Price-Performance Comparison: 3G and Tropos MetroMesh

Price-Performance Comparison:3G and Tropos MetroMesh Architecture

A Technology WhitepaperJuly, 2007

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Price-Performance: 3G and Tropos MetroMesh

Metro-Scale Mesh Networking DefinedTM

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A Technology Whitepaper

Executive Summary Multi-megabit symmetric broadband enables new productivity applications, such as the transmission of email attachments, high-resolution images, and full-motion video. Based on an overlay of existing 2G voice cell sites that are ubiquitously deployed by wireless carriers at roughly 1 per square mile, 3G technology enables them to offer broadband IP data service for the fi rst time. Although 3G radios are capable of supporting speeds greater than 1Mbps, today’s end-user experience for these 3G overlays is the range of hundreds of kilobits for download, with upload speeds comparable to dial-up (56 Kbps). 3G systems fall short of their maximum performance since the low-density cellular network is incapable of delivering adequate RF signal power at either end of the link. This is due to the physical attributes of path loss (environment limited) and/or limits on transmission power imposed by interference between adjacent cells (capacity limited). Higher speeds inevitably require more RF power delivered to the radio receiver, or more cell density. Dramatic increases in maximum transmission power (EIRP) being unlikely, the delivery of more power to the receiver necessitates an increase in cell site densities beyond that which is deployed today. The cost and complexity of existing 3G base stations, including high equipment capital cost, expensive site preparation, and the need to backhaul each cell with Telco links makes increased 3G cell densities problematic.

Tropos technology enables very cost-effective deployment and operation of cells at densities needed to support multi-megabit broadband (exceeding 20 per square mile). Cells are low cost and low power consumption, easy to install and do not require wired backhaul. A mesh of Tropos MetroMesh routers has been demonstrated to offer symmetric multi-megabit speeds to low cost, open-standard Wi-Fi clients outdoors across entire cities. Multi-megabit coverage can be extended indoors with inexpensive mass-market Wi-Fi bridges. In comparison to 3G built as an overlay to the cellular voice network, the Tropos metro-scale Wi-Fi solution delivers 5x the performance at ¼ the price. With forthcoming advances in open-standard radio and Tropos network architecture, the performance of the Tropos cellular mesh is expected to rise to 40x the performance of 3G in the coming years.

Executive Summary ...........................................................................................2Start with the end-user in mind .............................................................................2End-user speed is a function of received power and bandwidth ......................................4The physical constraints imposed by the 3G radio link .................................................53G broadband requires more received power than voice ...............................................53G broadband has much lower link budget compared to voice ........................................6Therefore, 3G broadband requires greater cell density ................................................8MetroMesh : 4x less cost and 5x faster than 3G ........................................................ 10The future of wireless broadband ........................................................................ 10For further reading .......................................................................................... 11

Start with the end-user in mindBy leveraging the tremendous industry momentum behind Wi-Fi, an open-standard supporting multi-megabit speeds, Tropos Networks today delivers multi-megabit broadband across metro-areas to laptops, PDAs and a wide range of client devices that already have



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Wi-Fi built-in. As open-standard radio evolves to faster speeds and lower costs, benefi t versus 3G technology can only widen. End-user speeds on a Tropos metro-area network today are 500kbps-2Mbps (symmetri-

cal), representing over a 5x performance improvement over typical 3G solutions that cost over 4x more to deploy. In the near future, speeds beyond 4Mbps will be common.

With the introduction of cellular-mesh technology to support dense-cells, the Tropos solution dramatically reduces the total cost of ownership (TCO) of a city-wide cellular network making it economical to deploy and operate a dense-cellular network required for delivering symmetrical multi-megabit broadband.

By supporting access to open-standard Wi-Fi devices today, Tropos connects mobile client devices with $0 cost and CPE devices selling for $50, without requiring any special client software.

Observed roundtrip latencies are in the range of 10-50ms, making VoIP practical. Tropos supports any applications that run on wired networks including video. Beginning with a throughput advantage of 5x over 3G today, the Tropos speed advan-

tage is expected to climb steadily to 10-20x by 2005 and 20-40x by 2006-2008 en-abling a whole new set of applications.

This paper explores the tradeoffs and limitations inherent to 3G data communications, by analyzing the performance of 1xEV-DO, a representative 3G technology. We will then compare this to what is achievable with Wi-Fi (802.11b/g) using the Tropos cellular mesh. The analysis of this paper can be generalized to other 3G technologies, such as the W-CDMA, and the main conclusions do not differ since the performance of 1xEV-DO and W-CDMA are within 10-20% of each other (see “Comparison of services with 1xEV-DO, W-CDMA and cdma2000” in [AGILENT]). Despite 3G radio support for multi-megabit rates: Today’s 3G deployments across the world are proving that 3G is capable of consistent-

ly delivering 150-300kbps on the download and even dropping down to the range of dial-up speeds (56kbps) at times, although speeds of 500-700kbps are observed much more infrequently. This makes 3G unsuitable for video transmission and large fi le transfers.

Upload is rarely better than dial-up speed, and hence the service is asymmetric, making 3G unable to support both enterprise applications involving email attachments, Micro-soft Outlook, VPNs, as well as recreational applications such as interactive-gaming, and music.

leading wireless carrier tests their 1xEV-DO network outdoors on the street to 80kbps upstream and 400kbps downstream, with much more variability expected in-doors.

Proprietary end-user client hardware and software ($200-$400) needs to be installed in order to access 3G data networks.



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Roundtrip latencies of several hundreds of milliseconds are observed in these net-works. Not only does this create an end-user perception of even slower than actual speeds for fi le downloads, it limits the use of time-sensitive applications involving VoIP (voice) and video.

1xEV-DO Wi-Fi

Standards

1xEV-DO is marketing-speak for IS-856, a continuation of the CDMA standard IS-95 and 1xRTT that are used on today’s CDMA-based cell phones. It is also known/associated-with the cdma2000 standards.

Wi-Fi is marketing-speak for IEEE 802.11b. Versions of 802.11 in different bands are denoted with a different letter extension: 802.11b is the most popular, but others include 802.11g, and 802.11a.

Frequency

A single 1xEV-DO data channel has a bandwidth of 1.25MHz deployed on one of many such channels originally allocated for voice in the 1900MHz PCS band licensed to cellular operators. Some operators may have licenses in 800MHz spectrum.

802.11b has three 22 MHz wide channels at 2.4GHz at the unlicensed ISM band.

Maximum Transmit Power

Maximum 55dBm EIRP at base station, and typically much less due to power controlTypically 20dBm EIRP at client

In United States, 36dBm EIRP at base station typically 15-20dBm EIRP at client. Lower in some countries.

Table 1: Wireless Standards

End-user speed is a function of received power and bandwidthThe apparent complexity of cellular communication systems often disguises the fundamental fact that the maximum speed (throughput) that may be delivered to and from end-user client devices depends on just a few factors, namely the bandwidth of the medium and the power (energy) present in the RF waves that is ultimately received by the receiver (i.e., received power).

In analogy to music played on a piano, the bandwidth is similar to the range of piano keys used to play the song (lowest to highest). The transmitted power is analogous to how loud the pianist plays. The more keys used, the richer the musical experience can be. The listener (receiver) can enjoy the song if the music is loud enough to hear (enough power received). If the listener is too far away, then the song will be hard to make out or may not even be heard at all. In that case, the pianist must either play louder (more transmit power), or the listener (receiver) must be moved closer to the pianist such that the received power is above the minimum required by the listener.

Figure 1: The Tradeoff of Bandwidth and Power



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The Capacity of Communication Channels: Shannon’s Law

C = B_log2(1+S/N)C = maximum number of bits that can be transmitted per second B = the channel bandwidth (Hz)S = the power of the received by the receiverN = the power of the present at the receiver, a function of B

Equation 1: Shannon’s Law (1948)

Essentially the same relations hold true in wireless communications, where the maximum speed with which data can be conveyed is dependent on the bandwidth and the signal power. The exact relation (Equation 1) which was discovered in 1948 by Claude Shannon (see [SHANNON]), forms the basis of modern communications. In practice, communication systems achieve only a fraction of the maximum channel capacity due to constraints imposed by the state-of-the-art in signal processing technology.

The physical constraints imposed by the 3G radio linkThird generation (3G) technologies are unlikely to ever stretch end-user data speeds beyond hundreds of kilobits. Why? With the aim of leveraging existing infrastructure investments, 3G technologies use the same narrowband RF channels and roughly comparable cell density as existing 2G networks. Second generation (2G) networks were designed to support voice communications requiring less than 10kbps delivered to and from the end-user handset. As Shannon’s Law would suggest, in order to scale end-user speeds up to 1Mbps (or higher), RF bandwidth and/or received power for 3G radios must increase by two to three orders of magnitude. Not only are licensed cellular (PCS, etc.) channels expensive and already deployed for voice service, allocating the many more licensed channels needed in theory to support greater data rates for a single end-user is not even an option envisioned by 3G technology standards for these reasons. Therefore, the only practical option available to cellular carriers deploying 3G is to increase received signal power at the handset and base station. As we will show in the sections below, increasing received signal power would require much greater cell density than traditional cellular architectures can economically support due to the physics of wireless signal propagation.

3G broadband requires more received power than voiceExamining Shannon’s equation, notwithstanding a dramatic increase in bandwidth (B) utilized by both infrastructure and client radios, the only way to increase speed is to increase the received signal power (S). In fact, this age old technique is precisely how 3G cellular radios have been adapted to achieve higher data rates while also using the same RF channel that is currently used for voice (<10kbps). Based on the signal power required at supported data rates for 1xEV-DO radios (see Tables 3-5, 3-9, 5-2, 5-4 of [QUALCOMM])



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and Tropos’ Wi-Fi radios, Figure 2 below plots the power required (y-axis) to communicate at various data rates (x-axis), both on log scales. A similar plot of required signal to noise ratio (SNR) for 1xEV-DO is given in Figure 3 of [AGILENT]. Compared to the power required for voice speeds, 1xEV-DO download (forward link) at 1Mbps requires roughly 100x more received power and 2.4Mbps requires roughly 1000x more received power. 1xEV-DO does not support upload speeds greater than 153 kbps, although future revisions of the standard will, with commensurately higher power requirements.

3G broadband has much lower link budget compared to voiceAn overwhelmingly large fraction of the RF power that leaves the transmitter is absorbed, scattered, refl ected, diffracted, and ultimately lost to the environment. What matters is the remaining fraction that reaches the receiver, since that is what becomes useful for communication. Hence, the further the signal must pass through the environment in order to reach the receiver, the lower the speed that may actually be supported. Conversely, greater speeds necessitate incurring fewer environmental path losses.

How much environmental path loss may be tolerated? The maximum tolerable path loss or link budget is given by the difference between the transmit power (EIRP) of the transmitter and the power required at the receiver, and represents how much power may be lost due to signal propagation in the environment. Taking into account the maximum transmit power for both the base station and client device, the maximum tolerable path loss is plotted in Figure 3 (see Tables 3-5, 3-9, 5-2, 5-4 of [QUALCOMM]). The decrease in tolerable path loss as data rate increases is a direct consequence of the increased power required to support higher data rates. Actual signal path loss is the identical in both directions. The asymmetry in tolerable path loss, which is greater than a factor of 10 between forward and reverse links for 1xEV-DO, is the cause of highly asymmetric download and upload speeds.

To understand how this link budget may be used up, we consider the various aspects of environmental signal propagation. In free space, without any obstacles, the signal level decreases as an inverse square law as distance increases from the transmitter. However, in

Figure 2: Minimum Received Power versus Speed

Figure 3: Maximum Tolerable Path Loss



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real environments the (average) signal level drops much more rapidly. This rapid drop off is modeled by the widely used Hata model (see [CDMA HANDBOOK]) where signal level drops off as a higher power of distance, known as the path loss exponent. It is typically denoted by n and ranges between 3.5 and 4.0. Both free space and Hata propagation are illustrated in the following diagram in the case of a base station transmitting to a handset (receiver). The reversed scenario of the handset to the base station is similar. The point at which the signal falls below the minimum signal power for the required data rate (speed) is known an outage as shown in Figure 4, to indicate that the service is no longer available.

The Hata propagation depicted in Figure 4 would suggest that there is a sharp cutoff distance where the outage occurs, before which communication is feasible and after which communication is no longer possible.

However, reality is more complex and RF waves may be refl ected, scattered, attenuated or blocked such that the receiver receives a number of signals with varying power levels from multiple directions. The resultant signal is the sum total of all signals received. Therefore, in addition to Hata propagation, another phenomenon, known as fading (see [CDMA HANDBOOK]), causes the signal value to appear as if it is fl uctuating randomly. The effect of fading, illustrated in Figure 5, is to potentially induce an outage to occur well before the distance predicted by the Hata model alone.

The most widely used model of fading is the Rayleigh model, which considers the received signal as the sum total of a very large number of individual signals with varying amplitudes. Under the Rayleigh fading model, there is a probability (likelihood) of outage occurring well before the maximum distance predicted by Hata-propagation, as illustrated by the Figure 6. A low probability outage (< 5%) is required for an acceptable level of service.

Figure 4: RF Propagation and Path Loss

Figure 5: RF Propagation and Fading



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Therefore, 3G broadband requires greater cell densityWhen compared with voice, the Figures 2 and 3 show that much less environmental path loss of signal power can be tolerated when communicating at broadband speeds. The ubiquitous cellular experience we are conditioned to expect from cellular voice is delivered by a signal capable of supporting less than 10kbps with an acceptable probability of outage. How can such a 2G cellular network be adapted to deliver a broadband cellular experience, when the signal power required is two to three orders of magnitude greater? Of the three ways listed below, increasing cell density is the only practical option.

1. Transmit power: Today’s cellular voice networks can operate at a maximum of roughly 200-400 Watts EIRP transmitted from the base station, and much less from the handset. This places upper limits on the propagation path loss. However, in urban areas cells are usually operated at much lower power to reduce cell size and maximize their capacity. In the case where cell size is reduced for expanded capacity, increasing the power would simply cause unwanted interference between cells.

2. Channel Bandwidth: One licensed channel is typically allocated to all data users within a cell. Allocation of additional channels by the operator would only add capacity for more users, not increase end-user speeds since the radios are not designed to use more than one channel per user. Even if radios could hypothetically be redesigned to exploit more channels by taking away channels allocated to voice services (each operator has a limited number), such a channel-hungry design would consume expensive licensed channels and take away from aggregate user capacity while only increasing speeds proportional to the number of channels consumed at best.

3. Cell Density: More cells offer more options (links) with which to connect to the end-user, which in turn implies greater speeds to the end-user. The only practical way to increase received signal level is by increasing the density of cell sites within the coverage region. This works to reduce the average loss through the environment that the signal incurs before reaching the handset (or base station), hence ensuring an overall increase in received signal strength and aggregate capacity. However, increased cell density in a 3G deployment requires expensive base station equipment, sites and backhaul to the Internet for each cell.

Figure 6: RF Propagation and Outage Probability



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How many more cells will be necessary to deliver megabit broadband starting with 3G upgrades to 2G cellular networks? To quantitatively address this question, we start with the fact that cellular operators have had thus far deploying 1xEV-DO in downtown areas where the cell density is already high compared to most other areas. As a result, overlays of 1xEV-DO in such downtown cellular deployments could be expected to deliver the maximum speed across all environments. Representative experience from 1xEV-DO deployments in Seoul and San Diego show that download speeds can at times reach up to 500-700kbps, but they can only reliably deliver 300kbps and 150kbps, respectively, meaning that they reach an acceptably low outage probability at these speeds. Upload speeds do not reliably reach much more than dial up in either case.

Based on the required signal power as shown in Figure 2, we can use the relationship between cell density and RF link budget (Equation 2) to calculate the increase in cell density needed to maintain a comparable outage probability for higher data rates. By employing an approach similar to the classic analysis of antenna diversity under Rayleigh fading channel (see [CDMA HANDBOOK]), it may be shown that this relation must hold true to maintain a constant probability of outage under fading. Without taking fading into account, this equation may be derived more simply from calculating the decrease in cell size given by the Hata model that follows from a decrease in link budget at higher data rates.

The Relationship of RF link budget and Cell-Density (to maintain a fi xed outage probability)

D ~ (PT/SR) -(2/n)

or equivalently,D ~ 1/(link budget) (2/n)

D = required cell density to support data rate R at a fi xed outage probabilityPT = maximum transmitted power by each cell at its antenna (EIRP)SR = required signal power at data rate Rn = path loss exponent (as in the Hata model)

Equation 2: RF Path Loss and Cell Density

The plot in Figure 7 reveals the multiplier on today’s cell density that would be required to raise 1xEV-DO download speeds in San Diego (150kbps) and Seoul (300kbps) up to multi-megabit speeds based on the required signal power in Figure 2. As we can see, 3-4x the current density is required to achieve >1Mbps downstream and 6-8x the current density is required for > 2Mbps downstream. Due to the asymmetry in the 1xEV-DO link budget, increasing upload speeds to the megabit range would require even greater cell densities. Our estimates for the increase in cell density required depend on the choice of path loss exponent (n). For illustration, we employing a typical path loss exponent of n=3.5 for urban areas in Equation 2.



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MetroMesh : 4x less cost and 5x faster than 3GWith the increases in cell density needed to support of megabit broadband speeds (even without symmetric service), the economics of the traditional cellular base station architecture is questionable. Not only must the cell infrastructure be purchased, installed and operated, but each cell requires backhaul entailing a heavy recurring expense.

To support multi-megabit speeds, a Wi-Fi based Tropos cellular mesh is the cost-effective alternative to 3G approaches. The self-organizing cellular mesh enables easy, quick installation of low-cost MetroMesh routers at the densities required for symmetrical megabit broadband at a fraction of the cost of operation. The small, low-power, low-cost MetroMesh routers are mounted in minutes on any available mounting asset, such as street lights, traffi c signals, and power poles. Backhaul can be supplied to fewer than as 5% of the MetroMesh routers (i.e. fractional backhaul), with the remaining cells routing their traffi c over the air at the highest possible throughput to and from the client to provisioned backhaul assets. Conversely any cell can be reconfi gured to inject wired or fi xed wireless backhaul into the cellular mesh.

In Table 2, we compare the price and performance of deploying Manhattan (34 square miles) using 3G (1xEV-DO) and Wi-Fi (Tropos). The estimates for 1xEV-DO are based on data from [AIRVANA].

MetroMesh Advantage Over 3G: 4x Less Cost 5x More Performance

The future of wireless broadbandWith 3G technology, end-user speeds are fundamentally limited by link budget and offer little potential for future growth. As we have seen, the 3G link budget is heavily constrained by cell density.

Figure 7: 3G Speed andCell Density

Table 2: Deployment Costs in Manhattan (34 Square Miles)



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Figure 8: Tropos MetroMesh Technology Curve

Today, the Wi-Fi based Tropos MetroMesh architecture offers a 5x speed advantage over 3G for delivering broadband to metro-areas at a fraction of the cost. Wi-Fi based radio technology is at the beginning of dramatic increases in spectral effi ciency as it transitions from 802.11b (DSSS) to 802.11g (OFDM) to 802.11n (MIMO) and 802.16 (WiMax). Due to forthcoming advances in protocols and network architecture, the Tropos cellular mesh technology holds immense potential for achieving further advances in end-user speeds. Based on these projections, in the coming years MetroMesh will deliver end-user speeds in the metro-area that rises to 10-20x in 2005 and 20-40x by 2006-2008 over what is possible with 3G technology, as shown in Figure 8.

For further reading1. [TROPOS] “Metro-Scale Wi-Fi Using Tropos

Networks’ Cellular Mesh Technology,” © 2004 Tropos Networks, http://www.tropos.com/pdf/Tropos_Tech_WP.pdf

2. [IEEE] “The IEEE 802.11 Handbook: A Designer’s Companion” by Bob O’Hara, Al Petrick, © 1999 IEEE

3. [SHANNON] “The Mathematical Theory of Communication,” by Shannon, Claude E. and Warren Weaver, © 1998 University of Illinois Press.

4. [CDMA HANDBOOK] “CDMA Systems Engineering Handbook” by Jhong Sam Lee, Leonard E. Miller, © 1998, Artech House.

5. [AGILENT] “Understanding Measurement of 1xEV-DO Access Terminals,” Application Note 1414, © 2003 Agilent Technologies http://cp.literature.agilent.com/litweb/pdf/5988-7694EN.pdf

6. [QUALCOMM] “The cdma2000 High Rate Packet Data System” by Qiang Wu and Eduardo Esteves, March, 2002 in Chapter 4 of “Advances in 3G Enhanced Technologies for Wireless Communications”, © 2002, editors Jiangzhou Wang and Tung-Sang Ng. http://www.qualcomm.com/cdma/1xEV/media/pub_papers/cdma2000_HighRatePacket.pdf

7. [AIRVANA] “Wi-Fi vs. 1xEV-DO,” © 2003 Airvana, http://www.airvananet.com/1xev/wifi .shtml

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www.tropos.com • [email protected]©2003-2007 Tropos Networks, Inc. All rights reserved. Tropos and PWRP are registered trademarks of Tropos Networks, Inc. Tropos Networks, MetroMesh, AMCE, TMCX, SABRE, CMDP, MESM and

Metro-Scale Mesh Networking Defi ned are trademarks of Tropos Networks, Inc. All other brand or product names are trademarks or registered trademarks of their respective holder(s). Information contained herein is subject to change without notice. The only warranties for Tropos products and services are set forth in the express warranty statements accompanying such products and

services. Nothing herein should be construed as constituting an additional warranty. Tropos shall not be liable for technical or editorial errors or omissions contained herein.contained herein.

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Price-Performance Comparison: 3G and Tropos MetroMesh