Even though the title is about femtocells, let me first cover flying or floating picocells. This ties back to my earlier blog about Lufthansa’s FlyNet service.
Using mobile phones on planes has long been a desire of many. Even though a lot of people find the period between closing of the cabin doors and landing at the destination a respite from others’ expectations for constant communication, there are many who genuinely believe that they need to be able to communicate whenever and wherever they want and this includes airplanes.
Emirates Airlines started offering this service on select airplanes as of 2008. Since its introduction service adoption has been quite significant. A recent article suggests Emirates’ service provider Aero Mobile was expecting to serve its 5th Million customer sometime last week (before Feb. 18th 2011). Over 220 Emirates flights per day provide GSM service. Aero Mobile has recorded as many as 227 registered phones on a plane. Certainly being registered doesn’t mean making or receiving a phone call or SMS. Looking at a presentation Aero Mobile CEO made back in 2009, traffic levels of 400-500 SMS for a 7-hour flight have been observed. Furthermore, Aero Mobile can easily increase the adoption rate by trimming the prices. As an example, introductory prices of $0.20 per SMS with Malaysian Airlines apparently generated a lot of traffic.
Flying picocells in Europe are regulated by Electronic Comunications Committee of European Conference of Postal and Telecommunications Administrations (CEPT). Back in 2006 after consultations CEPT has made a number of decisions on the use of airborne GSM systems (ECC/DEC/(06)07):
- Mandated the use of DCS1800 only
- Required that all devices on board are prevented from registering with networks on the ground
- Limited the use of devices to the period when the aircraft is at least 3000 m (~10,000 ft) above ground
- Put restrictions on radiated power from the on-board system as well as the terminal devices at various altitudes
Primary reasons for using DCS1800 only are the lower minimum handset transmit power (0 dBm compared to 5 dBm for GSM900) and higher attenuation through the fuselage as well as space between the airplane and ground systems. Since almost all European GSM handsets support both GSM900 and DCS1800 this restriction didn’t seem to be impacting adoption. Restricting the use of WCDMA seemed acceptable, especially in 2006, when satellite links limited backhaul speeds especially for the uplink direction. With the current rules in place it is possible to support EDGE for packet data traffic.
In order to prevent mobile devices from registering with ground-based networks, CEPT decision recommended either relying on a combination of fuselage loss and restricting usage to even altitudes higher than 3000 m or using an active jammer system, a euphemism called Network Control Unit (NCU) on the plane to overcome the signal a mobile device will receive from a ground base station.
Resulting system is similar to one shown in the following figure taken from an ETSI White Paper on this topic.
A leaky coax cable antenna laid out through the fuselage carries signal from the pico BTS and jamming signals from NCU. NCU power level is set such that it is 12 dB lower than the pico BTS power level. Furthermore, the combined power level of pico BTS and NCU for 1800 MHz would not exceed -13 dBm at 3000 m. This level is important since there may be multiple airplanes circling over an airport at the same time. Objective is by restricting the radiated power level from the airplane, it will be possible to reduce the likelihood of interference among multiple planes.
First Europe and then many parts of the world solved the problem of mobile communication in airplanes by simply mandating the use of one wireless standard and one frequency band. On the other hand, in the US a similar effort to place picocells on board have failed. Primary reason for this was the diversity of mobile communication standards in the US including CDMA, GSM and IDEN. (I exclude WiMax since there is no single-mode WiMax device.) In order to provide equal access, an airplane needs to be fitted with a picocell that can support all technologies. Considering the challenges of spectrum bands versus technologies, settling on one single frequency band (most likely PCS1900) and just targeting CDMA2000 and GSM would provide access to almost all mobile customers. However, FCC has decided to continue keeping its ban on using mobile phones in planes. On the other hand, FCC has allowed multiple systems using WiFi on board for broadband services. Multiple companies including Aircell, OnAir and Row 44 have started providing WiFi based Internet access. OnAir and Row 44 are similar to Aero Mobile with a satellite based backhaul. Aircell has quite a different system relying on ground-based towers with antennas pointing towards the sky, using EV-DO at 2+2 MHz in 800 MHz band. Considering EV-DO is a capped system in terms of its technology evolution, it is quite interesting to see what Aircell (aka GoGo) will adopt next. Apart from the choice of backhaul link, biggest benefit of operating a WiFi system is eliminating concerns about the interference that required significant regulation as CEPT has provided for GSM. We will come back to this topic.
Following its success of regulating flying picocells, CEPT has done the same for floating picocells, those base stations that are placed on board marine vessels. In a 2008 decision (ECC(09)093R1) which was later adopted by European Commission, it included the following:
- Allowed the use of both GSM and UMTS at 900 and 1800 MHz spectrum
- Limited the operation of the ship picocell to the periods when the ship is at least 2 nautical miles from the shore
- Allowed the use of only indoor antennas
- Required that Timing Advance (TA) for the picocell to be set to minimum
- Required that handsets on board while communicating with the ship picocell will not exceed 5 dBm (900) and 0 dBm (1800) transmit power
- Required that disconnection threshold will be set to as high as – 70 dBm
- Mandated that in open areas of a ship measured signal level will not exceed -80 dBm
In order to prevent land based mobiles accessing the ship’s picocell main techniques are the use of TA and a very high power level for disconnection threshold. Furthermore these will also prevent mobiles on a nearby ship to use a picocell on another ship which is not a desired form of communication. By restricting the allowed radiated power levels on board (both for mobiles and picocell), CEPT directives target a significant reduction in interference to land based base stations and mobiles. Following picture from the CEPT report describes the use of positioning to control the operations of the on board picocell.
In the US floating picocells have been a reality for a long time. As early as 2006-2007, all major cruise lines have equipped their ship with on-board systems. These systems are operated by carriers such as AT&T, Verizon (for US cruise lines) and allow other carriers to roam on their networks. T-Mobile with its UMA capable handsets has a distinct advantage over other carriers as long as on-board WiFi coverage is reliable and has no restrictions for UMA traffic, which is quite unlikely.
Unlike strict European regulations restricting the operations of on-board systems to areas beyond 2 nautical miles off the shore, US doesn’t seem to have any similar restrictions. I believe FCC treats this as a matter mobile operator owns and manages since they re-use their own spectrum for on-board systems. This was clearly a factor when Wayne Burdick, an AT&T customer, received a $28,067.31 bill after he watched a Chicago Bears football game on his computer according to an FCC filing by National Consumers Group on the topic of bill-shock. At the time, Burdick was on board a cruise ship docked at a U.S. port. Burdick’s AT&T mobile broadband card had connected to the ship’s microcell rather than the local preferred tower.
Lessons for User-Deployed Femtocells
European (CEPT) regulations on the use of flying and floating picocells are clear indications of the potential impact of interference between such systems and macro network. In case of user deployed femtocells, a more challenging environment exists since femtocells are managed by the end-user as opposed to the operator, their use is restricted to a closed user group, such as handsets in the household or small office. This prevents a macro network user being able to use the femtocell even when it is the best available cell. In such a situation the worst impact is inability to communicate with the macrocell due to interference from the femtocell. Such a problem is more likely for 3G or 4G networks when the wide-band nature of the spectrum requires a frequency re-use factor of 1, hence higher likelihood of interference. That’s why best femtocell deployments are limited to two scenarios:
- Used as a coverage enhancement solution outside the macro-coverage area (analogous to 2 nautical miles or 3 kilometers altitude rules for floating and flying picocells)
- Managed by the operator as opposed to the end-user (analogous to flying or floating picocells where all handsets are allowed and furthermore actively encouraged (remember on-board jammer) to communicate with the femtocell)
Due to the restrictions I listed above femtocells will be limited to coverage-enhancement solutions. Recent announcement from Telstra outlines the same argument for 3G. Similarly for operators that are short on 3G spectrum (limited to 2-3 carriers), it is not conceivable to allocate spectrum for femtocells to avoid interference.
In previous blogs I argued about the feasibility of WiFi traffic offload to solve the capacity crunch. Complementary approach is the deployment of operator managed high-density solutions. Especially with LTE, with the ability to dynamically allocate OFDM pilots as granular as 1-ms basis it will be possible to shift the capacity among multiple layers of a wireless network. Recent announcements from Qualcomm, ALU, Ericsson are all proposing various forms of heterogenous networking to solve the capacity crunch. This is a topic I will continue to cover since it will be the foundational change for communication in this decade.