WO2009066986A2

WO2009066986A2 - Capaqty enhancement in wimax mesh network

Info

Publication number: WO2009066986A2
Application number: PCT/MY2008/000152
Authority: WO
Inventors: Abdelhaleem Saeed Rashid; Bin Mohd Ali Borhanuddin; Mohamad Hafizal; Chee Kyun Ng
Original assignee: Mimos Berhad; Universiti Putra Malaysia
Priority date: 2007-11-22
Filing date: 2008-11-24
Publication date: 2009-05-28
Also published as: WO2009066986A3; MY147158A

Abstract

Capacity has become of primary importance in WiMAX mesh networks (WMN due to the ever increasing demand for multimedia services and the possibility o providing high speed wireless Internet. The major factor limiting capacity in such systems is interference originating from adjacent nodes, namely co-channel interference. The present invention is generally an analysis of co-channel interference for a spectrally efficient WMN configuration. A new channel allocation algorithm is proposed by using smart antenna system to eliminate the co-channel interference. This algorithm considers the number of subchannels in the OFDM spectrum in order to increase the system capacity. The new algorithm is designed to avoid the same subchannel of OFDM spectrum to be allocated in adjacent nodes. The system capacity can be increased when the co-channel interference is reduced.

Description

CAPAQTY ENHANCEMENT IN WiMAX MESH NETWORK

FIELD OF THE INVENTION

The present invention relates to a capacity enhancement in WiMAX mesh network and more particularly the present invention relates to an enhanced WiMAX using a new channel allocation algorithm.

BACKGROUND OF THE INVENTION

WiMAX or also known as Worldwide Interoperability for Microwave Access has been standardized as IEEE 802.16 standard which is initialized from a group called WiMAX Forum. The WiMAX Forum is an industry-led, non-profit corporation formed to promote and certify compatibility and interoperability of broadband wireless products based on IEEE 802.16 and ETSI HiperMAN wireless MAN standards.

WiMAX provide a broadband wireless access at high speed and a scalable solution for extension of a fiber-optic backbone. WiMAX base stations can offer greater wireless coverage of about 5 miles, with line of sight (LOS) transmission within bandwidth of up to 70 Mbps.

Beyond just providing a single last hop access to a broadband internet service provider (ISP), WiMAX technology can be used for creating wide-area wireless backhaul network. When a backhaul-based WiMAX is deployed in Mesh mode, it does not only increase the wireless coverage, but it also provides features such as lower backhaul deployment cost, rapid deployment, and reconfigurability. WiMAX mesh networks (WMN) create connectivity over multiple wireless hops. It becomes an interesting alternative to wired access in metropolitan areas.

*; Although single hop WiMax provides high flexibility to attain Quality of Service (QoS) in terms of data throughput, achieving the same in multi-hop WiMAX mesh is challenging. One of the major problems is dealing with the interference from transmission of the neighboring WiMAX nodes.

In the present invention, there is proposed a novel algorithm for channel assignment in WMN with smart antenna system. A narrower beam from smart antenna can spatially isolate the interference from other nodes besides in frequency domain. The reuse of sub-channel for other nodes is possible as long as the interest nodes are not experiencing the co-channels from transmitting node. Some constraints are imposed in this proposed algorithm. The relay nodes must be in static mode in order to occupy the smart antenna as its beam forming system. AU the nodes subscribed to the same node are assigned with different sub-channels. However, these sub-channels can be reused in other tier nodes. There is a time that all the nodes in different tiers are transmitting in the same direction which will cause co-channel interference. Therefore, different subchannels must be assigned to these nodes. Nevertheless, the sub-channel reuse assignment can provide a significant throughput to WMN. These double separate techniques in both spatial and frequency domains will create a dominant interference cancellation so that the throughput of WMN will be increase dynamically.

SUMMARY OF THE INVENTION

The present invention relates to a capacity enhancement in WiMAX mesh network to enhanced WiMAX. The said WiMAX mesh network is enhanced by using a new channel allocation algorithm and a smart antenna. The said smart antenna consists of an antenna array, combined with signal processing in both space and time. A beam forming is performed at the radio frequency level by controlling the amplitudes and phases of the feeding currents through attenuators and phase shifters. The smart antenna operates its beams in the baseband, thus the feeding currents of the antenna elements are directly proportional to the modulated baseband signals. The smart antenna is capable to cancel co-channel interference and wherein co-channel interference in the transmitting mode is reduced by focusing a directive beam in the direction of the desired user, and nulls in the directions of other interfering users. Each OFDM symbol is created by mapping the sequence of symbols on the sub-carriers.

In order to create the OFDM symbol in the frequency domain, the modulated symbols are mapped on to the sub-channels that have been allocated for the transmission of the data block. The number and exact distribution of the subcarriers that constitute a sub-channel depend on the subcarrier permutation mode. Each node is equipped with a smart antenna system and multiple beams generation is possible and wherein nodes in a WMN are either gateways or routers and wherein gateways are equipped with a base station interface and additionally offer connectivity to the wired internet network and wherein routers are equipped with base station and subscriber station interfaces which attributed to as relay station and wherein each relay station needs to have a connection to any gateway to be able to establish an internet link.

When a relay station node is in mobile mode its beams' direction to other nodes are no longer accurate. An exemption is given to end subscriber nodes in WMN wherein these nodes are the last node that does not support the next hop connectivity and wherein the mobility is possible for these nodes due to the flexibility of beam steering from its base station smart antenna. A task of channel allocation is to assign each communication link a channel on a given topology and wherein it is sufficient to allocate a channel between two nodes which is determined by base station interface of each node.

To allocate the channels, an algorithm is designed base on the topology information and interference values at each node. By using smart antenna in WMN in parallel transmission can be occurred along the topology in the network due to spatially separation.

BRTEF DESCRIPTION OF THE FIGURES

Figure 1 shows a wireless communication system impairments.

Figure 2 shows a frequency domain representation of OFDM symbol.

Figure 3 shows a subcarriers permutation scheme.

Figure 4 shows a simple WMN topology construction.

Figure 5 shows a ^a direction of a node from origin.

Figure 6 shows different sub-channels are assigned to nodes in the same direction but different tiers.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The IEEE 802.16 Working Group is the IEEE group for wireless metropolitan area network (MAN). The IEEE 802.16 standard published in April 2002 defines^the wireless MAN air interface. The IEEE 802.16 designed to operate in the 10-66 GHz spectrum and it specifies the physical layer (PHY) and medium access control layer (MAC) of the broadband wireless access (BWA) air interface systems. At 10-66 GHz range, transmission requires LOS. The IEEE 802.16 standard provides the foundation for a wireless MAN industry. However the physical layer is not suitable for lower frequency applications where non- line-of-sight (NLOS) operation is required. For this reason, the IEEE published 802.16a standard to accommodate NLOS requirement in April 2003. The standard operates in licensed and unlicensed frequency between 2 GHz and 1 1 GHz and it is an extension of the IEEE 802.16 standard.

The IEEE 802.16a standard allows users to get broadband connectivity without needing direct LOS with the base station. The IEEE 802.16a specifies three air interface specifications and these options provide vendors with the opportunity to customize their product for different types of deployments. The three physical layer specifications in

802.16a are: a) Wireless MAN-SC: it uses a single carrier modulation format. b) Wireless MAN-OFDM: it use orthogonal frequency division multiplexing (OFDM) with 256 point Fast Fourier Transform (FFT). This modulation is mandatory' for license exempt bands. c) Wireless MAN-OFDMA: it uses orthogonal frequency division multiple access (OFDMA) with a 2048 point FFT. Multiple access is provided by addressing a subset of the multiple carriers to individual receivers.

The IEEE 802.16 WiMAX standard provides mechanism for creating multi-hop mesh, which can be deployed as a high speed wide-area wireless network. IEEE 802.16a standard defined the basic signaling flows and message formats to establish a mesh network connection. Subsequently, the mesh mode specifications were integrated into the TEEE 802.16-2004 revision. Although single hop WiMAX provides high flexibility to attain QoS in terms of data throughput, achieving the same in multi-hop WiMAX mesh is challenging.

The major problem in WMN is dealing with the interference from transmission of the neighboring WiMAX nodes. Cross-layer design and optimization is known to improve the performance of wireless communication and mobile networks. Interference in wireless systems is one of the most significant factors that limit the network capacity and scalability of wireless mesh networks. Consideration of interference conditions during radio resource allocation and route formation processes impacts the design of concurrent transmission schemes with better spectral utilization while limiting the mutual interference.

A newly formed group within 802.16, the Mesh Ad Hoc committee, is investigating ways to improve the coverage of base stations even more. Mesh networking allows data to hop from point to point, circumventing obstacles such as hills. Only a small amount of meshing is required to see a large improvement in the coverage of a single base station. If this group^'s proposal is accepted, they will become Task Force F and develop an 802.16f standard. In comparison to IEEE 802.1 la/b/g based mesh network, the 802.16-based WiMax mesh provides various advantages apart from increased range and higher bandwidth. The time division multiple access (TDMA) based scheduling of channel access in WiMAX-based multi-hop relay system provides fine granularity radio resource control. This TDMA based scheduling mechanism allows centralized slot allocation, which provides overall efficient resource utilization suitable for fixed wireless backhau] network.

However, the interference remains a major issue in multi hop WMN. To provide high spectral usage, an efficient algorithm for slot allocation is needed, so as to maximize the concurrent transmissions of data in the mesh. The level of interference depends upon how the data is routed in the WMN.

The first and most fundamental challenge for broadband wireless comes from the transmission medium itself. In wired communications channels, a physical connection, such as a copper wire or fiber-optic cable, guides the signal from the transmitter to the receiver, but wireless communication systems rely on complex radio wave propagation mechanisms for traversing the intervening space. Several large and small obstructions, terrain undulations, relative motion between the transmitter and the receiver, interference from other signals, noise, and various other complicating factors together weaken, delay, and distort the transmitted signal in an unpredictable and time-varying fashion. It is a challenge to design a digital communication system that performs well under these conditions, especially when the service requirements call for very high data rates and high-speed mobility. The wireless channel for broadband communication introduces several major impairments such as multipath fading, delay-spread and co- channel interference, as shown in Figure 1. Multipath fading is caused by the multiple traveling paths that the transmitted signal can take to arrive at the receiving antenna The signals from these paths add with different phases, resulting in a received signal amplitude and phase that vary with antenna location, direction, and polarization, as well as with some time delay for some movement in the environment. In practice, for a multipath fading channel, synchronization would be very difficult to achieve between different users because very accurate timing synchronization at network level must be achieved, which is in general not easy. In a wireless channel model, three components are considered for a typical variation in the received signal level. The three components are mean path loss, slow fading (or lognormal fading), and fast fading (or Rayleigh fading). This increases the required average received signal power for a given BER.

Due to multipath nature of the channel, a number of multiple transmitted signals are received. Each path has a different length such that the time of arrival (TOA) for each path is different. This effect is called delay spread. When the delay spread exceeds about 10 percent of the symbol duration the significant inter-symbol interference (ISI) can occur. It will limit the maximum data rate of the system. On the other hand, it is clear that the signals with different delays will exhibit different power strengths statistically. To find out the delay spread power distribution, certain models are adapted to interpret the power level for each individual delay spread path. This power profile of each delay spread path is called power delay profile. Co-channel interference, where the main source of interference is coming from adjacent cells. Cellular systems divide the available frequency channels into channel sets, using one channel set per cell, with frequency reuse. This results in co-channel interference, which increases as the number of channel sets decreases (i.e., as the capacity of each cell increases). For a given level of co-channel interference (channel sets), capacity can be increased by shrinking the cell size, but at the cost of additional base stations.

Conventional base stations are using omindirectionai or sectored antennas. This can be regarded as a waste of power as most of it will be radiated in other directions than toward user. In addition, the power radiated in other directions will be experienced as interference by other users. The demand for increased capacity in wireless communication systems has motivated recent research towards the development of algorithms and standards that exploit space selectivity. As the results, there are many efforts on the design of "smart" antenna arrays and the associated beam forming algorithms. The principle reason for applying smart antenna is the possibility for a large increase in system capacity due to control and reduces interferences. This is accomplished through the use of narrow beams at the base station and as a result of the user's separations at the space.

Smart antenna consists of an antenna array, combined with signal processing in both space and time. Beam forming is performed at the radio frequency (RF) level by controlling the amplitudes and phases of the feeding currents through attenuators and phase shifters. Hence, beam forming is sometimes referred to as spatial filtering, since some incoming signals from certain spatial directions are filtered out, while others are amplified. Smart antenna operates its beams in the baseband, thus the feeding currents of the antenna elements are directly proportional to the modulated baseband signals.

Most important feature of a smart antenna system is its capability to cancel co- channel interference. Co-channel interference is caused by radiation from cells that use th^ same set of channel frequencies. Thus, co-channel interference in the transmitting mode is reduced by focusing a directive beam in the direction of the desired user, and nulls in the directions of other interfering users. Delay spread and multipath fading can also be reduced with employing smart antenna system that is capable of forming beams in certain directions and nulls in others, thereby cancelling some of the delayed arrivals. Usually, in the transmitting mode, the antenna focuses energy in the required direction, which helps to reduce multipath reflections and the delay spread.

In the frequency domain, each OFDM symbol is created by mapping the sequence of symbols on the subcarriers. WiMAX has three classes of subcarriers.

a) Data subcarriers are used for carrying data symbols. b) Pilot subcarriers are used for carrying pilot symbols. c) Null subcarriers have no power allocated to them including the DC subcarrier and the guard subcarriers toward the edge. Reference is now made to Figure 2 wherein it shows a typical frequency domain representation of an IEEE 802.16e-2005 OFDM symbol containing the data sufacarriers, pilot subcarriers, and null subcarriers. In order to create the OFDM symbol in the frequency domain, the modulated symbols arc mapped on to the sub-channels that have been allocated for the transmission of the data block. A sub-channel, as defined in the

IEEE 802.16e-2005 standard, is a logical collection of subcarriers. The number and exact distribution of the subcarriers that constitute a sub-channel depend on the subcarrier permutation mode as shown in Figure 3. The number of sub-channels allocated for transmitting a data block depends on various parameters, such as the size of the data block, the modulation format, and the coding rate.

In the time and frequency domains, the contiguous set of sub-channels allocated to a single user or a group of users, in case of multicast is referred to as the data region of the user(s) and is always transmitted using the same burst profile. In this context, a burst profile refers to the combination of the chosen modulation format, code rate, and type of forward error correction (FEC): convolutional codes, turbo codes, and block codes.

It is important to realize that in WiMAX, the subcarriers that constitute a subchannel can either be adjacent to each other or distributed throughout the frequency band, depending on the subcarrier permutation mode. A distributed subcarrier permutation provides better frequency diversity, whereas an adjacent subcarrier distribution is more desirable for beam forming and allows the system to exploit multiuser diversity. The task of topology construction is to connect the mesh nodes. Each node is equipped with a smart antenna system and multiple beams generation is possible. Nodes in a WMN are either gateways or routers. Gateways are equipped with a base station interface and additionally offer connectivity to the wired internet network. Routers ore 5 equipped with base station and subscriber station interfaces which attributed to as relay station (RS). Each RS needs to have a connection to any gateway to be able to establish an internet link. For example, Figure 4 shows a simple WNfN topology construction. In this figure the subscriber station interface of RS B subscribes to gateway A by using a channel determined from the base station interface of gateway A. In similar way. all LO other nodes, C. D and E subscribe to RS B with different channels given from the base station interface of RS B.

When each node is equipped with smart antenna system mobility is then due to be limited. This is because a RS node must be in static mode in order to direct its beams to

L5 other nodes. When a RS node is in mobile mode its beams' direction to other nodes are no longer accurate. However, an exemption is given to end subscriber nodes in WMN. These nodes are the last node that does not support the next hop connectivity. Mobility is possible for these nodes due to the flexibility of beam steering from its base station smart antenna system. Therefore, in our WMN topology model only the end subscriber

>0 nodes can be in either static or mobile mode. Other nodes which are considered as RS worked in static mode where mobility is not available. The task of channel allocation is to assign each communication link a channel on a given topology. In our WMN model, it is sufficient to allocate a channel between two nodes which is determined by base station interface of each node. To allocate the channels, an algorithm is designed base on the topology information and interference values at each node. Every node except the end subscriber nodes in WMN can become a host for other nodes. The host node is simply noted as PARENT node and the node joining the PARENT node is noted as CHILD node. Specifically, a CHILD node sends a REQUEST message to nearest PARENT node that willing to host it. The PARENT node then stores the particular information especially IP address of the CHILD node for the routing purpose. After the connection is completed, the updated configuration files are stored in both PARENT and CHILD nodes. The READY message from the PARENT node acts as a trigger on the CHILD node to set up the routing state. Once the routing state is set up. the CHILD node can start connect to the internet through its PARENT node. At the same time, the CHILD node also starts indicating its willingness to host other nodes in WMN.

One of the advantages of using smart antenna in WMN is parallel transmission can be occurred along the topology in the network due to spatially separation. However, practically the smart antenna cannot provide complete spatial isolation due to the presence of sidelobes beside the main beam. Sometime two or more nodes are located near to each others and the spatial isolations among them become impossible. Therefore, in this channel assignment scheme the second degree of separation is given from frequency domain in order to increase the throughput in WMN. The channel assignment is performed by a PARENT node for its CHILD node when both nodes are joined. Both node interfaces are initially configured on the default channel to enable connectivity. The PARENT node then selects a new channel by using a channel assignment scheme. In our WMN model with smart antenna system, channel selection must be done carefully. Since two (spatial and frequency) separation domains are used, the channel reuse is possible whenever there is full spatially separation. Most of the time, the mesh direction among the nodes is in single direction. In such case, different channels are allocated to the nodes along this mesh topology. And also, some time the nodes in WMN are located close to each other. In this case, much interference is experienced from either the sidelobes or the beams themselves. Hence, the different channels must also be assigned on these nodes.

For example, let assume that a node makes a direction of ^a from origin (^ ) as shown in Figure 5. If there are n number of CHILD nodes in WMN that are closed to

each other with ^a>

a, = [a_i , α₂ , a, , a_n ] < 10°

where i = 1 , 2, 3, ...., n. Then, the different subchannel in OFDM band of WiMAX system shall be assigned to each CHILD node from its PARENT node to avoid interference. On the other hand there is some possibility that nodes from different tiers of mesh are arranged in the same direction as shown in Figure 6. To avoid the interference within these nodes along the same direction, different sub-channels are assigned among them. Each PARENT node assigns the different sub-channels to theirs CHILD node using the channel assignment. Hence, in this case the isolation in frequency domain is more dominant than spatially separation.

However, referring to the main objective of using the integration of two separation (frequency and spatial) domains to increase the throughput in WMN, subchannels reuse algorithm is introduced. Some constraint must be imposed in this algorithm before it takes place. The minimum spatially separation, ^a related to 0

direction must be at least 10 among the nodes. This requirement is made to assure that the interference (sidelobes and co-channel) experienced in each node will be minima.

Assume that there are 36 sub-channels available in the WiMAX system. Since the channel reuse is allowed in the spatially base transmissions, the sub-channel assignment algorithm is designed such that the adjacent sub-channels are not assigned to the nodes that close to each other in order to avoid co-channel interference. Hence, we have come out a scheduling table base on the range of beam forming direction for the sub-channel assignment in WMN as shown in Table 1. In this table, the propagation zone is divided into 36 subzones with each size of 10o. The subzones 1, 2, 3, ...., 36 are defined as in the Table 1. The sub-channel assignment is then arranged in ascending manner (1, 2, 3, ...., 36). These sub-channels are also reused for the next tier of nodes in WMN with a condition that every tier cannot has the same sub-channel. And it is up to totally 36 tiers of hop. Therefore, in this scheduling table, the total nodes that can be accommodated are approximately 1296 (36 x 36) in one WMN system.

Table 1 : Sub-channel assignment in WMN.

Therefore, in the present invention a novel channel allocation scheme using smart antenna system for improving the throughput of WMN. Such a scheme exploits two separation techniques, both spatial and frequency domains to form a dominant co- channel cancellation algorithm. By exploiting the spatial separation offered from smart antenna and the frequency separation offered from multiple sub-channels, ours proposed channel assignment allows more channel reusability which suit achieves higher throughput.

Claims

1. A capacity enhancement in WiMAX mesh network to enhanced WiMAX characterized in that wherein the said WiMAX mesh network is enhanced by using a new channel allocation algorithm and a smart antenna and wherein the said smart antenna consists of an antenna array, combined with signal processing in both space and time and wherein a beam forming is performed at the radio frequency (RF) level by controlling the amplitudes and phases of the feeding currents through attenuators and phase shifters and wherein the smart antenna operates its beams in the baseband, thus the feeding currents of the antenna elements are directly proportional to the modulated baseband signals and wherein the smart antenna is capable to cancel co-channel interference and wherein co-channel interference in the transmitting mode is reduced by focusing a directive beam in the direction of the desired user, and nulls in the directions of other interfering users.

2. A capacity enhancement in WiMAX mesh network to enhanced WiMAX as claimed in Claim 1 wherein each OFDM symbol is created by mapping the sequence of symbols on the sub-carriers.

3. A capacity enhancement in WiMAX mesh network to enhanced WiMAX as claimed in Claim 1 wherein in order to create the OFDM svmbol in the frequency domain, the modulated symbols are mapped on to the sub-channels that have been allocated for the transmission of the data block.

4. A capacity enhancement in WiMAX mesh network to enhanced WiMAX as claimed in Claim 1 wherein the number and exact distribution of the subcarriers that constitute a sub-channel depend on the subcarrier permutation mode.

5. A capacity enhancement in WiMAX mesh network to enhanced WiMAX as claimed in Claim 1 wherein each node is equipped with a smart antenna system and multiple beams generation is possible and wherein nodes in a WMN are either gateways or routers and wherein gateways are equipped with a base station interface and additionally offer connectivity to the wired internet network and wherein routers are equipped with base station and subscriber station interfaces which attributed to as relay station and wherein each relay station needs to have a connection to any gateway to be able to establish an internet link.

6. A capacity enhancement in WiMAX mesh network to enhanced WiMAX as claimed in Claim 1 wherein when a relay station node is in mobile mode its beams' direction to other nodes are no longer accurate.

7. A capacity enhancement in WiMAX mesh network to enhanced WiMAX as claimed in Claim 1 wherein an exemption is given to end subscriber nodes in WMN wherein these nodes are the last node that does not support the next hop connectivity and wherein the mobility is possible for these nodes due to the flexibility of beam steering from its base station smart antenna.

8. A capacity enhancement in WiMΛX mesh network to enhanced WiMAX as claimed in Claim 1 wherein a task of channel allocation is to assign each communication link a channel on a given topology and wherein it is sufficient to allocate a channel between two nodes which is determined by base station interface of each node.

9. A capacity enhancement in WiMAX mesh network to enhanced WiMAX as claimed in Claim 1 wherein to allocate the channels, an algorithm is designed base on the topology information and interference values at each node.

10. A capacity enhancement in WiMAX mesh network to enhanced WiMAX as claimed in Claim 1 wherein using smart antenna in WMN in parallel transmission can be occurred along the topology in the network due to spatially separation.