1. What are the main advantages and disadvantages of the cellular concept?


Advantages:

  • Total bandwidth available to the system MB Hz
  • If there is only one high power base station, the system can serve M users simultaneously
  • Due to low transmitted power the same set of frequencies can be reused in another cell situated far enough to avoid interference
  • If the same set of frequencies is reused K times, the system can serve KM users simultaneously
  • Due to small size of the cell not only the BS but also the mobile station can transmit with lower power (longer battery operation) giving lower interference between cells

Disadvantages:

  • Cellular concept is very complex due to the impossibility of dividing the area into equal size cells.
  • To decrease the co-channel inference in the cell area, sectorization is used. This has its drawbacks:
    • It requires more Base Station equipment.
    • There is an increase in control signalling due to the Main Station changing sector.
    • The efficiency of trunking decreases – set of channels now divided among the sectors – no of served users will remain the same as in unsectorized cell only if the no of users located in each sector is proportional to the no of channels assigned to each sector

2. What are the methods of increasing capacity of cellular systems?

The methods of increasing capacity:

Cell sectorization:

  • decreases the S/I
  • the clusters can consist of smaller no of cells
  • the available channels can be used more frequently in the same area.
  • increases the reuse factor by 1/N resulting in higher system capacity.
  • Price

-    tripled no of antennae and BS and

-    more frequent handover between cells and sectors.

-    decreases he trunking efficiency unless the users are distributed evenly in all sectors

Cell splitting:

  • Subdividing the congested cell into smaller ones
  • Each subcell has its own BS
  • The height and power of antenna is reduced
  • Increases system capacity since channels are reused more often
  • If cell radius is R/2 the area is reduced by 4
  • To cover the area we need 4 times more cells
  • Therefore no of channels per area is increased by 4
  • Larger cells used in the areas with low traffic intensity, smaller in areas of high demand for capacity

Using microcell zones:

  • Increased no of handoffs results in an increased load on the switching and control links elements of the system
  • Lee proposed a concept of microcell zones
  • Cell is divided into 3 or more zones all served by the same BS
  • Zones connected by coaxial cable, optical fiver of microwave links
  • Mobile travelling within the cell is served by the zone with the strongest signal
  • Superior to sectorizing as zones are placed at the edge of the cell and any channel can be assigned to any zone

3. What mechanisms are used in GSM in order to overcome problems associated with radio transmission (e.g. signal fading) Name 3?

1. Frequency Hopping

  • In cells where users have severe multipath problem
  • Slow frequency hopping – on frame-by-frame basis
  • Max. rate 217.6 hopes per second
  • There can be up to 64 frequencies in a hopping sequences

2. Equalisation

  • Performed by receiver using information from the training bits transmitted in mid-amble if every TS

3. Enhanced Full Rate Encoder

4. What are the main advantages of using CDMA?

CDMA has increased capacity over other methods. The no. of users in CDMA is around 5 times higher than no. of users in traditional FDMA systems.

Due to utilisation of the RAKE receiver and the same frequency band in neighbouring cells, the Main Station can receive signals simultaneously from 2 cells( at the border between cells). This is known as a soft handover and can be considered a type of path diversity.

CDMA is useful to the military as it hides the presence of signal transmission. To the external observer, these signals look random.

The whole spectrum is used in each cell.

There is a precise power control by the Base Station sending command to the Main Station. This aids controlling inference.

5. What is the main capacity limiting factor in CDMA? How can it be overcome?

  • Inference is the main capacity limiting factor in CDMA. Using the appropriate processing can eliminate inference. Interference in CDMA is reduced using:

—       Speech activity detector (as in GSM)

—       Strong error correction coding and interleaving – reduces required signal to noise ratio (SNR) – therefore transmit power can be reduced and

  • Reduction of interference increases the capacity of the system
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