Fetmo Access Point: Is the interference problem a big Red Herring?

If Femtocells were to operate on a frequency that is shared with the macro network in a restricted mode (i.e. only specific users can access a particular Femtocell), there is a concern that interference between the two layers may create a doomsday scenario.

For some reason, many think that the uplink interference from Femto users will jam the macro recievers and cause macro uplink coverage to deterioate. Some technologists and vendors have responded to this challange by developing extremely complex techniques. I have seen documents on both FemtoForum and 3GPP talking about things like:
- an interface between the macro layer and femtocells to exchange radio specific contexts.
- A "cognitive pilot channel" proposal that seems to be at least 15 years ahead of its time.
- Proposals to use uplink interference cancellation technology in femtocells.
- Suggestions that only LTE access points and release 8 (or 9) handsets will eliminate the interference problem.

All of the above is very interesting for a technologist. Ofcourse there is always a way to make things better, but there are also other considerations: value added, cost, market segment, technology application, market drivers ...etc, otherwise we would be all driving a Ferrari or a Porsche.

Looking at this interference issue, I can't help point out a number of things:
- I believe that more than half of femtocell deployments will end up using a dedicated carrier.
- In the absence of a femtocell, the user will go on the macro network and will create an amount of noise rise anyway. When you put a femtocell close-by the user will radiate less power and create less noise rise to the macro site. There is at least a 25-30dB reduction in path loss, so the femto users will radiate much less power than if they are using the nearest macro cell.
- A residential femtocell will only support a very limited number of users, so the interfernce limit ishardly ever reached and additional uplink interference from macro users operating in the vicinity will hardly block the femtocell uplink.

Is is possible that theso called interference problem is nothing but a big red herring?

Advanced Cellular Technologies Episode 4: Evolution of UMTS/HSDPA

3GPP is constantly thinking of ways to improve the performance of UMTS to compete with new emerging technologies that promise enhanced performance over legacy systems.

There are two evolution philosophies within 3GPP. One philosophy is start from scratch and design a new system with new architecture and air interface which do not have any legacy constraints. Another philosophy is to fine-tune existing architecture and introduce advanced features to the existing air interface to enhance its performance to the best level possible while maintaining backward compatibility. The driver for this second approach is to capitalize on existing infrastructure investment. While 3GPP is devising strategies to evolve UMTS on the long term to achieve a set of target performance goals, 3GPP is also thinking of ways to develop the existing UMTS standard to reach its full potential.

Near-term 3GPP enhancements
The near-term enhancements to UMTS are addressed in 3GPP Rel’6 and Rel’7. These primarily focus on the issues directly related to market deployment and the customer experience and to fine-tune and incrementally improve the performance of UMTS earlier standard releases to ensure that UMTS/HSDPA products and services are of high quality, perform correctly and are as fully featured as intended. Some of the goals of these enhancements are:

- Call set-up delay reduction for improved end-user experience.
- Signalling flow optimization.
- Support of real-time packet services such as VoIP.
- Increased system efficiency and overhead reduction.

Medium-term HSDPA evolution: HSPA+
HSPA+ is a manifestation of the evolved HSPA philosophy where the existing HSPA implementation is pushed to its limits by introducing various improvements to increase the efficiency of the system while maintaining backward compatibility. Some of these improvements are:
- Higher order modulation
- The use of advanced receiver such as Equalizers and IC.
- Possibly, the introduction of MIMO receivers and Receive diversity.

The main driver for HSPA+ is to capitalize on existing HSPA investment in infrastructure by focusing on backward compatibility and upgrade simplicity.

Long Term Evolution (LTE) of 3GPP
The main objective for LTE is to enhance the 3GPP standard to become a highly competitive packet-based radio access technology. 3GPP promises huge increases in performance and capacity with LTE. From a performance point of view, the main goals of LTE are:

Flexible spectrum usage with scalable system bandwidth from 1.25 MHz up to 20 MHz
Increased spectrum efficiency and peak data rates at cell edge. Target peak rates of 100 Mbps/DL and 50 Mbps/UL.
Reduced latency for both user and control plane: less than 10ms round trip delay for user plane between UE and the serving RAN node, less than 100ms transition time for control plane between inactive state and active state.

LTE philosophy is more “revolutionary” than HSPA+ in scope. In order to achieve the ambitious goals set for it, LTE takes a fresh look at system architecture and air interface access without the constraints of legacy systems. Therefore system architecture will noticeably change with a new radio access layer.

Although many aspects of LTE are still under discussion in 3GPP, some of the main attributes are:

• OFDM-based air interface (OFDM=Orthogonal Frequency Division Multiplexing).
• Flat IP system architecture. (Often this is described as SAE: System Architecture Evolution, and is a separate study item in 3GPP).
• Higher level modulation and state of the art receiver technology.

Advanced Cellular Technologies Episode 2: Steered Beams

Steered beams are essentially the combination of two technologies: Beam-forming and beam steering.

Beam-forming is achieved by using phased array antennas to create a narrow beam directed at each mobile in the downlink. The antenna elements form a phased array antenna that have very small spacing (usually half a wavelength). By virtue of the small spacing, transmitted signals from the array antennas are added constructively in one particular direction and are added destructively (or nullified) in all other directions. The net effect is a narrow pencil-shaped beam. In contrast, transmit diversity antennas have to be spaced at multiple wavelengths to reduce the correlation between the propagation paths and produce a diversity gain.

Beam steering is achieved by altering the signal phases that feed each of the antenna array elements. This alters the direction in which the signals are added constructively. Feedback information from the mobile terminal is required to continuously alter the phases in such a way that makes the beam follow the mobile as it is moving.