS contains core functionalities of the MAC layer such as system access control, bandwidth allocation, connection establishment, connection maintenance, and Quality of service (QoS) handling. Finally, the Privacy Sub layer (PS) provides authentication, key exchange and encryption. The high flexibility of IEEE 802.16 is due to its capability of supporting multiple connections per terminal, and also multiple QoS levels per terminal.
V. SIMULATION RESULTS AND ANALYSIS
1. Simulation Models
OPNET 14.5 [11] was used for simulating the different hybrid models that show the effects of the various ways of integration the HAP and satellite networks on the IP-broadband services. The simulations aim to compare the performance of the scenarios, in order to find the most suitable architecture.
The following three different scenarios shown in Figure 5 for integrating the networks are simulated:
• Scenario 1: HAP-HAP which represent a network
architecture that use HAP platforms in a standalone.
• Scenario 2: HAP-GEO which represents network
architecture that uses HAP and a GEO satellite in
providing services and applications to users.
• Scenario 3: HAP-LEO-GEO which represents
network architecture that uses HAP and LEO/GEO satellites in providing services to users.
Figure 5a: Standalone HAPs simulation model
Figure 5b: Hybrid HAPs-Satellite (GEO) simulation model
Figure 5c: Hybrid HAPs-LEO-GEO simulation model
Each of the three scenarios contains the following segments:
• The HAPs Network: including both the Stratospheric Airships and the users.
• The Gateway (GW): This is the first router in the
terrestrial network.
• The Satellite Communication Network
• The Public Network: representing the IP Internet
Core Backbone
• IP Backbone Services: includes five servers to
support Email, HTTP, FTP, Voice and Video
services.
The HAPs networks provide broadband traffic to the users that are placed within the HAP cell (as a very high Base-station), supporting wireless (802.11) or WiMAX (802.16) users in each scenario. The hybrid models are divided to different segment including User Segment, Sky Segment, Ground Segment and Space Segment.
New network entities were coded in the hybrid network models for each scenario. Each of these nodes contains a number of processor modules that model the different functional entities within that module. The details of these processes are out of scope of this paper. The different node models that are present in the different scenarios are:
• Ground Segment:
− Network of Servers (IP core backbone)
− Internet
− Gateway
• Sky Segment:
− HAP Networks (Stratospheric Airships)
• Space Segment:
− Satellites (GEO, LEO, or both)
• User Segment:
− WiMAX LAN User Device
− Wi-Fi LAN User Device
Each scenario simulates various applications and services that are specified in the application and profile attributes. The application model contains the definitions of the different applications that can be used by the different profiles. Figure 6(a), (b) illustrates the actual configurations of the HAP entity, showing the altitude (height) and the spectrum.
(a) HAP Antenna Attributes
(b) HAP Frequencies Attributes
Figure 6: HAP Configurations
2. Results Analysis
The main objective of the simulations is to evaluate the performance of IP-broadband services and applications over hybrid HAPs-Satellite networks. The following different parameters shall be used to compare the performance of the different scenarios:
• Delay,
• Response time,
• Throughput and
• Traffic capacity.
(a) Email Download Response Time
(b) Email Upload Response Time
Figure 7: Email Service Performances
TABLE I. EMAIL APPLICATION PERFORMANCES
Application Scenario Simulation Duration (sec.)
User
100
200
300
Email
(Download
Response
Time)
(1) Client 1 0.8 0.7 0.6
Client 3 0.9 0.9 0.7
(2) Client 1 3.5 3.5 3.5
Client 3 3.6 3.6 3.6
(3) Client 1 4.5 4.5 4.5
Client 3 4.6 4.6 4.6
Figure 7(a) and 7(b), show the average time taken to upload and download the email services in the three scenarios. Table I shows the average Email download response time for the Cient1 and Client 3 who are present in different HAP networks. It can also be seen that the response time is larger in the scenarios including LEO and GEO satellites systems. While the Email download response time for scenario 3 is around 4.5 sec (for both clients), it goes down to around 1sec and 3.5sec in the scenario 1 and scenario 2 respectively. This leads to more than 60% improvement from scenario 3 to 1. This is mainly due to the huge Round Trip Time (RTT) in the satellite links which governs the overall delays in the network.
(a) Video ETE Delay
(b) Voice ETE Delay
Figure 8: Video and Voice Services Performances
Figure 8(a) and 8(b) show the end-to-end (ETE) delay for voice and video applications for Client 1 and Client 3 in the three scenarios. With the increase in the complexity in the network architecture, the end-to-end delay also increases. Figure 8(a) shows that for scenario 1, which represents the HAP system without including satellites in the architecture; the end-to-end delay for the video application is less than that for scenario 2 by 0.3s and approximately by 0.45s than the scenario 3. This gives more than 50% and 70% improvement for scenario 1 than the other scenarios 2 and 3 respectively. Figure 8 (b) shows the end-to-end delay for the voice application for the three scenarios. Also, giving the same progress for scenario 1, from the other scenario (1, 2), which is nearly the same what it shown in the video application. It shows that scenario 1 is most suitable for serving user as it has the least end-to-end delay as compared to the other scenarios.
(a) Clients Load on HAPs
The simulation were also used to measure how the performance of the stratospheric platform (HAP Airship) in the different architectures. The performances intensity of the HAP system in some layers, such as the internet protocol layer and the data link layer (WiMAX MAC Layer) was analysed.
Figure 9(a) shows that that when the number of clients is fixed, the data throughput (highlighting the HAP load) is identical for all the scenarios.
(b) WiMAX Layer Delay on Airships (HAPs)
Figure 9: HAP Performances
In contrast, it was also seen in Figure 9(b) that the delay over the HAP varies for the three scenarios with the delay for scenario 1 being the least as compared to each of the other scenarios. In the other hand, Fig. 10(a) shows the average time that takes in the queue before sending the service requests in all the scenarios, which shows as before that scenario 1 is better and quicker in demanding the services and applications from the main IP-servers, nearly less than 0.1msec in scenario 1 from 2 and 0.15msec from scenario 3. But in Fig. 10(b) it shows that throughput of traffic being sent to clients is more in scenario 3 which the system is using three platforms of telecommunications (HAP-LEO-GEO). Where it was improved (increased) to approximately 20% and 7% than the other scenarios (1, 2) respectively.
(a) Time Queue Requesting services
(b) Broadcasting IP-Broadband Services to Clients from HAPs
Figure 10: Traffic Performances over Hybrid Scenarios
VI. CONCLUSIONS
The telecommunication industry has significantly changed during the last few decades due to the different communication factors; among them are the integration of wireless mobile communication systems, expansion of wireless networking and user demands on IP-broadband services. However, it has always been a desire for telecommunications engineers and researchers to develop a powerful hybrid infrastructure with many features like low delays, very high data rates and extensive coverage area, especially reaching rural regions and remote spots where user density is low. These schemes work as prototypes for the future Broad Wireless Association networks, which will replace other wireless multimedia access networks. However, the HAPs which use the WiMAX technology onboard are being considered as an important link and challenging topic for future wireless communications. This paper described the advantages of integrating HAPs with satellites networks and the different possible network architectures for such hybrid systems. It was seen the while such hybrid systems provided several benefits of rapid deployment, large coverage, etc. it also results in higher delays as compared to when no satellites are used. However it can be seen that the advantages of such a system outweigh this increase in delay which is still within acceptable limits.
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