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Friday, March 18, 2011

mobile broadband: The Femtocell Concept

And exactly what’s a femtocell? In a wider sense, a femtocell is a low-power base station communicating in a licensed spectrum, offering improved indoor coverage with increased performance; functioning with the operator’s approval; offering improved voice and broadband services in a low-cost, technology-agnostic form factor. Providing indoor coverage can be a difficult task, especially due to the propagation path loss of the outer walls of the premises as well as the inter-floor loss. These losses can aggregate to a considerable amount, thus making high-speed 3G data access indoors extremely challenging. Relying on a base station physically located a few kilometers away in distance is not necessarily the best method to effectively deliver high-speed data services to an indoor user— especially since these high-speed data services typically have lower progressing gain and/or use higher-order modulation, such as 64- QAM, to arrive at the high-throughput performance. The small coverage footprint coupled with the friendly indoor propagation environment will create an atmosphere of high SNR to provide improved performance to support multimedia services at a reasonable price target.

While providing superior system quality of service (QoS) performance, improving cellular coverage is absolutely pertinent, although it can be a daunting task when one tries to satisfy not only the outdoor and highly mobile user but also the indoor and leisurely mobile user. The wireless user will encounter a vastly different experience due to the physical nature of the propagation phenomenon. It is well known that the lower frequency bands have better propagation characteristics than the higher frequencies and will allow signals to penetrate buildings to reach the indoor users. Moreover, the lower frequency bands improve the link budget, thus allowing the use of higher-order modulation, lower processing gains, etc., which results in higher data throughput to the user. This is part of the reason for the almost absolute about-face from the technology providers racing toward the higher frequency bands to their attempting to revive the lower-frequency bands such as 450 MHz and 700 MHz.

The femtocell or personal base station concept is realized when a cellular service provider places a base station in the home to not only provide better indoor coverage but also to alleviate traffic from the public macrocells. Hence as a user enters his or her home, the cellular phone will recognize the presence of the femtocell and then register to it. This will alert the public macrocell that any further communication to this user will be via the home ISP network. In this case, your cellular phone can behave as a traditional cordless phone; in other words, in addition to its typical cellular traffic, it will now see the traffic from the home usage. The Femto user is still accessible by the cellular service provider but has freed up resources in the public macrocell that can now be used by additional users that are physically located outdoors. In doing so, the service provider must allow access into their private core network to provide the capability of sending user traffic to the home. This access is provided in the form of a gateway, specifically a femtocell gateway. This provides a dual benefit. First the network operator can now alleviate a fraction of their backhaul traffic to the ISP network. This freed-up capacity will be easily consumed by new users entering the network. The second benefit is to the end user—a higher data rate link can now be established to your phone. Now here is where it gets exciting: a higher data rate will ignite an influx of creative applications to be written for target cell phones.

Below is a sample sample network overview of the femtocell deployment. The homes are expected to have a broadband modem connection (i.e., XDSL, cable, or fiber) through their Internet service provider (ISP) to the Internet. The cellular specific data will be funneled through the femtocell and enter the femtocell (mobile) gateway for access back into the cellular network. For the Third- Generation Partnership Program (3GPP) network, the gateway would interface to the core network; this interface is called Iu-h.

Cellular phones are sold for operation within specific frequency bands, since these phones are meant to operate in private frequency bands for particular cellular service operators. What this means is that the femtocell will not be allowed to transmit in regions where the service provider doesn’t have service nor the rights to that particular frequency band. The owners of these licensed frequency bands are responsible for ensuring emissions satisfy the respective regulatory requirements. Hence, knowing the geographic location of this femtocell is extremely important. This is one particular reason that the femtocells have GPS capability: in order to report back to the cellular service provider the exact location the user is intending to power on the femtocell. This will supply the service provider with control needed to restrict the femtocell’s operation. Moreover, knowledge of the geographic location is also used to support emergency services, as well as lawful interception and a host of other reasons. We wish to quickly follow up by noting that GPS is not the only method available to provide location information; service provider IP addresses and other means are also available. I believe a combination of all of these will lead to an accurate and satisfying experience. Cellular service providers have paid exorbitant prices for the regional licensed spectrum; hence, they have the legal rights to use the spectrum. Moreover, from one service provider to the next the spectrum properties (bands, regulations, etc.) differ not only nationally, but also from one country to another. 

For naming purposes, let us call the network used in the femtocell a private network, while the network used for typical cellular communications a called the public network. Consider the illustration below is an illustration showing the possible combinations of the private and public networks.

Here the public macrocell is shown by a single, large oval coverage area. Within this area are four Home NodeBs (HNB), using the 3GPP nomenclature. They are identified as follows:

• HNB-A is geographically located near the macro-NodeB.
• HNB-B is located near the cell fringe.
• HNB-C is located in an area where cell coverage is spotty.
• HNB-D is co-located with HNB-B.

The HNB-A position is located near the public, high-power NodeB. If the macrocell is using the same frequency band as the private cell, then the downlink of the private and public networks can see an increase in interference. As a result of this increase in the downlink interference, UEs located within the HNB-A coverage will see a degraded downlink from the public macrocell. When moving indoors, however, the public macro-signal becomes attenuated by the factors already discussed, whereas the indoor private femtocell signal is increased. Here the outermost wall is used in a positive manner and extremely welcome. This wall will not only attenuate the signal entering the home from the public cell but also attenuate the signal exiting the home from the private cell to help reduce downlink interference within the private and public networks, respectively.

The HNB-B position is located near the cell edge. If the macrocell is using the same frequency band as the private cell, then we would generally expect to have smaller downlink interference due to the increased propagation loss on the downlink. In this scenario the use of the femtocell has increased the downlink throughput due to the better SNR of the femtocell compared to the public macrocell offering. However, the UE is located at the macrocell edge and will transmit with higher power than the UE associated with the HNB-B. Here the HNB-B uplink will experience a larger rise in interference, since the two UEs are not both associated with the HNB. Here interference mitigation techniques should be applied carefully so as not to allow an increase in the HNB transmit power to overcome this shortcoming, since closed loop power controlled systems have the potential to be unstable (or closely approach it, thus requiring QoS intervention).

The HNB-C position is located at the cell fringe, where we have included the possibility that cell coverage can be nonexistent. If the macrocell is using the same frequency band as the private cell, then the downlink interference is expected to be small, but the uplink can be significant, depending on the location of the public UEs. In this case the femtocell has increased the cell coverage and also improved the available data rates to the end user.
The HNB-D position is located near HNB-B, where we have purposely needed to include interference generated by neighboring femtocells operating in either the same frequency or adjacent frequency. Here both femtocells experience uplink and downlink interference from the macrocell. We must note for multiple cases, however, that the interference from HNB can now deteriorate performance of users in the macrocell. Hence users operating near a few hundred HNBs, for example, may experience some sort of performance degradation within close proximity. The 3GPP standard's group is working diligently to minimize this occurrence. Although this single-cell example was used to convey the potential interference the femtocell would need to overcome, similar issues arise when multicell deployment scenarios are considered. Finally, when the adjacent frequency bands are considered, interference issues still exist and should be carefully planned. Let’s consider the apartment complex scenario where many users are operating within the building and potentially the adjacent complex. Users associated with the macrocell can easily have degraded performance not only outside but also indoors due to the rise in cochannel interference (CCI).

To fully support the femtocell concept, a few components need to be defined: personal base station, handset, ISP, gateway, and cellular network. The figure below provides an example of a single femtocell architecture.

Here we have a single UE communicating to the HNB, which has a coverage area that can extend slightly beyond the home premises. This HNB plugs into a broadband modem to
access the Internet. Access back into the cellular network is available
through the femtocell gateway.

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