Two issues of femtocell

The first big issue for femtocell is increasing coverage.

We know network coverage is limited in mobile network. There are two main reasons for this.

1. Some areas have such a small population that it is not economical for an operator to install and operate a conventional base station

2. The high frequency mobile signals used in 3G systems are heavily attenuated as they pass through walls (3G networks need much stronger signals with low interference to provide high data speeds). In addition, the attenuation means that one indoor user consumes at least the same power as 10 outdoor users. In the other words, even if a user lives in an urban area, they still may not get coverage in all of the home.

Femtocells are significantly cheaper than conventional base stations so they can be installed in consumer's homes and even in the street to boost coverage.

The second big issue for femtocell is mobile broadband.

These days , we consume more traffic data and voice data. With mobile broadband traffic doubling every year, the strain on mobile networks is enormous. While mobile voice usage is approximately constant, data use is rocketing and the growth shows no signs of slowing down. Unlike their fixed cousins, mobile operators cannot simply roll out fibre-optic networks to increase capacity; the rely on radio spectrum, which is an extremely limited resource.

参考：Ingenia Articles

Femtocell Interference Management in real life

Today, I read prof. Simon Saunder article on Femtocell and the following struck me.

A major technical challenge that femtocell designers initially faced was the need to manage potential interference. It takes up to two years to install conventional base stations, during which time radio engineers meticulously plan a station's position and radio characteristics to avoid interference. However, such an approach is not viable in the case of femtocells, deployed potentially in their million at random. Automating a process conducted by radio engineers was no mean feat and simply would not have been possible a few years ago.

Fortunately, the fact that the walls of buildings keep 3G signals out and keep the femtocell’s signals in provides strong inherent interference mitigation for indoor femtocells. Extensive studies have shown that proper implementation of a few key techniques to reduce interference can take advantage of this attenuation in an intelligent manner. Such techniques include frequent monitoring of the cell’s surrounding radio environment combined with adaptive power control. Indoor users gain faster data rates, as do outdoor users who now operate on less congested cells, while it costs less for operators to deliver higher overall network capacity. Large-scale, real-world deployments are demonstrating that these techniques work in practice and even allow new approaches, such as operating 3G networks in the same spectrum as 2G networks.

AT&T has deployed femtocells on the same frequencies as both the hopping channels for GSM macrocells and with UMTS macrocells. They have tested thousands of femtocells, and found that the mitigation techniques implemented successfully minimise and avoid interference. The more femtocells are deployed, the more uplink interference is reduced.

It is very interesting to see that the interference is not causing any problems in real life.

(From today, I will try to introduce more information about femtocell Interference as possible as I can. ^~^ .)

Frequency Selective Fading

In any radio transmission, the channel spectral response is not flat. It has dips or fades in the response due to reflections causing cancellation of certain frequencies at the receiver. Reflections off near-by objects (e.g. ground, buildings, trees, etc) can lead to multipath signals of similar signal power as the direct signal. This can result in deep nulls in the received signal power due to destructive interference. For narrow bandwidth transmissions if the null in the frequency response occurs at the transmission frequency then the entire signal can be lost. This can be partly overcome in two ways.

By transmitting a wide bandwidth signal or spread spectrum as CDMA, any dips in the spectrum only result in a small loss of signal power, rather than a complete loss. Another method is to split the transmission up into many small bandwidth carriers, as is done in a COFDM/OFDM transmission. The original signal is spread over a wide bandwidth thus, any nulls in the spectrum are unlikely to occur at all of the carrier frequencies. This will result in only some of the carriers being lost, rather then the entire signal. The information in the lost carriers can be recovered provided enough forward error corrections is sent.

50 Billion Connected Devices by 2020

In the recently concluded LTE World Summit, this statement seemed to have gained lots of attention. Everyone quoted this left, right and center. The interesting thing was that some said that this would happen by 2025 and some also said 2030.

While we can make a generic statement that there will be some 50 Billion connected devices sometime between 2020 and 2030, not everyone was sure how they would be connected.

My understanding is that a device is connected if it has a valid IP (IPv6) address. That means that the PC's at home are included and anything connected over WiFi are included as well.

So by this definition, it wont surprise me if we probably have 100 Billion connected devices by 2030.

参考　3g 4g wireless blog