and cost effective radio range extension that it may allow for airport areas ... coordination with multihop relay base station (MR-BS); otherwise the RS is in.
IEEE 802.16j Multihop Relays for AeroMACS Networks and the Concept of Multihop Gain Behnam Kamali, Mercer University School of Engineering Robert J. Kerczewski, NASA Glenn Research Center
Abstract The potential benefits and challenges of applications of IEEE 802.16j-based relays in AeroMACS networks are discussed at the outset. Perhaps the most important advantage of application of multihop relays in AeroMACS networks is the flexible and cost effective radio range extension that it may allow for airport areas shadowed by large constructions and natural obstacles with virtually no increase in the required network power levels. With respect to PHY layer RSs may be classified as Transparent Relays (TRS) and Non-Transparent Relays (NTRS). While a TRS essentially functions as a repeater and bears no logical connection to the subscriber station (SS), a NTRS operates as a “mini base station (BS)” and is physically and logically connected to the SSs that it serves. Regarding MAC sublayer functionalities, RSs may operate in centralized or distributed modes. Distributed mode means that the RS is capable of scheduling network resources in coordination with multihop relay base station (MR-BS); otherwise the RS is in centralized mode. The RS can be in distributed or centralized mode with respect to security arrangements as well. The NTRS relays may further be divided into two categories; time-division transmit and receive relays (TTR) and simultaneous transmit and receive (STR) relays; both of which are supported by IEEE 802.16j standard. The TTR relay communicates with its subordinate and superordinate nodes using the same radio channel. The employment of relays in an AeroMACS network requires no alteration in the subscriber system. The key concept of “multihop gain” is introduced. Under a reasonable set of assumptions and using a simple analysis, multihop gain is quantified in the form of an equation that provides a raw measure of this gain in Decibel.
Presentation Outline •
Introductory Remarks on Multihop Relays
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Relay Classification
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Transparent versus Non-Transparent Relays
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TTR versus STR Relays
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Multihop Relay for AeroMACS
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Multihop Gain
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Concluding Remarks
Introductory Remarks on Relays •
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The IEEE 802.16j amendment to IEEE 802.16-2009 introduces multihop relay as an optional deployment that enables network performance enhancements in a variety of ways; including but not limited to radio outreach extension and capacity improvement. The amendment expands IEEE 802.16 standard, specifying OFDMA physical layer and medium access control sublayer enhancements to IEEE 802.16 for licensed bands to enable the operation of relay stations while maintaining subscriber station specifications intact. In all broadband cellular RS-augmented standards, the main idea is to complement the base station (BS) with less complex, less costly, and easier-to-install relay stations instead of adding new BSs to the network. The augmented BS, known as multihop relay base station (MR-BS), covers an extended area beyond what the BS alone covers which is called “multihop relay cell” (MR-cell). MR-BS manages all communications resources within a MR-cell through a centralized or distributed procedure.
A simple relay network consisting of two relay stations Traffic and signaling between MR-BS and SS may be routed through “access RSs” or via a direct link between MR-BS and the SS. The physical channel between the MR-BS and a relay is called relay link, and the channel between an access relay and a SS is termed as access link. The protocols on the access link; including those related to mobility, remains the same as the ones in IEEE 802.16-2009, however, new functionalities are specified on relay links to support the multi hop features.
RS Relay Link
Access Link
MR-BS SS
Relay Link
Access Link
RS
Relay Classification • • • •
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Relays may be classified according to their PHY layer and MAC sublayer functionalities. In terms of PHY layer processing, relay stations may be classified as Transparent Relays (TRS); and Non-Transparent Relays (NTRS). A TRS essentially functions as a repeater that is transparent to the SS and bears no logical connection to it. A NTRS operates as a “mini BS” and thus is physically and logically connected to the SSs that are connected to it. The NTRS transmits preamble and broadcasts control messages. With respect to MAC functionalities, RSs can be characterized on the basis of their scheduling arrangements and security capabilities. In these respects the RS may operate in centralized or distributed modes. A TRS always operates in centralized mode with respect to both scheduling and security arrangements. The main function of TRSs is network throughput enhancement.
Relay Classification •
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A NTRS in distributed scheduling and security mode may provide radio outreach extension, higher bandwidth efficiency, as well as throughput enhancement in a WiMAX network. In centralized scheduling mode all information related to the access link (bandwidth request, channel measurement, etc.) is forwarded to the MS-BS for generation of proper DL-MAP and UL-MAP.
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TRS and NTRS have different applications and advantages and disadvantages; therefore one has to evaluate their tradeoff values before a relay mode is adopted.
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Low cost and complexity are important advantages of TRSs. TRSs cannot be implemented in multihop architecture, they can only be used to transmit signal to the SS as the final hop. The TRSs cannot extend the network radio outreach.
Relay Classification •
The NTRS and its superordinate MR-BS may use different OFDM sub-channels, enabling control over network interference level. However, augmenting a wireless network with NTRSs requires a careful planning phase with a meticulous frequency reuse format to lower the overall interference in the network.
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The NTRS is far more complex than the TRS to the extent that it can affect the cost of the network. NTRS requires guard intervals that increases overhead.
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NTRSs have fix allocation for access links and relay links. This is a major drawback for statistical IP network in which bandwidth requirement and channel condition is changing dynamically for each SS. This drawback worsens significantly in multi-hop topology. There is also additional overhead due to retransmission of frame control by the MR-BS to the NTRS
TTR versus STR Relays •
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Depending on assigned radio channels to the relay link and the access link, the NTRS relays may be further divided into two categories; time-division transmit and receive relays (TTR) and simultaneous transmit and receive (STR) relays; both of which are supported by IEEE 802.16j standard. The TTR relay, sometimes termed as “single radio RS” also known as “inband relay”, communicates with its subordinate and superordinate nodes using the same radio channel. The STR, also known as “dual radio RS” and “outband relay”, communicates with its subordinate and superordinate nodes using different radio channels, i.e., different OFDM subchannels. The STR relays are capable of simultaneous signal transmission and signal reception, i.e., transmitting signal to MR-BS while listening to a SS on the UL, and transmitting to a subscriber station at the same time that they receive signal form MR-BS on the DL.
Multihop Relay for AeroMACS 1. When a portion of an airport is significantly shadowed by a new obstacle, a TRS or NTRS can be added to the airport network to provide higher capacity and acceptable QoS to the shadowed area. Adding a relay to an already established network does not require network reconfiguration and radio resource reallocation. 2. When a heavy load of traffic is expected in a certain location, or when an incident has occurred that requires a wireless link over a limited period of time, RSs may be deployed temporarily to provide coverage or additional capacity. 3. If a station is outside of the airport area but needs connection to the AeroMACS network, an RS (as opposed to a BS) can be used to establish the connection. For example, a wake vortex detection sensor may need to be located a mile or more from the end of a runway, which may be well outside the airport boundary. 4. Coverage to single point assets on the airport surface that are outside of the BSs coverage area can readily be rendered by an RS. This may be particularly suitable for airport security equipment such as cameras.
Multihop Gain •
The application of multihop relay configuration enables a reduction in path loss, and therefore a link budget “gain” is resulted. We designate this gain as “multihop gain”. The multihop gain can then be translated into one or more of the following system enhancements for AeroMACS:
1. Radio outreach extension 2. Improvement in throughput and network capacity 3. Reduction in transmit power
Item 3 is one of the primary concerns in AeroMACS application and deployment regarding the issue of interference into co-allocated applications.
Multihop Gain The application of multihop relay enables a reduction in path loss, and therefore a link budget “gain” is resulted. We designate this gain as “multihop gain”. Under the following assumptions, a simple analysis can provide a raw measure for the multihop gain measured in Decibel. • • • • •
RS and SS receivers have the same sensitivity shown by SP dB. Propagation path loss between the MR-BS and RS is represented by LBR dB Propagation path loss between the RS and SS is represented by LRS dB Direct propagation path loss between the MR-BS and SS is represented by LBSS dB Under these conditions it can be shown that the multihop gain; in dB, can be calculated from the following equation.
GMH
>
LBSS 10 log 10
LBR 10
10
LRS 10
@
Multihop Gain •
This equation is intuitively satisfying and demonstrates that multihop gain depends on the propagation path loss between various stations in the network (which in turn depends on positioning of the relay stations and the terrain), in other words it varies from one propagation environment to the other. The final conclusion is that the multihop gain is directly affected by the following factors in the system:
1. Relay stations positioning in the network 2. Propagation characteristics of the terrain through which signal travels 3. Transmit power setting and distribution Pathloss graphs can be plotted for various propagation loss models for airport surface case. One of the variables that can be taken as a parameter, is the factor D shown in Figure 3.
Multihop Gain
d1
d2
MR-BS
SS
d0
d 0 d d1 d 2 o d 0 D d1 d 2 Figure3. Multihop Gain Scenario
D d1
Concluding Remarks Regarding the generated multihop gain the following important conclusions can be drawn. •
It is possible to achieve either of the following gains without increasing the total transmit power: – Extend the radio coverage range for a required received signal strength (RSS) – Enhance the RSS at a particular point in the network
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2. Reduction in transmit power can be realized while maintaining the same RSS. This in turn reduces the interference to co-allocated applications. For AeroMACS this is highly desirable, since interference to Mobile Satellite System (MSS) that shares the same spectrum will be reduced.
