Draft 802. 20 Permanent Document




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IEEE P 802.20™/PD/V

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Draft 802.20 Permanent Document


<802.20 Evaluation Criteria – Ver 01>

This document is a Draft Permanent Document of IEEE Working Group 802.20. Permanent Documents (PD) are used in facilitating the work of the WG and contain information that provides guidance for the development of 802.20 standards. This document is work in progress and is subject to change.


Contents


1 Overview 3

2 Link level and System Level Analysis 3

3 Link level Modeling 4

4 System Level Modeling 5

5 Channel Modeling 9

6 Equipment Characteristics 9

7 Output Metrics 10

8 Payload Based Evaluation 14

9 Fairness Criteria 15

10 Appendix A: Definition of terms 16

11 References 16





<802.20 Evaluation Criteria>

1Overview

1.1Scope


. [Alan Chickinsky, alan.chickinsky@ngc.com], [Joanne Wilson, joanne@arraycomm.com, Todd Chauvin, Chauvin@arraycomm.com and Larry Alder, alder@arraycomm.com].

This document describes the evaluation criteria used by the IEEE 802.20 working group to evaluate different candidate air interface proposals for the alternatives for inclusion into IEEE 802.20 standard. This document and the IEEE 802.20 requirements document form the basis for decisions.

Although the IEEE 802.20 standard defines operations at the Link and Physical layer of the ISO Model, many of the criteria in this document extend to other ISO layers. The evaluation criteria based on other ISO layers are for information use only. Informational areas of this document are used when other methods are insufficient to determine an alternative.

1.2Purpose


. [Joanne Wilson, joanne@arraycomm.com, Todd Chauvin, Chauvin@arraycomm.com and Larry Alder, alder@arraycomm.com].

This document presents the criteria used for the evaluation of air interface (i.e. combined MAC/PHY) proposals for the future 802.20 standard. As such, the evaluation criteria emphasize the MAC/PHY dependent IP performance of an 802.20 system. While other system aspects, e.g. TCP/IP and the backhaul network, will affect system level performance, these criteria are intended for the evaluation of the MAC/PHY aspects of the system only.

An “802.20 system” constitutes an 802.20 MAC/PHY airlink and the interfaces to external networks for the purpose of transporting broadband IP services.

1.3Organization of the Document


[Alan Chickinsky, alan.chickinsky@ngc.com]

2Link level and System Level Analysis


[Joanne Wilson, joanne@arraycomm.com, Todd Chauvin, Chauvin@arraycomm.com and Larry Alder, alder@arraycomm.com].
A great deal can be learned about an air interface by analyzing its airlink to a single user. For example, a link-level analysis can reveal the system’s noise-limited range, peak data rate, maximum throughput, and the maximum number of active users. Extension of the link-level analysis to a multi-user single-cell setting is generally straightforward and provides a mechanism for initial understanding of the multiple-access (MAC) characteristics of the system. Ultimately, however, quantifying the network-level performance of a system, i.e. system level performance, although difficult, carries with it the reward of producing results that are more indicative of the viability of the system and its expected worth to a service provider.

Since system level results vary considerably with the propagation environment, the number and spatial distribution of users loading the network, and many other fixed and stochastic factors, the assumptions and parameters used must be reported carefully lest the quoted network-level performance be misleading.

Given the charter of 802.20 as a mobile broadband wide area system, it is important to understand the system’s performance in a network setting where multiple base stations serve a large mobile customer base. In a macro-cellular deployment as required by the PAR, multiple basestations are required to cover a geographic region. In practice, cell radii may range from 0.5 km to 15 km. The proposed systems must cope with the considerable effects of intra-cell and inter-cell interference that arise in network deployments.

Ultimately, the system level performance is the key metric that will drive much of the system level economics. For example, while the per-user peak data rate is an important service metric, a more important one is the achievable service level as a function of the network loading. While link-level performance quantifies what is possible, system level performance quantifies what is likely.


3Link level Modeling


[Joanne Wilson, joanne@arraycomm.com, Todd Chauvin, Chauvin@arraycomm.com and Larry Alder, alder@arraycomm.com].

Single user link-level analysis is an analysis of the performance of a single user terminal (UT) in an assumed propagation environment. This is an important metric for understanding the air interface and yields important information about the system including:

  • the effectiveness of link-adaptation and power control,

  • the noise-limited range,

  • the SNR requirements to support various classes of service,

  • the tolerance to multipath and fading, and so on.

However, it should be clear that relying solely on link-level performance can lead the working group to drawing erroneous conclusions. Due to variability in the propagation environment and inter-cell interference, single-user link-level analysis cannot be directly extrapolated to network-level performance.

3.1Modeling assumptions


.

3.2Performance metrics


b/N0. AC Proposal: FER vs SINR is the product of link-level simulation in AWGN channel. Systems with adapative modulation should produce a set of curve (one curve per modclass). A second family of curves is the link-level throughput vs. SINR. This is derived by combining the FER from the first curve with the number of bits/symbol for each of the modulation classes at a fixed FER of 1 percent.>

4System Level Modeling

4.1Cell layout


[Joanne Wilson, joanne@arraycomm.com, Todd Chauvin, Chauvin@arraycomm.com and Larry Alder, alder@arraycomm.com].

Hexagonal tessellation of cell sites shall be used.

To faithfully model inter-cell interference, we suggest that statistics be gathered only for cells that are interior to the network. Two possible scenarios are:



  • Two tier: 19 basestations, statistics collected only from the interior cell

  • Three tier: 37 basestations, statistics collected only from the interior 7 cells

This simple guideline protects the statistics from bias due to unrealistic performance around the edges of the network where inter-cell interference is artificially small due to the finite number of cells.
Distribution of users

Most users of wireless systems experience very good link-quality near the basestation. For this reason, the distribution of users throughout the network is integral to the quoting of network-level performance results. Absent the desire to highlight specific abilities of an air interface, users should be distributed uniformly throughout each cell of the network.


User usage model

The following user terminal usage parameters must be specified:



  • distribution of indoor vs. outdoor users

  • mobility profile across the user base


4.2Antenna Pattern and Orientation -->

4.3Propagation Model




4.4Fading Models

4.4.1Slow Fading Model



4.4.2Fast Fading Model



4.5Traffic Modeling

4.5.1Traffic Mix




4.5.2Traffic Models




4.6Higher Layer Protocol Modeling



4.6.1HTTP Model

4.6.2TCP Model


[Farooq Khan, fkhan1@lucent.com]

Many Internet applications including Web browsing and FTP use TCP as the transport protocol. Therefore, a TCP model is introduced to more accurately represent the distribution of TCP packets from these applications.


4.6.2.1TCP Connection Set-up and Release Procedure


The TCP connection set-up and release protocols use a three-way handshake mechanism as described in Figure 1 and Figure 2. The connection set-up process is described below:

  1. The transmitter sends a 40-byte SYNC control segment and wait for ACK from remote server.

  2. The receiver, after receiving the SYNC packet, sends a 40-byte SYNC/ACK control segment.

  3. The transmitter, after receiving the SYNC/ACK control segment starts TCP in slow-start mode (the ACK flag is set in the first TCP segment).

The procedure for releasing a TCP connection is as follows:



  1. The transmitter sets the FIN flag in the last TCP segment sent.

  2. The receiver, after receiving the last TCP segment with FIN flag set, sends a 40-byte FIN/ACK control segment.

  3. The transmitter, after receiving the FIN/ACK segment, terminates the TCP session.


Figure 1: TCP connection establishment and release for Uplink data transfer


Figure 2: TCP connection establishment and release for Downlink data transfer


4.6.2.2TCP slow start Model


The amount of outstanding data that can be sent without receiving an acknowledgement (ACK) is determined by the minimum of the congestion window size of the transmitter and the receiver window size. After the connection establishment is completed, the transfer of data starts in slow-start mode with an initial congestion window size of 1 segment. The congestion window increases by one segment for each ACK packet received by the sender regardless of whether the packet is correctly received or not, and regardless of whether the packet is out of order or not. This results in exponential growth of the congestion window.

4.6.2.3TCP Flow control Model



4.7Backhaul Network Modeling

4.7.1Network Delay models



4.7.2Network Loss models



4.8Mobility Modeling



4.9Control signaling modeling

4.9.1DL signaling models



4.9.2UL signaling models



5Channel Modeling

5.1Channel Mix




5.2Channel Models



6Equipment Characteristics


[Joanne Wilson, joanne@arraycomm.com, Todd Chauvin, Chauvin@arraycomm.com and Larry Alder, alder@arraycomm.com].

6.1Antenna Characteristics



6.2Hardware Characteristics


The assumed hardware parameters of both the basestation and the user terminals are necessary to interpret the quoted results. For example, differences in specification (both BS and UT) significantly affect performance results:

  • maximum output power

  • noise figures

  • antenna gain, pattern, and height

  • cable loss (if applicable).



6.3Deployment Characteristics

Relevant system-level parameters used for an 802.20 deployment include:



  • number of carriers

  • total spectral bandwidth

  • system frequency allocation

  • sectorization (if applicable)


7Output Metrics


Two good criteria for evaluating the network-level performance of an MBWA system are its ability to cover the worst served users and the aggregate throughput that can be delivered within the cell. In this section, we propose statistics for quantifying these aspects of network-level performance.



7.1System Capacity Metrics


This section presents several metrics for evaluating system capacity. Specifically, respondents are required to provide:

  • User data rate CDF for specified load and basestation separation (Section 7.1.1: Fixed load/coverage operating point: Service Distribution)

  • Plot of aggregate throughput vs. basestation separation for stated minimum service levels. (Section 7.1.2: Aggregate Throughput)

  • Plot of number of active users per cell vs. basestation separation for stated minimum service levels (Section 7.1.3: Network performance under Varying Load/Coverage)

  • Spectral Efficiency for stated load coverage operating points (Section 7.1.4: Computing Sustained Spectral Efficiency)

Since the nature of IP traffic and applications will evolve over time and is uncertain, respondents are required to provide this evaluation using the traffic agnostic infinite queue model. Here all active users are assumed to want to transmit and receive and infinite amount of data. The network is assumed to supply and consume an infinite amount of data and there is no assumption on data retransmission or throttling by the network. The results presented for the uplink and downlink capacity should be achievable simultaneously by the system. If the results for uplink and downlink cannot be achieved simultaneously by the system, the respondent should indicate so.

7.1.1Fixed load/coverage operating point: Service Distribution


Let the load/coverage point be fixed at , where (by definition) the number of active users per cell1 (), and the (common) inter-basestation separation () for a hexagonal tessellation of cells is specified. This operating point implies a distributionof data rates for each user that the system is able to deliver within the cell area. We propose that the distribution be sampled separately in uplink and downlink directions (Monte-Carlo simulation) with statistics gathered only from the interior cells of the network.

Figure 3 shows a qualitative example of a cumulative distribution function (CDF) of the distribution of downlink data rates in the interior cells of a network for a specified load/coverage operating point . This graph shows the distribution of data rates on the ensemble of random placements of active users in each cell of the network and all other stochastic input parameters. The CDF is not complete without specification of the assumed probability distribution of user placement.




Figure 3: Service Distribution for a fixed load/coverage operating point







7.1.1.1Minimum Service Level


From a service integrity standpoint, the lower tail of the resulting service CDF contains important information. Continuing the example of Figure 3, 90% of the active users will be served with a minimum service level of 566 kbits/sec at the load/coverage operating point. The notation emphasizes that the minimum service level is a function of the load/coverage operating point.

7.1.2Aggregate Throughput

For each placement of users, the aggregate throughput is the sum of the data rates delivered to the active users in a cell. The per-user data rate is computed by dividing the total number of information bits received by the time-duration of the simulation. The respondent should provide a graph of the aggregate throughput vs. basestation separation for constant minimum service levels (See Section: 7.1.3) . This graph would be of the same for as Figure 4 with the vertical axis being aggregate through put instead of number of users.



7.1.3Network performance under Varying Load/Coverage


The CDF of Figure 3 characterizes the ability of the system to serve active users at a fixed load/coverage operating point. Studying the behavior of the system with varying network load gives additional insight. One interesting approach is to compute the minimum service level on a grid of points in the load-coverage plane. Sample contours of constant minimum service level are shown in Figure 2. This example (synthetically produced for illustrative purposes), reveals the tradeoff between the basestation separation () and the number of active users per cell ().
For example, to guarantee an expected minimum service rate of, say, 1024 kbits/sec across 90% of the cell area, few active users (less than 5) can be supported per cell at the noise-limited inter-basestation separation of 6 km. Conversely, many active users per cell (more than 20) can be supported in the interference-limited case when the basestations are closely spaced.


Figure 4: Contours of constant minimum service level




7.1.4Computing Sustained Spectral Efficiency


In the present setting, the sustained spectral efficiency () can be computed in a meaningful and straightforward manner. A moment’s reflection will reveal that rather than being a single number, spectral efficiency is a family of numbers parameterized by the load/coverage operating point (Section7.1.1) and the assumed minimum service level.
For a specified operating point and a minimum service level, the expected aggregate throughput () is defined as the expected sum of the data rates delivered to the active users in the cell. For example, in the downlink direction, the expected aggregate throughput (per-cell) is defined

where is the downlink rate to the user and is the statistical expectation. A similarly defined statistic applies in the uplink direction. The total expected aggregate throughput is the sum of uplink and downlink: .


The sustained (total) spectral efficiency is computed

/cell

where is the total system bandwidth. Similarly, the spectral efficiency is computed in the uplink direction as



/cell

where is the (effective) bandwidth reserved for uplink traffic. The spectral efficiency in the downlink direction is similarly defined.




8Payload Based Evaluation


[Marianna Goldhammer, marianna.goldhammer@alvarion.com]

The payload-based evaluation method for MAC-Modem-Coding capacity and delay performance assessment is described below.


8.1Capacity performance evaluation criteria


In order to evaluate the different proposals capacity performance, it is useful to define evaluation scenarios. The evaluation parameters are:

  • Channel spacing: 1.25MHz and 5MHz

  • Modem rate (max rate & minimum coding, medium rate & medium coding, minimum rate & maximum coding);

  • MAC frame duration: 5ms

For capacity evaluation, the payloads associated with every type service are:

  • 30 bytes for VoIP, G.729 codec, 30ms period

  • 1518 bytes for long IP packets;

  • 64 bytes for short IPv4 packets;

  • 40 bytes for video-conference, 64kb/s (64kb/s*5ms/8 =40bytes)

  • 240 bytes for video-conference, 384kb/s

  • T.B.C. bytes for multi-media streaming.

The computation shall take into account the influence of the MAC overheads, MAC granularity, interleaver, coding block, etc.

In order to simplify the procedure, only one type of traffic is assumed for all the Base Station subscribers. For every type of traffic shall be calculated the subscriber number, separately for up-link and down-link


8.2Payload transmission delay evaluation criteria


The delay is an important factor for real-time services.

The payload transmission delay shall be evaluated according to the same procedure and parameters, as specified for capacity evaluation. The computation shall take into account the influence of the MAC granularity, interleaver, coding block, etc.

The delay will be calculated between the moment in which the payload enters the MAC and the moment in which the payload exits the MAC, on the other side of the wireless link. The processing power of the implied devices will not be taken into account.

The calculation shall be done separately for up-link and down-link, assuming the number of subscribers resulted from capacity calculation.


9Fairness Criteria




10Appendix A: Definition of terms




10.1Number of Active Users Per Cell


For the purposes of this analysis, an active user is a terminal that is registered with a cell and is seeking to use air link resources to receive and/or transmit data within the simulation interval. Evaluating service quality as a function of the well-defined concept of the number of active users per cell is a natural way of comparing how well disparate MBWA systems behave under increasing network load.

10.2Inter-basestation separation


For the purposes of defining network load, it is natural to treat inter-basestation distance as a parameter. Closely spaced deployments will stress the interference-limited performance of the network while widely spaced deployments will stress the range-limited performance. In any case, users of an 802.20 system will likely experience different link quality at locations throughout the cell that depend both on the distance from the basestation and the inter-basestation separation. Thus, we include inter-basestation separation in our definition of the load/coverage operating point.

11References


  1. IEEE C802.20-03/32, Selected Topics on Mobile System Requirements and Evaluation Criteria.

  2. IEEE C802.20-03/33r1, Criteria for Network Capacity.

  3. IEEE C802.20-03/35, Evaluation Methodology for 802.20 MBWA.

  4. IEEE C802.20-03/43, 802.20 Evaluation Methodology Strawman - 00.




1 See Section 10.1 for definition of active users



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