LTE rollouts are now happening all across Asia and have the potential to completely reshape how networks perform. Many LTE networks incorporate a technology called multiple-input multiple-output (MIMO), which splits data transmission into multiple streams and sends them at the same time on the same frequency using multiple antennas. The expression 2×2 MIMO means that there are two antennas transmitting in the downlink to two antennas receiving in the handset.
What makes this development so exciting is that MIMO offers a way around a classic limiting factor of RF communications known as Shannon’s Law, which dictates how much throughput can be delivered down a given amount of bandwidth. As Figure 1 shows, you can only expect to get to within 3dB of a bandwidth’s theoretical maximum in a practical application. With 2×2 MIMO you can potentially double the capacity over a traditional 3G implementation, which otherwise would be bound by Shannon’s law.
For LTE systems to maximise the potential of MIMO, interference must be minimised, making sector sculpting even more important for new 4G, LTE networks. Today, wireless consumers demand faster speeds and uninterrupted calls. In order to achieve a robust network that provides prompt, reliable data transmission with seamless voice capabilities now and well into the future, these networks will need to utilise the right antenna and sector sculpting techniques.
Shannon’s Law dictates that you can only expect to get to within 3dB of a bandwidth’s theoretical maximum in a practical application.
Let’s start with the basics of the antenna and why it is so important. The antenna is one of the most critical parts of a wireless system and is often the most visible. Antennas come in all shapes and sizes, because each one is built for a specific purpose but they share a common link: they are the key to how well – and how far – communications can be shared.
What makes cellular networks different from other types of communications is the principle of cell reuse. Cell reuse is a way of increasing network capacity by “reusing,” or reassigning, individual frequencies on the fly, within a particular cell.
Consider the shape of cells and how they fit together. Typically, cells are represented as interlocking hexagons. Depending on the density of the area served, these hexagons can be miles across or cover just a few hundred feet.
Because of this incredible flexibility of distance, channel sensitivity is limited by external interference rather than noise issues, as older radio communications have traditionally been. The specialised pattern shaping of sector sculpting, available with directional antennas, both in azimuth (horizontal direction) and elevation (vertical space), enables precise coverage with minimal interference with neighboring cells.
A critical performance metric considers the relationship from one cell to its neighboring cell. This metric defines the overlap of energy between the cells and between the sectors that make up the cells. Known as sector power ratio, this is a comparison of signal power registered outside and inside a desired receiving area as a consequence of an antenna’s radiation pattern. The lower the ratio, the better the antenna’s interference performance.
Higher sector power ratios indicate a higher amount of interference between antennas in adjacent coverage areas. When competing signals overlap, interference can increase and reduce performance – such as dropping a cell phone call while moving from one cell to another. Cellular networks require precise sectorised planning to prevent this kind of problem.
The cellular network concept requires either frequencies or codes to be re-used over and over again throughout the network in order to support the massive amount of voice and data traffic. Operators use sector sculpting techniques to minimise interference between cells operating on the same frequencies or codes. Low levels of interference containment result in long distances between cells using the same frequency resources. By containing interference with sector sculpting, frequency resources can be reused closer to each other, whilst increasing spectrum efficiency, capacity and network performance.
As the cell sites become denser, the coverage area from each cell is typically decreased to reduce cell to cell interference. One way of decreasing the coverage area from a cell site is to lower the antenna radiation center. However, this is often undesirable since it could place the antenna below many surrounding obstacles such as buildings or foliage. Since most cellular networks now use sector antennas, a second option is beam tilting. This is done by tilting the vertical pattern of the antenna downwards, reducing the coverage on the horizon—where interference to neighboring cell sites takes place.
The easiest way to accomplish this is to mechanically tilt the entire sector antenna using adjustable brackets available from most antenna suppliers. However, a major drawback is that it does not reduce coverage consistently on the horizon over the sector. It reduces it more in the boresite direction and less at other angles away from the boresite, so the result is not a consistent reduction of the cell coverage. A quantifiable term for this phenomenon is known as “pattern blooming.”
Advanced networks typically deploy a more elegant method of tilting the sector antenna’s vertical pattern using electrical downtilt. This method keeps the antenna mounted upright and accomplishes the beam tilting by changing the electrical phase delivered to each element. The resulting pattern is tilted consistently over the entire 360 degrees yielding consistent reduction of the cell coverage. For electrically downtilted antennas, there is no increase in pattern blooming regardless of the amount of tilt.
Another benefit of electrical downtilt is that it can be accomplished remotely in advanced antenna systems. This feature will become even more important as more sophisticated technologies, such as LTE, mature.
One option associated with LTE is called self-organizing network (SON) where the network will continually re-optimise itself based on demand. The dynamic coverage adjustment associated with electrical downtilt is a key feature in the deployment of the SON concept.