ECE 525E - GSM Cell Planning Quiz

Explanation

Frequency reuse is the fundamental concept in cellular networks that allows the same set of frequencies to be used in different geographic areas (cells) separated by sufficient distance to keep interference within acceptable limits. This concept dramatically increases system capacity compared to a single transmitter serving an entire area.

TDMA and CDMA are multiple access techniques, while FDD is a duplexing method. While TDMA is used in GSM, it is not the concept that enables frequency sharing across different geographic areas.

Explanation

The co-channel reuse ratio (q) is defined as the ratio of the distance between co-channel cells (D) to the cell radius (R). This ratio determines the level of co-channel interference in the system. A higher q value means less interference but also lower capacity (since frequencies are reused less frequently).

For hexagonal cells, q is related to the reuse pattern N by the formula: q = √(3N)

Explanation

For a reuse pattern with cluster size N=7, the co-channel reuse ratio is calculated as:

q = √(3N) = √(3×7) = √21 ≈ 4.58

The N=7 pattern (with i=2, j=1) is the classic GSM reuse pattern that provides a good balance between capacity and interference. It means that the same frequency is reused only once in every 7 cells, with the distance to the nearest co-channel cell being approximately 4.58 times the cell radius.

Explanation

The capacity of a cellular network is inversely proportional to the reuse factor N. A smaller N means frequencies are reused more often, resulting in higher capacity. N=3 provides the most aggressive frequency reuse among the options, offering the highest capacity.

However, smaller N values come with increased co-channel interference. N=3 (often implemented as 1x3 reuse with frequency hopping) is typically used in dense urban areas where capacity is more critical than interference mitigation.

Explanation

The Erlang B formula calculates the probability that a call will be blocked due to all channels being busy (blocked calls cleared system). A Grade of Service (GoS) of 2% means that during the busiest hour, 2 out of 100 call attempts will be blocked because no channels are available.

This is different from call dropping (which happens during an active call) or poor voice quality. Typical GSM networks aim for 1-2% blocking probability during busy hours.

Explanation

Cell splitting involves dividing a congested cell into smaller cells, each with its own base station. This increases capacity because:

1. Smaller cells serve fewer subscribers
2. Frequencies can be reused more often in a given area
3. Transmit power can be reduced, decreasing interference

Sectorization uses directional antennas to divide a cell into sectors, while cell splitting physically creates new, smaller cells. Frequency hopping and channel borrowing are other techniques but don't involve creating new cells.

Explanation

With a reuse factor of N=7, the total available channels are divided among 7 cells in a cluster. Therefore:

Channels per cell = Total channels / N = 124 / 7 ≈ 17.7

In practice, the channels would be distributed as evenly as possible, with some cells getting 17 channels and others getting 18. This calculation assumes all channels are available for traffic, though in reality some channels are reserved for control functions.

Explanation

The Okumura-Hata model is an empirical propagation model derived from extensive measurements in urban areas. It's widely used for GSM macrocell planning (cell radii 1-20 km) in the frequency range 150-1500 MHz.

Key features of the Okumura-Hata model:

• Takes into account frequency, base station height, mobile height, and distance
• Has corrections for urban, suburban, and rural environments
• Simple to use and computationally efficient
• Extended by COST-231 for frequencies up to 2 GHz

Explanation

Sectorization (typically 120° with 3 sectors per cell) provides two key benefits:

1. Interference Reduction: Directional antennas reduce the number of interfering cells from 6 (in omnidirectional cells) to 2 (in 3-sector cells)
2. Capacity Increase: The same set of frequencies can be reused more aggressively, effectively increasing capacity by a factor of about 2.5-3 compared to omnidirectional cells

Sectorization does not increase coverage area (it may actually slightly reduce it due to antenna patterns) and requires careful frequency planning for each sector.

Explanation

1x3 frequency reuse is an aggressive reuse pattern where:

• "1" indicates that each frequency is reused in every cell
• "x3" indicates that cells are divided into 3 sectors (120° each)

This pattern is possible with frequency hopping, which randomizes and averages interference. In practice, not all frequencies are used in every cell; a subset is assigned to each cell with careful planning to minimize adjacent channel interference.

1x3 reuse provides high capacity but requires precise power control and advanced interference management techniques.

Explanation

When cell radius is halved (R → R/2), the area of each new cell becomes:

New area = π(R/2)² = πR²/4 = (Original area)/4

Thus, 4 new cells fit into the area of the original cell. Assuming uniform traffic distribution, each new cell carries approximately 1/4 of the original traffic, so the total capacity in the area increases by a factor of 4.

In practice, the increase may be slightly less due to increased handovers and the need for guard channels between cells.

Explanation

While many factors are considered in cell site selection, proximity to residential areas is generally NOT an advantage for the following reasons:

• Residential areas often have zoning restrictions against towers
• Residents may have concerns about health effects (though scientifically unfounded at typical power levels)
• Maintenance access is not significantly easier in residential areas
• Land costs may be higher in residential areas

Key factors in site selection include: coverage requirements, terrain, availability of infrastructure (power, backhaul), zoning regulations, environmental impact, security, and total cost of ownership.