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| A Note from the
Publisher:
We continue to enjoy the feedback from
our readers regarding our first year of Rural Signals. Our team
of technical consultants strives to bring you understandable
information regarding current issues facing the wireless telecommunications
industry. In this issue of Rural Signals, our team offers up
additional information and insight regarding several issues
that are sure to be of interest to wireless broadband stakeholders.
VoIP is here today, and it is growing, but it carries its share
of network requirements of which operators should be aware.
Judy Deng takes a look at how Quality of Service can make a
big difference in VoIP networks, and some techniques for improving
Quality of Service.
Contemporary and future broadband services and applications
require spectrum, and lots of it. WiMax could be one vehicle
for delivering those services. David Fritz reports on where
WiMax might find some spectrum in the U.S., and why it won’t
be easy.
Speaking of spectrum, even the smallest slivers of it can be
actively contested, especially with the potential to deploy
broadband services. This is typified in the Upper 700 MHz frequency
band, where a group of spectrum managers have petitioned the
FCC with proposals to rearrange the spectrum blocks. Len Garavalia
dives into the technical aspects of the proposals, and provides
a discussion of the proposed techniques to limit interference
between the blocks. Keeping an eye on the more established Lower
700 MHz band, Len then adds some tips on how to determine when
certain operations might require prior FCC approval.
Going beyond spectrum, wireless operators face yet another
challenge in the delivery of services: ubiquity. Malick Sohrab
explains some exciting developments with IMS and UMA, solutions
that could literally bridge the gap between mobile and local
networks with a single device.
In a previous issue of Rural Signals, we started a series on
understanding how Enhanced 911 solutions work in mobile networks.
In the second part, Jim Egyud literally goes to the next Phase:
understanding how E911 Phase II location technologies work,
and why achieving the FCC’s location accuracy requirements
can be so difficult. Keep your eyes on Rural Signals for reports
of further E911 technology developments that could particularly
benefit the rural carrier.
Enjoy Rural Signals and, as always, your feedback is priceless.
Bennet & Bennet, PLLC
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Help for the Acronym Challenged
Are you tired of getting bogged down trying to remember and understand all of those acronyms, those little abbreviations that us technical folks so love to use? Well, we get tired of them also, and we're the technical consultants! In our effort to make your reading experience an easy one, Rural Signals is pleased to now offer a glossary of several technical acronyms used in this issue. In the articles, you will find links associated with the first appearance of a term. Simply click on the link for the definition, or you can scroll to the bottom of the article.
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Quality of Service on the VoIP Network
What Every VoIP Provider Should Know
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By Judy Deng
Voice over Internet Protocol (VoIP) has probably gathered more attention in 2005 than in any previous year, with many carriers providing it and still more investigating it. As a definition, VoIP is a technique of sending voice samples and signaling data as packets over IP networks. This implementation leads to the integration of voice with data networks. The consolidation of separate voice, fax, and data resources offers an opportunity for significant savings via reduced investment on equipment, efficient bandwidth usage, and network management. VoIP opens up another convenient pathway, and an affordable one at that, to chat with the rest of the world. However, carrying voice over packet networks also carries with it some quality of service (QoS) issues unique to the packet networks. We encourage VoIP providers, especially prospective rural ones, to take a close look at their networks in light of VoIP's needs and quirks.
First and foremost, IP is a "connectionless" technology, i.e., IP packets do not take a specific path as they traverse the network, and there is no guarantee on bandwidth. It is especially important to understand that the protocol will not, in and of itself, differentiate network traffic based on the type of flow to ensure that the proper amount of bandwidth and prioritization level are defined for a particular type of application.
With this as a backdrop, the QoS factors involved in VoIP applications are: network availability, bandwidth, delay, jitter, and lost packet compensation.
1. Network availability
It should go without saying that network availability significantly affects QoS. Simply put, if the network is unavailable, even during brief periods of time, the user or application will experience unpredictable or unreliable performance. While data networks that achieve 99.9 percent availability are considered to be operating extremely well, people expect much greater reliability from their phone systems because they are accustomed to the QoS provided by the PSTN and their private PBX-based networks. These connection-oriented, circuit-switched networks generally deliver "five 9s", i.e., 99.999 percent availability.
2. Bandwidth
Bandwidth is probably the second most significant factor that affects QoS. Bandwidth allocation can be categorized as "available bandwidth" or "guaranteed bandwidth". Many network operators oversubscribe the available bandwidth on their networks in a play to maximize the return on investment in infrastructure or bandwidth. This is, of course, akin to an airline overbooking a flight. In the airline world, the overbooked traveler usually ends up on a later flight. In the IP world, the jammed pipe also results in delays. The end results can include high latency, slow throughput, poor QoS, and some very aggravated customers.
Some network operators offer a service that provides a guaranteed minimum bandwidth. Because the bandwidth is guaranteed, the service is priced higher than available bandwidth service. The network operator must then ensure that those who subscribe to the guaranteed bandwidth service get what they pay for. The operator must take a look at the throughput and latency required for sufficient QoS of VoIP and other services in determining the guaranteed bandwidth to offer. While the bandwidth required for each VoIP session is relatively low, the challenge is to make the bandwidth available regardless of the network utilization and overall traffic. Depending on the type of voice-compression method used, each one-way VoIP transmission requires between 32 kbps (kilobits per second) and 64 kbps of bandwidth. Some compression methods can reduce the required bandwidth below 8 kbps.
3. Delay
Delay is most problematic when it occurs in conjunction with one or both of two other conditions: echo and talker overlap. Echo is caused by the signal reflections of the speaker's voice from the far-end telephone equipment back into the speaker's ear. Echo becomes significant when the round-trip delay, or latency, is greater than 50 milliseconds. As an echo can be perceived as a significant quality problem, VoIP system providers must address the need for echo control and implement some means of echo cancellation. Talker overlap occurs when two persons in a conversation speak at the same time (affectionately known as stepping on someone else's speech). Talker overlap becomes even more significant if the one-way delay becomes greater than 250 milliseconds. Planning for and mitigating end-to-end delay are essential practices for reducing the effects of delay through a packet network.
4. Jitter
Jitter is the variation in the delay of packet delivery. Jitter has a pronounced effect on real-time and delay-sensitive applications. The voice application requires the audio to play out at a constant rate. If the next packet does not arrive within the playback time, the application may replay the previous voice packet until the next voice packet arrives. However, if the next packet is delayed too long, it is simply discarded when it arrives, resulting in a small amount of distorted audio. Removing jitter requires collecting packets and holding them long enough to allow the slowest packets to arrive in time to be played in the correct sequences.
5. Packet Loss
Packet loss can be an even more severe problem. Too much traffic in the network causes the network to drop packets. In current IP networks, all voice frames are treated like other data frames. Under peak loads and congestion, voice frames will be dropped equally with regular data frames. Unfortunately, voice frames are time sensitive and cannot be appropriately corrected through the process of retransmission like the data frames.
Loss can also occur due to errors introduced by the physical transmission medium. In particular, wireless connections such as mobile or fixed networks could have a high bit error rate (BER) due to environmental or geographical conditions such as fog, rain, RF interference, cell handoff during roaming, and physical obstacles such as trees, buildings, and mountains.
QoS Technologies and Practices for the Network Provider
Service providers should ensure that their networks are properly engineered to support VoIP. The network QoS must be measured and monitored to ensure that the network adequately supports real-time services. Increasing bandwidth may be a necessary first step for accommodating the real-time applications, but it might still not be enough to avoid jitter during traffic bursts. Even on a relatively unloaded IP network, delivery delays can vary enough to continue to adversely affect real-time applications. Fortunately, QoS techniques can be applied to an IP network to make it capable of supporting VoIP with acceptable, consistent, and predictable voice quality.
There are a number of protocols and algorithms that can provide quality of service for IP networks. QoS techniques do not create bandwidth, but manage it so that it is used more effectively to meet the wide range of application requirements. QoS technologies generally encompass bandwidth allocation, prioritization, and control over network latency, and provide some level of predictability and control beyond the current IP "best effort" service. The three most common QoS assurance practices are:
- Resource Reservation (RSVP, also known as Integrated Services): provides quality of service by using a protocol to explicitly reserve bandwidth on a per flow basis.
- Differentiated Services (DiffServ): provides a course and a simple way to categorize and prioritize network traffic flows.
- Multi Protocol Labeling Switching (MPLS): a connection-oriented form of IP networking - packets have labels added and are forwarded along pre-constructed label-switched paths by routers modified to switch MPLS frames.
QoS technologies are recommended in all types of networks. Low bandwidth connections often require more complex QoS technologies to provide adequate performance levels while maximizing the bandwidth efficiency.
Significant migration of voice service from the traditional circuit switched networks to IP-based packet switched networks has been occurring over the past few years. While the challenges to this integration of voice and data are substantial, the potential savings make the investment in a high QoS implementation compelling, especially as bandwidth demands increase and more voice services utilize IP-based technology.
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In the VoIP world, both perceived and measured QoS drops can occur for many reasons, with customer satisfaction at stake. If you should have any questions regarding the reasons and QoS techniques discussed here, or regarding VoIP QoS in general, please do not hesitate to contact Judy Deng.
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Potential Spectrum Options for WiMAX in the United States
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By David Fritz
Worldwide Interoperability for Microwave Access ("WiMAX") is a topic frequently discussed as the up and coming low cost solution to fixed wireless broadband access. Proponents of WiMAX believe that standardization, vendor interoperability, and large-scale production of silicon will drive this technology into the forefront of the broadband market place within the next few years.
The WiMax Forum, the standards body with over 333 member companies, has moved forward with its first certification testing, which targets the spectrum profiles of frequency bands at 3.5 GHz and 5.8 GHz. Table 1 provides a quick look at the profile characteristics of these bands:
Table 1: 2005 WiMax Forum Certification Profiles (IEEE802.16-2004):
| Band |
Channel Width |
Duplex Method |
| 3.5 GHz (3400-3600 MHz) |
3.5 / 7.0 |
TDD / FDD |
| 5.8 GHz (5725-5850 MHz) |
10 MHz |
TDD |
Source: www.wimaxforum.org
The Forum chose these profiles as the starting points for WiMAX certification due to their usability by a significant portion of the world's population. For the United States, the 5.8 GHz profile is open for unlicensed use (i.e., not requiring FCC approval at a specific location), but the 3.5 GHz profile does not fit into the FCC's spectrum utilization chart for commercial broadband services. Of the licensed and unlicensed spectrum options available in the US, what bands will attract the WiMAX Forum's attention next?
Unlicensed Spectrum Options
In the US, the 5.8 GHz (5150-5350; 5470-5850 MHz) spectrum is part of the Unlicensed National Information Infrastructure (U-NII), consisting of 580 MHz, in which the WiMAX certification profile falls within the upper 125 MHz band. As unlicensed spectrum, the 5.8 GHz band has the positive aspects of immediate use, no spectrum costs, and interoperability for certified equipment. However, limiting factors such as high frequency "line-of-sight" propagation characteristics, significant power restrictions, and little control of interference with other users severely reduce the appeal of this band for any type of robust WiMAX deployments.
Fixed Wireless Access (FWA), at 3.65 GHz (3650-3700 MHz), covers 50 MHz of spectrum slated for future unlicensed use by the FCC. Issues stemming from incumbent users requiring protection zones, unclear contention-based protocol requirements, and the registration of all fixed and base transmitters are some of the many details with which the FCC is wrestling in order to make this band available to the public for use. As unlicensed spectrum adjacent to the WiMAX Forum's 3.5 GHz profile, the FWA band is somewhat attractive to vendors and companies interested in WiMAX. There are some industry proponents of WiMAX pushing the FCC to offer up the FWA band as licensed spectrum (i.e., spectrum in which specific usage requires FCC licensing), while others hope the spectrum will stay the course for unlicensed status and become available for deployment soon. Either path will require some additional time for the details to settle out, but industry stakeholders hope that conclusions on the outstanding issues will be reached in 2006.
Licensed Spectrum Options
For licensed spectrum, one of the most underutilized bands with potential for broadband deployment is the allotment to the Wireless Communications Service (WCS) at 2.3 GHz (2305-2320 & 2345-2360 MHz). WCS spectrum was auctioned off in 1997 and acquired by its original bidders at significantly lower costs than spectrum in other FCC auctions. WCS band licensees include companies such as Sprint/Nextel, Bellsouth, Comcast, and Verizon, companies that could drive the technology choices for both fixed and mobile broadband solutions in this band. Adjacent interference created from high-powered terrestrial Digital Audio Radio Service (DARS) sites is one negative issue facing the WCS licensees. The FCC's upcoming 2007 build-out deadlines for the WCS licensees will likely spur deployments in the near future. For WiMAX, 2.3 GHz certainly has significant potential as a future profile as pre-certified equipment is accepted and deployed by U.S. and foreign companies.
The 2.5 GHz band (2500-2690 MHz), currently allocated to the Broadband Radio Service (BRS) and the Education Broadband Service (EBS), is likely a target of the WiMAX Forum for a future U.S. certification profile. Formerly known as Multipoint Distribution Service (MDS), Multichannel Multipoint Distribution Service (MMDS) and Television Fixed Service (ITF), BRS spectrum was originally auctioned off by the FCC for delivery of one-way video and data programming. However, the FCC expanded the rules to include two-way broadband radio services. To accommodate the added technology allowances, the BRS band is currently going through a three-year re-banding process to consolidate the spectrum, as discussed in our Spring 2005 issue of Rural Signals.
The post-transition plan, as adopted June 10, 2004, regroups the BRS licenses into one high-power and two low-power contiguous bands, deviating from the original plan of interweaving smaller 5 MHz license segments. The culmination of the complex re-banding process initiated regionally by licenses, the creation of irregular market boundaries by pre-MDS/MMDS/ITFS grandfathered licenses, and the partitioning of the spectrum for the Advanced Wireless Services (AWS) auction, have put this band into a bit of a quagmire for the next couple of years. Broadband deployments by Sprint/Nextel, Bellsouth, and Clearwire, having major holdings in the BRS spectrum, will likely drive the future for WiMAX in this band.
One more possibility that has received attention is the 700 MHz licensed C-Band, with its paired 6 MHz blocks forming what many licensees feel is 12 MHz of valuable spectrum that initial bidders acquired very inexpensively. For many of the populated areas of the U.S., this spectrum cannot be utilized until the incumbent co-primary broadcasters are forced to vacate it or significant legal filings are made to gain FCC regulatory approval for 700 MHz licensees to operate in close proximity to the incumbent broadcasters. Recent moves in Congress to clarify and put a true fixed deadline on the digital TV transition (possibly 2009) have increased interest in this spectrum and its potential for a future WiMAX certification profile. Many WiMAX proponents see 700 MHz, with its propagation characteristics conducive to mobile applications, as a place for future fixed, nomadic, and mobile WiMAX profiles. One uphill battle for WiMAX at 700 MHz might be the competing technology interest in using that spectrum for wireless broadcast services, such as Qualcomm's MediaFlow product. However, with a number of pre-WiMAX deployments already in operation, 700 MHz will certainly be of great interest to WiMAX supporters.
Licensed Spectrum Yet To Be Auctioned
On the auction horizon, the remainder of the 700 MHz spectrum and the first Advanced Wireless Services spectrum (AWS1) are the two large bands to which virtually all of the wireless industry is looking to fill spectrum needs. The remaining 700 MHz spectrum includes Upper (746-764 & 776-794 MHz) and Lower (698-710, 716-722 & 728-740 MHz) bands, yielding a total of 66 MHz, while the AWS1 spectrum (1710-1755 & 2110-2155 MHz) offers a tantalizing 90 MHz. With no fixed auction date scheduled yet for either band, it appears AWS1 will be the first to auction with a tentative time frame of June 2006. With large carrier interest in 3G rollouts for existing networks and the market demand for more bandwidth, it is anticipated that AWS1 and the remaining 700 MHz bands will garner significant competitive bidding, driving up the cost to acquire licensed spectrum. How this high cost and competitive bidding will factor into WiMAX's marketed, low cost deployment model, will certainly play out in the minds of potential owners bidding on the spectrum in these auctions. As it seems with all auctioned spectrum in the US, incumbent clearing, compatibility with global spectrum allocations, and large company technology choices will all play a significant factor in broadband wireless access deployments for these specific bands.
Will the Market Cart Drive The Spectrum Horse?
In all, there are many potential spectrum options for WiMAX in the U.S. Unfortunately, most of the options seem to be unique to the U.S. market, making it difficult for the WiMAX Forum to latch onto a certification profile prior to any wide company acceptance and dedication to deploy WiMAX in a specific spectrum band. The initial certification roll-out of 5.8 GHz by the WiMAX Forum will likely not make a huge impact on the U.S. fixed broadband market. For future certification profiles, both fixed and mobile, it appears that the U.S. will likely derive profiles based on market demand and acceptance of pre-WiMAX solutions, with "official" Forum Certifications in tow. Today, many pre-WiMAX solutions are being pushed by vendors with the promise of future certification upgrades. Out of character with the WiMAX marketed model, but in the true "American way," first crack at the market share will likely take priority over the slow but vitally important process of equipment interoperability certification.
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If you have any questions related to spectrum purchasing, leasing, utilization, or market options for broadband wireless access, please contact David Fritz or anyone at Bennet & Bennet for further assistance. (Editor's note: For a discussion of the evolution towards WiMax, check out Judy Deng's article in our Summer 2005 issue of Rural Signals.)
Glossary of Acronyms:
FDD - Frequency Division Duplexing
GHz - Gigahertz - one billion Hertz (Hz), or one thousand MHz (Megahertz)
MHz - Megahertz - one million Hertz
TDD - Time Division Duplexing
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Upper 700 MHz Rebanding
Guard Band Managers Place Spectrum Blocks in a State of "Flux"
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By Len Garavalia, with Introduction by Jim Egyud
It's official: spectrum is valuable. This little truism is exemplified by the Upper 700 MHz frequency band, where both public safety and commercial interests abound and the FCC tries to strike a reasonable balance among them. With a number of different interests and stakeholders looking to deploy various systems in the spectrum, both the licensees and the FCC are keeping a close eye on both the frequency allocations and how to ensure applications on adjacent spectrum blocks do not interfere with each other. Even the small pieces of spectrum originally intended to serve as interference-control buffers between Public Safety and commercial operations, called the guard bands, have become a focal point for potential use.
The FCC assigned the small guard bands as the Upper 700 MHz A and B blocks, and with an eye towards balancing their potential usage with their role as buffer between larger commercial and public safety systems, placed significant usage and licensing restrictions on those bands, including a prohibition of technologies using cellular-like architecture. The FCC then entrusted the guard bands to a small group of entities called, appropriately enough, the Guard Band Managers.1 These caretakers currently have options to lease this somewhat encumbered spectrum to other users, predominantly unaffiliated third parties. With relatively small pieces of spectrum having limited application, the Guard Band Managers have quite naturally petitioned the Commission with suggestions to further the spectrum's usability, including a proposal to permit cellular-like architecture. Of course, the proposals would also permit usage by the Managers themselves. The Managers' series of White Papers to the FCC propose both rebanding of the Upper 700 MHz spectrum, alternatives for interference protection of adjacent channel licensees, and recommendations for permitting cellular-like architecture within the guard bands.
Recipients of Bennet & Bennet's informational memos recently received an extensive discussion of the Guard Band Managers' proposals and their potential implications. In this article, Len Garavalia focuses on the technical aspects of the proposals, including the spectrum allocation options and a rather novel out-of-band interference mitigation approach using power flux density limits.
1 The A and B-Block licensees include Access Spectrum, L.L.C., Pegasus Guardband, L.L.C., PTPMS II Communications, L.L.C., and Columbia Capital Equity Partners, L.P.
Rebanding Options Abound
There is certainly no shortage of proposals for how the FCC should re-band and re-allocate the Upper 700 MHz spectrum. In our Fall 2005 issue of Rural Signals, we discussed a proposal by the Rural Telecommunications Group, Inc. (RTG), which seeks to have a block of the spectrum made available on an MSA/RSA basis, as shown in RTG's Upper 700 MHz Band Plan. By contrast, the Guard Band Managers suggest several options for distributing the spectrum itself among the various blocks.
In their initial White Paper submitted to the FCC on August 3, 2005, the Guard Band Managers proposed three options for rebanding. Two of the three options would eliminate the B-Block after rebanding. Their subsequent White Paper proposed specific FCC rule changes and detailed their proposals. In various combinations, two of the proposals would increase the A-block spectrum, two would increase Public Safety spectrum, and two would eliminate the B-block altogether. Depending on the ultimate distribution and assignment of spectrum, the A-block or C-block, or both, would be restricted by certain interference mitigation measures, as proposed by the Managers and discussed later in this article. For a graphical depiction comparing the Managers' proposed spectrum changes and the current FCC plan, please see the attached diagram of Band Plan Options. Bennet & Bennet's memo provides a more detailed discussion of those options. The final spectrum allocation could have a significant impact on the licensees who are adjacent to the Public Safety band depending on whether or not the Guard Band Managers' interference mitigation techniques are also adopted.
The Guard Band Managers have not addressed repackaging the commercial C or D-Blocks into cellular market sizes (MSAs and RSAs), an action which would affect the proliferation of rural broadband deployments. By contrast, RTG's Upper 700 MHz band plan proposed MSA/RSA geographic licensing for the C-Block spectrum. It should be noted that RTG's Upper 700 MHz proposal did not shift the frequency allocations, but only assigned MSA/RSA geographic license sizes for the C-Block spectrum.
Out-of-Band Emission Limits With a Broader Objective
The Guard Band Managers have proposed alternatives for interference mitigation to protect Public Safety licensees operating in the Upper 700 MHz frequency band while increasing guard band usability. They have proposed that the FCC adopt out-of-band emissions rule changes for the guard band blocks similar to the FCC's existing adjacent channel interference mitigation measures applicable to the commercial (C and D) blocks and Public Safety broadband services in the Upper 700 MHz spectrum.
The Guard Band Managers point out that the formulas for existing out-of-band emission limits do not account for a radio access network operating within the guard band itself, especially one that could operate with broadband channel bandwidths. This is significant since newer digital technologies (e.g., narrowband CDMA, TD-OFDM, TD-SCDMA, etc.), which could potentially be used within the guard band spectrum, would likely utilize a bandwidth of 1.00 to 1.75 MHz. This, in turn, is important because the amount of power in a digital channel is distributed across its entire bandwidth. If we compare narrow and wide bandwidth channels, where the channel-wide transmitter power is equal, such as a 6.25 kHz vs. a 1.25 MHz channel, there is more power measured on a per Hertz basis within the 6.25 kHz channel than within the wider 1.25 MHz channel. Therefore, the Guard Band Managers suggest that a wide-band channel "leaking" over the band edge would have less power per Hertz than the currently allowed narrower channel and, accordingly, would have a lower potential for interference that may affect adjacent-channel licensees.
The Guard Band Managers propose out-of-band-emission limits for the proposed guard band frequency blocks, which must protect the narrowband usage of the Public Safety spectrum, by applying limits similar to those specified by the FCC for the commercial C and D-Blocks. Under those limits, for any frequency outside of the C or D spectrum, the emission must be attenuated below the transmitter power, as described by the formula:
43 + 10 log(transmitter power in Watts) [calculated in dB]
However, as proposed for application outside of the guard band, the emission would be measured over a 100 kHz bandwidth. The Guard Band Managers assert that their proposed out-of-band-emission limits would meet or exceed the current ACCP (Adjacent Channel Coupled Power) limits applied to the Public Safety spectrum and claim that, in practice, the signal attenuates more than defined by the formula as the frequency separation increases from the center of the undesired channel.
Power Flux Density: Is It a Practical Solution for Cellular Architecture?
Since the prohibition of cellular system architecture could severely hamper deployment of broadband services in the rebanded A and/or B-Block spectrum, the Guard Band Managers propose to delete the portions of the FCC's Rules which prohibit the use of cellular architecture on frequencies in the Upper 700 MHz A and B-Blocks and replace these rules with power flux density (PFD) limits. PFD represents the amount of power crossing a certain area perpendicular to the direction of wave travel.
The notion of PFD might sound foreign to wireless operators that are used to basing power measurements and received signal levels on sheer Watts, mW or dBm. However, PFD is by no means a new concept, even within the FCC's 700 MHz rules. In the Lower 700 MHz bands, the FCC requires PFD limits of 3000 microwatts per square meter (uW/m2) out to one kilometer (1 km) from the base station. For the guard bands, the Managers propose a much more stringent PFD limit of 25 uW/m2 on the ground over the area extending from the base of the antenna structure out to 1 km. Notably, in two of the three rebanding options, this PFD limit would also apply to the C block sought by potential rural operators.
The Guard Band Managers argue that a power flux density restriction at ground level would satisfy the FCC's "near-far" concerns, where Public Safety personnel attempt to communicate from a location at the edge of their serving transmitter's service area (far), but a commercial operator's transmitter located in close proximity (near) has a relatively strong field strength and could cause harmful interference. Because of the high number of sites in a system with a cellular architecture, the statistical probability of Public Safety personnel operating in a "near-far" situation would increase dramatically.
To compensate for proximity to a greater number of sites in a cellular scenario, signal measurements using PFD would take advantage of typical cellular antenna characteristics, whereby the PFD on the ground could be limited through the antenna's height and its vertical radiation envelope (pattern). As a practical example, let's look at PFD calculations if we assume that a broadband licensee proposes a tower site with an antenna centerline height of 250 feet above ground level operating at 250 Watts EIRP (152 Watts ERP), and utilizes an antenna pattern that has good "null-fill" or is tilted downward to provide management of intra-system interference. At a distance of 1 km from the tower, the angle below the antenna centerline to a point on the ground is 4.3 degrees. For the typical high gain panel antenna, this depression angle is within its main beam, or within 3 dB of its highest gain and therefore its maximum EIRP. If so, the PFD at that point on the ground would be less than 25 uW/m2 (see the PFD Diagram, Figure 1).
If we back into the depression angle by calculating where an exact measurement of 25 uW/m2 would occur, we find an angle below the antenna of only 4.8 degrees. This is still within the main lobe of radiation of most panel-type antennas. Note that the distance from the tower to this location would be 889 meters (see Figure 2 in the PFD Diagram). This point is significant. At every point closer to the tower, where the 250 Watt EIRP is maintained, the flux density would remain at or above 25 uW/m2.
For example, if we look at an antenna system with 10 degrees of electrical downtilt, that angle below the horizon would project the maximum EIRP at points located 431 meters from the tower. The resultant PFD would be 104 uW/m2, failing the proposed 25 uW/m2 limit (see Figure 3 in the PFD Diagram). This could prove challenging for the operators, as it would not be unusual to use 10 degrees of downtilt to minimize intra-system interference and manage cell footprint. In fact, at least one commercially available antenna for the 698-794 MHz band has an available 10-degree electrical downtilt (see this vertical gain pattern of a sample antenna with such a tilt).
In addition to a more stringent PFD limit, the Guard Band Managers' proposal would also appear to apply it in a different manner. While the FCC's existing PFD limits only apply to those facilities that operate with an ERP in excess of 1 kW, the Guard Band Managers' changes would seem to apply at all ERP levels. Also, rather than all guard band and commercial blocks being affected, only the guard bands and 1 MHz of the C-block that would be adjacent to the Public Safety spectrum would receive the limits. By contrast, the Managers did not propose PFD limits on the edge of the commercial D-Blocks, even where they abut the Public Safety spectrum.
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We will provide an update on the Commission's decisions and responses in future editions of Rural Signals. If you would like to submit comments to the FCC regarding the proposals, please contact Carri Bennet. If you are wondering how a particular system or facility might fit into the different rule proposals discussed here, or for more information about the 700 MHz band plans and technologies, please contact Len Garavalia.
Glossary of Acronyms:
CDMA - Code Division Multiple Access
dBm - Decibel referenced to a milliwatt, i.e., the amount of gain above or below one milliwatt
EIRP - Equivalent Isotropic Radiated Power
ERP - Effective Radiated Power
kHz - kilohertz, or one thousand Hertz, a measure of frequency in the electromagnetic spectrum
MHz - Megahertz, or one million Hertz, a measure of frequency in the electromagnetic spectrum
MSA - Metropolitan Statistical Area
mW - milliwatt, or one-thousandth of a Watt, a typical unit of received radio signal power
RSA - Rural Service Area
TD-OFDM - Time Dispersive Orthogonal Frequency Division Multiplexing
TD-SCDMA - Time Division Synchronous Code Division Multiple Access
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Rural Signal: Another note about 700 MHz FCC Filings
Lower 700 MHz C-Block: When Is Prior FCC Approval Required?
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Since our previous Rural Signals note about Lower 700 MHz C-Block filings, we have received multiple inquiries regarding potential operations in that spectrum at sites located in proximity to adjacent channel TV/DTV broadcasters. Licensees have asked,
"If I'm proposing a base station utilizing a TDD technology and that base station is within the height, power and distance tolerances of Table E, can I operate without obtaining prior approval from the FCC?"
The answer is a definite "it depends"! There are two possibilities that would not require prior approval:
- The base station and the subscriber units are operating on the same frequency, adjacent to the TV/DTV incumbent, and the subscriber units are fixed stations. These subscriber units should be treated as individual base stations, and the height, power and distance tolerances of Table E should be maintained. Each fixed subscriber station can operate as close as 60 miles to the adjacent broadcaster as long as the subscriber station's ERP is less than 50 Watts, and its Height Above Average Terrain (HAAT) is less than 400 feet.
- The base station and subscribers operate on the same frequency as the incumbent, but the subscriber units are mobile. In this case, footnote 3 from Table E states that the licensee is in compliance if the base station is located at least "the minimum distance [of] 145 km (90 miles) [from the incumbent TV/DTV licensee] where there are mobile units associated with the base station."
If either of these scenarios applies, then yes, the proposed facilities can likely be operated in accordance with FCC Rule Section 27.60(b)(1)(i), and prior Commission approval is usually not required. Please consult with Bennet & Bennet for a specific analysis of your market.
As discussed in the Spring 2005 and Fall 2005 issues of Rural Signals, all Lower 700 MHz licensees must evaluate TV/DTV channels 53, 54, 55, 58, 59 and 60 for potential interference to incumbent stations. Even where a proposed base station transmitter site might be located outside of the Grade B contours of these stations, there are still specific geographic separation requirements that must be maintained for interference protection. On the other hand, there are some instances where proposed base stations could receive prior FCC approval to operate within an incumbent's Grade B contour.
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There are plenty of nuances to the Lower 700 MHz licensing process. For an evaluation specific to your licensed Rural or Metropolitan Service Area, or if you would like more information, please contact Len Garavalia.
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IMS and UMA
Bridging the Gap Between Mobile and Local Networks
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By Malick Sohrab
Delivering High Bandwidth IP Services Via IMS
For mobile operators looking for ways to generate revenue from the next generation of mobile services, the integration of IP Multimedia Subsystems (IMS) architecture promises to efficiently deliver IP-based services such as streaming video, music downloads, interactive gaming, video conferencing and other high bandwidth applications. The reliance of these services on data bandwidth has led the industry to move towards offering high bandwidth wireless mobile data services to meet those requirements. Second Generation (2G) CDMA networks are now evolving to Third Generation (3G) and beyond via technology paths such as 1xRTT and EVDO, while networks on the GSM/GPRS track evolve toward this goal through EDGE to UMTS.
The perceptions by some in the industry that customers will want a single converged point of access for voice and IMS services, and that emerging technologies such as VoIP, Broadband, and WLANs (or other unlicensed wireless networks) may pose threats to mobile service providers, seem to be pushing the provision of IMS services. The growth of VoIP, Broadband, and WLANs is already chipping away at the mainstream landline service industry. In fact, some already view public WLAN hotspots as a threat to mobile 2.5G/3G data services, and VoIP over private WLANs as a threat to mainstream mobile voice services at the customer premises (home, office, business environment, café, etc.).
In an effort to pursue the provision of IMS services, mobile operators will have to juggle three things:
- the high bandwidth data needs of these services, which the limited bandwidth and high latency of the operators' cellular and PCS networks probably could not support;
- the cost of migrating to new UMTS/EVDO type technology and infrastructure that will not only support these services, but also provide them via reliable coverage to the customer premises; and
- the perceived threat that emerging VoIP, Broadband, and WLANs offer at the customer premises level.
One trend in the industry suggests embracing these new technologies instead of perceiving them as a threat, and leveraging them to overcome some of the barriers in bandwidth and cost (in new equipment and coverage) that may keep mobile operators from pursuing IMS type services. Making this a possibility is the concept of interoperability and convergence of mobile services with broadband, unlicensed WLANs (e.g., WiFi), and VoIP.
Bridging the Gap Via UMA
The 3rd Generation Partnership Project (3GPP) emerged from an agreement among a number of telecommunications standard bearers, including ETSI, to produce a globally applicable technical specification for 3G mobile systems based on evolved core GSM networks and their supported access technologies. 3GPP's scope evolved to include maintenance and development of GSM as well as related, evolving technologies such as GPRS and EDGE. To pursue a convergence of cellular and PCS with WLANs, 3GPP established a standard called Unlicensed Mobile Access (UMA) to enable mobile operators to leverage broadband, WLAN and VoIP technologies to deliver IMS services indoors at the customer premises at a better quality and a potentially lower cost. Thus, UMA extends access to GSM and GPRS mobile services over unlicensed spectrum WLAN technologies such as Bluetooth and 802.11 (WiFi).
By deploying a UMA solution, mobile operators can allow seamless mobility (roaming and handover) between their wireless networks and WLANs, while offering a single point of access for voice, data, and all other IMS services. The 3GPP UMA standard promises the full indoor customer premises experience of voice, data, SMS, MMS, and IMS services, while allowing seamless hand-offs of active voice and data sessions with the outdoor cellular network.
Establishing UMA will require furnishing the customer with a UMA-enabled dual-mode device or handset which, when within range of a WLAN to which it has access rights, is allowed to connect. Upon connection, the customer device contacts the UMA Network Controller (UNC) over the broadband WLAN for authentication and access to the mobile (GSM) voice and data (e.g., GPRS) services. The UNC is the key intelligent core mobile network component that manages the UMA access network and access from mobile subscribers to voice, data and IMS services from various WLAN locations. Upon authentication of a subscriber request for access to the UMA network, the customer location information is updated and all communication functionality is transferred to the WLAN from the mobile network. This transition or handoff from the cellular network to the WLAN is seamless to the customer during a voice or data session upon detection of an accessible WLAN, and works similarly back to the cellular network upon loss of access to the WLAN.
By taking advantage of the ever growing number of unlicensed WLAN hotspots at home, at the office, and at the marketplace (hotels, restaurants, etc), the UMA option offers mobile operators on the GSM and GPRS paths a convenient and potentially easy way to leverage already available unlicensed spectrum and technology to the advantage of the mobile operators' networks, especially where cellular coverage may be unreliable or insufficient. By providing convergence with broadband access, the operators will open up potential services in these WLAN locations that would otherwise be limited by the lower bandwidth and high latency of the mobile networks. Interestingly, those cellular and PCS customers that are itching to do away with multiple devices for access to fixed and wireless voice, data, and IMS services may be overjoyed to cut the fixed access cord, and stick to a single mobile point of access. Of course, it is also important to ensure that all this can be offered at a cost breakpoint that is attractive to the customer, as promised by the UMA proponents.
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Network operators will have to sort through a number of technical issues to make such a convergence a reality. For more information regarding IMS, UMA, and their issues, please do not hesitate to contact Malick Sohrab.
Glossary of Acronyms:
1xRTT - The first in a series of CDMA2000 1x digital standards (sometime referred to as 2.5 G) designed to evolve IS-95 CDMA voice and data networks along the 3G path.
3GPP - 3rd Generation Partnership Project
CDMA - Code Division Multiple Access
EDGE - Enhanced Data rates for GSM Evolution, a data transport technology associated with advanced GSM-based networks
ETSI - European Telecommunications Standards Institute
EVDO - Evolution Data Only or Optimized - a fully mobile high-speed 3G CDMA data-only standard evolving 1xRTT, CDMA networks to higher data speeds.
GPRS - General Packet Radio Service, a data transport technology used over 2G GSM networks.
GSM - Global System for Mobile Communications
IMS - IP Multimedia Subsystems
IP - Internet Protocol
MMS - Multimedia Message Service
SMS - Short Messaging Service
UMA - Unlicensed Mobile Access
UMTS - Universal Mobile Telecommunications System
VoIP - Voice over Internet Protocol
WLAN - Wireless Local Area Network
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Understanding E911 Technology, Part II
Locating the Caller is the Rest of the Battle
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by Jim Egyud
In the Summer 2005 issue of Rural Signals, I surmised some reasons why a test 911 call on the west side of Manhattan got routed to a PSAP (Public Safety Answering Point) across the Hudson River in New Jersey. Wondering to myself if the public understood the technology enough to know what to make of the somewhat hyped media report of the misdirected call, I provided an overview of how Enhanced 911 (E911) Phase I works, and how some Phase II calls use Phase I mechanisms for deciding which PSAP should receive the call. Using the Phase I methodology, the E911 architecture decides which PSAP gets the call based on the cell site location that picked it up. In this second part of our series, we will look at how E911 calls might route differently using Phase II technology, and how different Phase II technologies attempt to locate the caller.
Phase II Routing By Caller Location
In the previous article, I noted that the area served by a cell site in the metropolitan New York area is typically smaller than the area within the jurisdiction of a PSAP. Thus, it is reasonable to make an immediate routing decision based on serving cell site location, so as to avoid those delays of a few seconds that might occur if the routing decision were to wait for a location fix by the Phase II technology. However, the situation could be reversed, particularly in the case of a rural "high site", where the coverage area extends across a PSAP boundary. In such a case, the caller might be located within PSAP A's service area, but the nearest cell site with usable coverage might be located in PSAP B's service area. In this case, using the Phase I method, the typical system routes the call to PSAP B, which must then transfer the call to PSAP A after learning of the caller's location during the conversation. While many PSAPs prefer this risk over the time lag sometimes associated with the more accurate Phase II routing, the technology continues to improve.
For Phase II, the FCC requires that the call be delivered with an estimate of the caller's location. To do this, wireless carriers utilize a Position Determining Entity (PDE), also known as a Serving Mobile Location Center (SMLC) in GSM networks. When alerted to a 911 call, the PDE starts with the identity of the caller's handset and the cell site, provided by the switch or the GMLC/MPC, to locate the caller in the network. Depending on the type of technology used, the PDE may also use other specific information gleaned from the call. The FCC has set up two categories of Phase II technologies: handset-based, which specifically requires a modified customer handset, and network-based, which relies instead on enhancements to the provider's system and/or cell sites. For the provider, the technology choice can depend on several factors, including its air interfaces, topology, size, and budget.
Phase II Technologies: Survey The Options
Understanding the various Phase II technologies, their applications, and their limitations can be as complicated as, well, finding a needle in a haystack. Fortunately, for CDMA carriers, the choice is usually an easy one. In what has become an industry standard, CDMA carriers in the U.S. typically use a handset-based Phase II technology whereby the PDE must summon handset readings of the GPS satellite constellation, enhanced by CDMA network-measured timing data that provides a form of triangulation to the handset. While network modifications are minimal, usually confined to switch software upgrades, the provider must meet certain benchmarks with respect to activation and penetration of the GPS-capable handsets to its customer base. As often reported in our sister publication, Rural Spectrum Scanner, these benchmarks are proving quite difficult to attain, even for the large carriers, as long-time customers are not always willing to trade in their old reliable, and often more powerful, phones. Moreover, the FCC has placed more stringent accuracy requirements on those providers using the handset-based solution.
While E911-capable handsets have not been developed for the U.S. GSM and TDMA markets, licensees using those air interfaces have a few options among a small handful of network-based technologies:
- The most common technologies, in the Time Difference of Arrival (TDOA) family, triangulate to the caller's position by calculating the differences in how long it took the handset's signal to reach different antenna locations. These technologies typically utilize receivers that tap the received signals from the existing cell site antennas, or can be placed on other tower sites if more measurement points are needed for triangulation and, hence, accuracy.
- Other technologies, such as Angle of Arrival (AOA), calculate the handset's position based on signal phase, from which they derive the angle or direction from the measurement site. Such calculations can rely on fewer receive points, but phase measurements require specialized antenna arrays mounted on a tower or pole. Each element in the array receives the signal's waveform at a different phase; calculations of the phase differences among the antennas reveal the angle from which the signal arrived. While such technologies conceivably require fewer reception points to calculate the handset's location, the additional load of the antenna array and its cabling on a tower must be considered.
- Yet another family of technologies has been categorized as "software-based." Such solutions do not require special signal reception equipment on towers, but compare handset-reported signal levels of multiple cell sites to a previously defined coverage model in order to estimate the handset's location. In other words, using normal call and handover functionality of the network, if the handset reports signal from Sites A, B, and C at different levels, the program looks at the pre-defined coverage model for the locations where the handset might see those sites' signals at those levels. If you think about it, this is really another form of triangulation: if the handset only receives sufficient signal from a single cell (such as in a remote area), a given signal level could be seen in any direction from that site. Therefore, In a similar manner as TDOA-based technologies, accuracy increases as signal is measured from additional sites. The number of possible handset locations decreases as the handset sees specific signal levels from more cell sites.
As a common thread, the accuracy afforded by all network-based technologies depends not only on signal levels, but also on signal strengths associated with more than one antenna site. In the wireless world, sufficient signal strength at multiple sites depends on several factors, including network layout, the proximity of antenna sites to each other, terrain, foliage, antenna configurations . . . I could go on and on. For the rural provider, achieving the FCC's required accuracy levels from a disperse deployment of remote sites or a linear string of sites along a highway can be nearly impossible without deploying additional antenna sites that would not otherwise be needed for adequate coverage to their end users. While the nationwide and other large carriers are well on their way to building out their Phase II solutions, many small carriers face a daunting task: meeting the FCC's accuracy requirements with an eclectic and sparse distribution of cell sites, and doing so in an affordable manner.
What's a Rural Carrier To Do?
For a rural carrier, the best Phase II solution depends on the carrier's unique situation, technology, and, for those needing a network-based solution, their wireless system layout and architecture. Such carriers would do well not to rely on the promotional claims of any vendor, but should consult with those that are knowledgeable of all options and their respective issues. Carriers should also become aware of the "hidden" costs of any solution, including "sniffer" systems that some network-based solutions need to monitor the carrier's network for call data, typically at the Base Site Controller (BSC) or along its transport path.
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The Bennet & Bennet, PLLC team has extensive knowledge of the various E911 architectures, technologies, rules, and cost recovery issues. If you should have any questions about E911 technology, or if you are a carrier wanting to learn more about the options available to you, please do not hesitate to contact Jim Egyud.
Glossary of Acronyms:
CDMA - Code Division Multiple Access
GMLC - Gateway Mobile Location Center - the GSM equivalent of an MPC
GSM - Global System for Mobile Communications
MPC - Mobile Positioning Center, a Service Control Point that matches cell sites with PSAPs, provides E911 call routing information to the mobile switch, and delivers Phase I E911 call data to the PSAP.
TDMA - Time Division Multiple Access
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If you have come across Rural Signals on-line and do not already receive our free quarterly e-mailed version, simply e-mail the Editor, Jim Egyud, by clicking here. Thank you for your interest.
Questions??? Call Rural
Signals Editor Jim Egyud [(202) 371-1500], and refer to
Vol. 1, No. 4.
About Rural Signals
Rural Signals is a quarterly publication of Bennet & Bennet, PLLC's technical consulting service division. Rural Signals is delivered by e-mail four times a year and features technical discussions on current spectrum related happenings affecting rural America. For subscription information or to inquire about specific rural spectrum issues, please call/fax/e-mail Rural Signals Editor Jim Egyud at 202-371-1500 or 202-371-1558 (fax).
While it is our intention to provide valuable information to readers of Rural Signals, the transmission of this newsletter does not create an attorney-client relationship. You should not act upon any information contained in Rural Signals or at www.bennetlaw.com without first seeking the advice of an attorney.
Copyright 2005 Bennet & Bennet, PLLC
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