1. Reuse Distance:
The first mobile radio systems were actually noise-limited systems with few users. Therefore, it was advantageous to put each BS on top of mountains or high towers, so that it could provide coverage for a large area. The next BS was so far away that interference was not an issue.
2. Cell shape :
The most natural choice would be a disk (circle), as it provides constant power at the cell boundary. However, disks cannot fill plane without either gaps or overlaps.
Hexagons, on the other hand, have a shape similar to a circle and they can fill up a plane, like in a beehive pattern. Thus, hexagons are usually considered the “basic” cell shape, especially for theoretical considerations. We stress, however, that hexagonal structures are only possible under the following circumstances:
• The required traffic density is independent of the location. This condition is obviously violated whenever the population density changes.
• The terrain is completely flat and there are no high edifices, so that path loss is influenced only by the distance from the BS.
3. Cell Planning with Hexagonal Cells :
For the case of hexagonal cells, some interesting conclusions can be drawn about the relationship between link margin and reuse distance.
Consider the hexagon whose center is at the origin of the co-ordinate system. Proceed now hexagons in the y direction, turn 60◦ counter clockwise and proceed hexagons in that new direction.
4. Methods for Increasing Capacity:
•Increasing the amount of spectrum used.
•More efficient modulation formats and coding.
•Better source coding.
•Discontinuous Voice Transmission.
•Adaptive modulation and coding.
•Reduction of cell radius.
•Use of sector cells.
•Use of an overlay structure.
•Partial frequency reuse.
1.7 Multiple Access Schemes
It is often desirable to allow the user to send simultaneously information to the base station while receiving information from the base station.
A cellular system divides any given area into cells where a mobile unit in each cell communicates with a base station. Its main aim is to be able to increase the capacity of the channel. i.e., to handle as many calls as possible in a given bandwidth with a sufficient level of quality of service.
There are several different ways to allow access to the channel. These includes mainly the following:
1) Frequency division multiple-access (FDMA)
2) Time division multiple-access (TDMA)
3) Code division multiple-access (CDMA)
4) Space Division Multiple access (SDMA)
FDMA,TDMA and CDMA are the three major multiple access techniques that are used to share the available bandwidth in a wireless communication system.
Depending on how the available bandwidth is allocated to the users, these techniques can be classified as narrowband and wideband systems.
Frequency Division Multiple Access(FDMA):
FDMA is the initial multiple-access technique for cellular systems in which each individual user is assigned a pair of frequencies while making or receiving a call as shown in Figure.
One frequency is used for downlink and one pair for uplink. This is called frequency division duplexing (FDD). That allocated frequency pair is not used in the same cell or adjacent cells during the call so as to reduce the co channel interference.
FDMA is usually implemented in a narrow band system. The symbol time is large compared to the average delay spread. The complexity of the FDMA mobile systems is lower than that of TDMA mobile systems. FDMA requires tight filtering to minimize the adjacent channel interference.
•FDMA/FDD in AMPS
•FDMA/TDD in CT2
• The transmitter (TX) and receiver (RX) require little digital signal processing. However, this is not so important in practice anymore, as the costs for digital processing are continuously decreasing.
• (Temporal) synchronization is simple. Once synchronization has been established during the call setup, it is easy to maintain it by means of a simple tracking algorithm, as transmission occurs continuously.
• If an FDMA channel is not in use, then it sits idle and cannot be used by other users.
• The bandwidths of FDMA channels are narrow (30kHz).
• Intersymbol interference is low.
• It needs only a few synchronization bits.
• FDMA systems are costlier because of the single channel per carrier design.
• It need to use costly bandpass filters to eliminate spurious radiation at the base station.
• The FDMA mobile unit uses duplexers since both the transmitter and receiver operate at the same time. This results in an increase in the cost of FDMA subscriber units and base stations.
• FDMA requires tight RF filtering to minimize adjacent channel interference.
Time division multiple-access (TDMA):
In digital systems, continuous transmission is not required because users do not use the allotted bandwidth all the time.
In such cases, TDMA is a complimentary access technique to FDMA. Global Systems for Mobile communications (GSM) uses the TDMA technique. In TDMA, the entire bandwidth is available to the user but only for a finite period of time.
TDMA requires careful time synchronization since users share the bandwidth in the frequency domain. The number of channels are less, inter channel interference is almost negligible. TDMA uses different time slots for transmission and reception. This type of duplexing is referred to as Time division duplexing(TDD).
•TDMA/FDD in GSM
•TDMA/TDD in DECT
TDMA shares a single carrier frequency with several users, where each user makes use of non-overlapping time slots. TDMA uses different time slots for transmission and reception.
Adaptive equalization is usually necessary in TDMA systems, since the transmission rates are generally very high as compared to FDMA channels.
The preamble contains the address and synchronization information that both the base station and the subscribers use to identify each other.
TDMA Frame structure
• Trial bits specify the start of a data.
• Synchronization bits will intimate the receiver about the data transfer.
• Guard Bits are used for data isolation.
Efficiency of TDMA:
The efficiency of a TDMA system is a measure of the percentage of transmitted data that contains information as opposed to providing overhead for the access scheme.
Then, the frame efficiency is
Code division multiple-access (CDMA):
In CDMA, the same bandwidth is occupied by all the users, however they are all assigned separate codes, which differentiates them from each other (shown in Figure).
CDMA utilize a spread spectrum technique in which a spreading signal (which is uncorrelated to the signal and has a large bandwidth) is used to spread the narrow band message signal.
The narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal (pseudo-noise code). The chip rate of the pseudo-noise code is much more than message signal. Each user has its own pseudorandom code word.
Direct Sequence Spread Spectrum (DS-SS):
This is the most commonly used technology for CDMA. In DS-SS, the message signal is multiplied by a Pseudo Random Noise Code.
Each user is given his own code word which is orthogonal to the codes of other users and in order to detect the user, the receiver must know the code word used by the transmitter.
Hybrid Spread Spectrum Techniques:
The hybrid combinations of FHMA, CDMA and SSMA result in hybrid spread spectrum techniques that provide certain advantages.
Hybrid FDMA/CDMA (FCDMA): An alternative to the CDMA technique in which the available wideband spectrum is divided into a smaller number of sub spectra with smaller bandwidths.
CDMA uses CO-Channel Cells. All the users use the same carrier frequency and may transmit simultaneously without any knowledge of others. The receiver performs a time correlation operation to detect only the specific desired code word. All other code words appear as noise.
•Low power spectral density.
•Signal is spread over a larger frequency band.
•Other systems suffer less from the transmitter.
•Interference limited operation.
•All frequency spectrum is used.
•The code word is known only between the sender and receiver. Hence, other users cannot decode the messages that are in transit.
•Reduction of multipath effects by using a larger spectrum.
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