Macro, Micro, Femto and Pico Cells

A macrocell is a radio coverage cell in cellular networks. The coverage distance varies depending on the frequency, bandwidth of the signals and physical obstructions in the area. Macrocell antennas must be properly mounted on ground-based masts, rooftops or other existing structures and at heights for an unhindered, clear view of the surroundings. Its performance can be increased by increasing the efficiency of the transceiver. Macrocell base stations have power outputs of typically tens of watts. Since this type of cell offers the largest coverage area, it is placed in stations along highways and rural areas where large stretches rarely have service within a few kilometers.

A microcell is served by a low power base station, which covers a limited area such as a mall, a hotel, or a transportation hub. A microcell is usually larger than a picocell but the distinction is not always clear. A microcell uses power control to limit the radius of its coverage area. The range of a microcell is usually less than two kilometers whereas standard base stations may have a range of up to 35 kilometers.

A picocell is regular base station with lower transmit power than macro cells. They are usually equipped with omni-directional antennas, i.e., not sectorised and are deployed indoors or outdoors, often in a planned (hotspot) manner. The transmit power ranges from 250 mW to 2 W for outdoor deployments and 100 mW or less for indoor deployments. The range of a picocell is 200 meters or less.

A femtocell (also called a home base station) is usually a consumer-deployed (unplanned) network nodes for indoor application with a network backhaul facilitated by the consumer's home digital subscriber line (DSL) or cable modem. It is equipped with omni-directional antennas and the transmit power is 100 mW or less. The range of a femtocell in on the order of 10 meters. A femtocell may be classified as open or closed depending or whether it allows access and usage of the consumer's home DSL /cable modem to all terminals, or a restricted set of terminals.

Femtocell networks offer the following benefits.

• Better coverage and capacity. Its short transmit-receive distance means femtocells can significantly lower transmit power, prolong headset battery life and achieve a higher signal-to-interference-plus-noise ratio (SINR). These, in turn, results in improved reception and higher capacity. The reduced interference means more users can be packed into an area in the same spectrum region, thus increasing the area spectral efficiency. Femtocells allow reduced transmit power while maintaining good indoor coverage.
• Improved macrocell reliability. If the traffic originating indoors can be absorbed into the femtocell networks using the IP backbone, the macrocell base station can redirect its resources toward providing better reception for mobile users.
• Cost benefits. Femtocell deployments reduce the operating cost and capital expenditure costs for operators. the cost of a macrocell network is untenable when subscriber growth does not match the demand for data traffic. Deploying femtocells reduces the need for adding macro-BS towers.
• Reduced subscriber turnover. Poor in-building overage causes customer dissatisfaction, why may lead to them switching operators. The enhanced home coverage provided by femtocells reduces home user dissatisfaction.

The capacity benefits of femtocells are due to:

• Reduced distance between the femtocell and the user, which leads to higher received signal strength,
• Lower transmit power and mitigation of interference from neghbouring macrocell and femtocell users due to outdoor propagation and penetration loss,
• A femtocell serves between one to four users. Therefore, it can allocate a larger portion of its transmit power and bandwidth to each user. This is unlike a macrocell with a much larger coverage area and a larger number of users, which makes it more difficult to guarantee QoS. Femtocells allow more efficient usage of limited power and frequency resources.

The first two points involves improving capacity through increased signal strength and reduces interference.

Source
V. Chandrasekhar, J.G. Andrews, "Femtocell Networks: A Survey", IEEE Communications Magazine, Vol 46 Issue 9, September 2008

Cellular Networks

The basic technology for mobile phones and personal communication systems is a cellular network. A service area is divided into cells. A base station is located in the middle of each cell.

Each cell is allocated a frequency and adjacent cells are allocated a different frequency to avoid interference. The transmitting power is controlled to avoid interference with neighbouring cells. The same frequency is used in non-adjacent cells – this is known as frequency reuse.

When the number of users in a cell increases, the frequency allocated to the cell is no longer enough. There are a number of approaches to overcome this problem.

• Add new channels: Usually, when a system is set up, not all channels are utilised. Growth and expansion is systematically managed by adding new channels when the number of users increases.
• Frequency borrowing: A congested cell borrows frequencies from adjacent cells.
• Cell splitting: Cells in areas with high density traffic are divided into smaller cells. For this purpose, the power level must be reduced to limit signal transmission to within one cell.
• Cell sectoring: A cell is divided into 3 or 6 sectors and each sector is allocated a subset of channels for that cell. Involves replacing a single omni-directional antenna with 3 directional antennas (120^o sectoring) or 6 directional antennas (60^o sectoring). Each sector uses a directional antenna at the BS and is assigned a set of channels. The number of channels in a sector is the number of channels in a cell divided by the number of sectors. Reduces transmission power, longer battery lifetime.