3.5 GHz Small Cell NPRM

About a year back, the FCC issued a notice of proposed rulemaking (NPRM) inviting comments on its plan of establishing what it calls "a new Citizens Broadband Service in the 3550-3650 MHz band". The 65-page detailed proposal can be accessed from this link, with some of the key points being:

• 100-150 MHz of contiguous spectrum being made available with small cells as a target application.
• A three-tiered shared access model managed by a spectrum access system (SAS). The FCC cites dynamic database as a starting point for SAS but is soliciting other interference mitigation ideas.
• The tiered shared access model is based on Qualcomm's Authorized Shared Access (ASA) and consists of the following three access tiers: 
• Incumbent Access: Only authorized federal and legacy 3.5 GHz Band users
• Priority Access: Critical, QoS dependent users at specific, targeted locations. One of the key aspect that FCC is seeking comments on is "who these eligible users should be" and they suggest it could include hospitals, utilities, state and local governments
• General Authorized Access: Wide range of residential, business, and others, including wireless telephone and Internet service providers
• Specific interference mitigation schemes, deployment strategies, and the design of the SAS database has not been specified, and are areas where the FCC is seeking comments. 

As expected, the NPRM has attracted a lot of attention from various businesses, industry groups, researchers, and individuals. Till date, more than 200 comments have been filed; a few interesting tidbits about the comments filed: 

• Google has submitted, by far, the most number of comment documents - 12 so far, totaling more than 120 pages!
• Most wireless service providers and their component suppliers are arguing for opening the Priority Access tier to wireless/mobile service providers. 
• Several comments, including those from Alcatel Lucent, WiMax Forum, and BLiNQ Networks have made the case for including small cell backhaul as a major use-case for the spectrum
• Notably, Microsoft has argued for ensuring that the Priority Access tier is limited to mission-critical users. It has requested the commission to make sure "that the licenses do not provide any single company or set of companies with excessive control over the spectrum."
• TV Broadcasters, including CBS, Fox, Time Warner, Viacom, and Disney have a problem with the FCC's plans. It says the new band "would be very problematic for incumbent C-Band (3700-4200 MHz) users, which generally includes the entire video distribution industry -- both broadcast and cable -- throughout the United States."

It seems like the FCC is quite active in trying to finalize its proposal for the 3.5 GHz band and is organizing, among other initiatives, a workshop to discuss the "Creation of a Spectrum Access System" on Jan. 14th, 2014. The key details that remain to be seen are which entities will be eligible for the Priority Access tier and what restrictions will be applied to the General Access users. 

Internet.org

After cringing at the stereotypical videos of poor kids on bikes, a barber doing his job, and tea plantation workers on Internet.org, I was trying to find out what exactly do they have in mind beyond the hyperbole of "making internet access available to the next 5 billion people". 

It turns out, most of the technical stuff they are considering are pure wireless stuff, including topics such as using spectrum more efficiently and utilizing white space bands. Some key point from Facebook's white paper: 

• Wireless repeaters: "...when an operator broadcasts a signal, it loses fidelity as it penetrates building walls. This not only requires operators to build out much more infrastructure in greater proximity than should be necessary, but it also means that data needs to be retransmitted when the signal breaks down, and the overall infrastructure is taxed by sending the same data multiple times."

• Edge Caching: "It is also possible to build technology that caches data inside an operator’s data center and makes it faster and cheaper for the operator to serve that data. "

• White Space: "Specifically, there is a policy movement to reallocate spectrum that has been used as a buffer around TV broadcasting."

• Reduce data usage in apps: "At Facebook, we’re investing heavily in opportunities to reduce our overall data use and help other apps reduce their data use as well. Some of the areas we’re focused on are caching, data compression and simple efficiency optimizations."

• Data caching: "People with feature phones are very cost conscious, so one of the most important things we’ve done has been to make this experience use as little data as possible by caching data efficiently so we can be very careful about which data we ever have to request from our servers."

• Data Compression: "Implementing compression in large scale apps or developing services that you route all your data through and compress everything would yield large data use savings."

• Efficient coding to reduce data: "Since most developers of large scale services are based in developed countries where data usage is a less important aspect of performance than, say, speed or server efficiency, we’ve found that many frequently used apps have had little or no data usage optimization."

• Peer-to-Peer Wireless: "There are also more speculative approaches we’re investigating, including enabling people to download some News Feed stories and photos from their friends’ nearby phones over Wifi Direct and other local network technologies."

• Zero-rating data: "We’ve already seen results where attaching free data for Facebook — what we’ve historically called zero-rating — increases both phone sales profits and data plan profits."

Looking at these (still hazy) details, and the list of partners, it seems like the Internet.org initiative will be more about incremental technical advancements in the general area of wireless networks and mobile Internet. In any case, I think this initiative is only partly inside Facebook's current comfort zone and we might see a lot of false starts and new developments as they get more comfortable with the whole providing-cheap-connectivity space.

What TV White Space might do for LTE and the cellular industry

While most of the early discussions/papers/press-releases on TV White Spaces (TVWS) described the new available spectrum as being useful for local networking akin to Wi-Fi or for M2M communications, it was only time before the biggest stake holders in all things spectrum - the cellular operators (and others in their ecosystem)  took a keen interest in it. An example of their growing interest in the topic is the following stats from the just concluded 3GPP Radio Access Network (RAN) Plenary which had presentations from a variety of stake holders (providers, network equipment suppliers, handset vendors, etc.) about the things they feel are important for LTE Release 12 and beyond:

• More than 15 companies talked about leveraging unlicensed band for use with cellular technologies.
• More than 6 companies specifically discussed possibilities related to TVWS.

Key reasons for the new found interest: 

1. Quite obviously the acute spectrum crunch: providers are looking at all means for sending more bits/sec to their customers and TV White Spaces offers high capacity 'beach-front' quality spectrum at little cost of ownership.

2. LTE PHY/MAC design: The time-frequency resource block (RB) based resource allocation in LTE is quite suitable for TVWS band operation. Resources can be assigned to users with fine granularity (1 RB = 180 KHz); an occupied TV band between two unoccupied ones can be easily skipped by just blacklisting the corresponding RBs; and being a scheduler based system, the dynamism in the available spectrum can be handled by gracefully lowering capacity as opposed to a Wi-Fi system where availability of less than 20 MHz (1 channel's worth) of spectrum means no operation at all. 

3. Increasing heterogeneity of cellular deployments: With micro/femto deployments gaining momentum, there is a need for new technological solutions on both the access and backhaul side of things. So while replacing licensed spectrum would be far-fetched in traditional macro-cells, it can be considered from the ground-up in these brand-new kinds of deployments. 

In this light, TVWS is being talked about in the 3GPP and the SDR Forums with regard to a no. of different use cases such as unlicensed band off-loading, last-mile backhaul links for small cells, for access connections in rural areas, and for device-to-device communication assisted by the cellular operator. Going forward, it will be interesting to see which of the use cases gains more momentum and what new co-existence challenges it brings to other technologies operating on the TV White Space bands.  

A few interesting links

Technical paper proposing/simulating the use of TVWS for mixed macro/femto deployments: Using TV White Space for Interference Mitigation in LTE Femtocell Networks 

Huawei's announcement of TD-LTE trial over TVWS: Huawei to Launch TV White Space LTE TDD System Trial

Blog post highlighting the alignment of LTE and TVWS: LTE and TV White Space, Related set of slides: link

Technical paper studying the use of LTE femotocells for indoor coverage over TVWS band: Interference Study for Cognitive LTE-Femtocell in TV White Spaces

European Commission sponsored technical paper on the technical challenges required to extend LTE for TVWS operation: Extension of LTE Operational Mode over TV White Spaces

Technical paper on a cognitive TD-LTE framework for operation over TVWS: CR Enabled TD-LTE within TV White Space: System Level Performance Analysis

Detailed technical paper from Ericsson research on the feasibility of license-exempt LTE operation over TVWS: License-exempt LTE systems for secondary spectrum usage: Scenarios and first assessment

CogEu feasibility report on LTE over TVWS (page 53): Cognitive radio systems for efficient sharing of TV white spaces in European context

TV White Space devices come to life

Late last year, FCC approved Spectrum Bridge's database service and KTS Wireless' device for operations in the TV White Space in Wilmington, NC. This comes after some months of uncertainty over whether there will be any unlicensed white space at all !  Although that danger seem to have been averted, the exact bands which will be made available for unlicensed use still remains unclear.

The KTS Wireless device is stated to operate at only 1.5-3.1 Mbps for a 6 MHz channel and does not seem to use any of the emerging white space standards. Primary applications right now are video surveillance, public safety and wireless Internet. While this will provide a proof-of-concepts for the database-backed spectrum use regime, the real test will be when many different devices operate through the use of many different database providers.

Meanwhile the WiFi Alliance is not too happy about TV White Space being called Super Wifi !

On ice skating and open bands !

Unlicensed bands are like skating rinks

If you have never seen or heard about a public skating rink, you could think the whole idea was crazy - toddlers, grandparents, teenagers, speed maniacs, all without helmets or knee-pads skating in a common area with no lanes, minimal rules and no guarantees that you won't get hurt! But the fact is, it does work and the reason it works is precisely the absence of specific rules.

Open wireless bands such as the 2.4 GHz ISM bands too have a large common spectrum-area, different kinds of participants - high bandwidth WiFi, narrow band but hopping natured Bluetooth, etc. It has a few transmission rules and most importantly just like the skating rink, no guarantees about interference from other users. You can and probably will collide but overall the system seems to work fairly well.

The idea of spontaneous order

Daniel B. Klein speaks about the same skating rink analogy with respect to society and economy. He argues that "intuition leads us to think that complex problems require complex, deliberate solutions. In a roller rink, the social good depends on getting the patterns to mesh. But no one is minding that good..... but in promoting my interest in avoiding collision with you, I also promote your interest in avoiding collision with me." And that is probably the key: coincidence of interest

Imagine a "rink master" in the rink who sits at the center and communicates instructions to individual skaters - 'Move right in 2 seconds', 'Increase your speed by 5mph' or 'Shift lane in 5 seconds'. Such a system could only work if the rink master precisely knows the capabilities and desires of each participant, not to mention the unwieldy task of calculating the optimum decisions for hundreds of people and communicating it to them in time.

This idea directly ties with the centralized vs distributed decision debate in wireless communications. And from the skating rink analogy, we can see that if we want to accommodate a large number of devices with widely varying access capabilities and data rate or delay requirements, an open free-for-all approach can work well with even very limited guidance. The coincidence of interest in avoiding collisions and the statistical multiplexing of available data help create spontaneous order in the wireless domain.

What's essential for co-existence

There are two important things that minimizes accidents in the skating rink:

1. A basic set of rules: 'No pushing or sudden stopping', 'No loose articles of clothing', 'Fixed direction of movement', etc.
2. Visibility: More than anything, the reason why we don't see constant collisions is that everyone can see what others are doing and decide what's the best action to take in response.

In unlicensed band operation, we have an analogous set of rules that each device has to adhere to but sometimes the visibility of what others are doing is very limited. In particular what I think would really help promote co-existence is a basic sense of the wireless environment - more than what the device can see itself like what is the channel occupancy on other channels, what kinds of devices are operating nearby, how much of the channel are they occupying etc. Listen before talk and other MAC schemes on those lines, essentially make use of the local visibility and delay their transmissions based on what they see on the channel. However the end-result is far from optimum. Bianchi's analysis, for example, shows that there is a specific transmission probability at which the optimum value of the system throughput is achieved. And this transmission probability is a function of the number of participants in the interference range of a device, which is an unknown from the point of view of each device.

The final point

The key point that I want to convey here is that even though decentralized, distributed decisions seem to be the best solution for unlicensed operation, there could be great gains by providing each network/device a certain sense of the wireless neighborhood through external means (a control channel ?). Without an improved co-existence mechanism between secondary devices in the TV White Space, the whole ecosystem of this new unlicensed band could be in trouble.

Startups in the TV White Space domain

While a host of big companies are already involved in various aspects of TV White Space (see a recent survey on TV White Space Cognitive Radio Patent portfolio); a stream of startups are emerging with an eye on the TVWS applications. Brief info on some of them:

1. Spectrum Bridge: Formed in 2007, is leading the field trials for TVWS deployment and is one of the Database Admins assigned by FCC. Focus: database, application software, value added services, and full network deployment

2. Adaptrum: Small Silicon Valley Startup, formed in 2004 with ties with UC Berkeley. Built one of the 1st White Space hardware for FCC approval along with the big players - Microsoft, Philips and Motorola. Focus seems to be hardware implementation but they also mention real-time resource monitoring, automated resource management, and self forming and optimizing networking capability in their company profile.

3. Neul: UK based startup founded in 2010. Aims to use the TV White Space for machine-to-machine communication like smart-grid, asset tracking, vehicular systems, remote health monitoring etc. Team looks impressive, not much impact as yet.

4. Key Bridge Global: A 2001 company which joined the TVWS bandwagon as a database administrator, though the revenue model is unclear for all the database providers. Have created a subsidiary called dsa, with some generic details about TVWS on the website for now.

A long list of interested parties can be obtained from the current members list of Wireless Innovation

Links to links

List of blogs, literature reviews on similar topics:

• Blog from Cognitive Radio Technologies, James Neel and Jeff Reed - Very informative, lots of relevant links, updated regularly.

• Blog from Gonzalo Vazquez Vilar, Researcher from Spain

• Blog from Michael Marcus, popular name in the wireless/spectrum related domains

• Blog from Spectrum Bridge, leading the initial deployments and database handling

• Blog from Amit Jain, VP, Product Management, Femtocells at Airvana