Radio Interface Techniques
In wireless cellular systems, mobiles have to share a common medium for transmission. There
are various categories of assignment, the main four include:
FDMA (Frequency Division Multiple Access),
TDMA (Time Division Multiple Access),
CDMA (Code Division Multiple Access)
OFDMA (Orthogonal Frequency Division Multiple Access).
Frequency Division Multiple Access
In order to accommodate various devices on the same wireless network, FDMA divides the
available spectrum into sub-bands or channels. The concept of FDMA is Using this technique a dedicated channel can be allocated to a user, whilst other users occupy other channels, i.e. requencies.
In a cellular system mobiles typically occupy multiple channels; one for the downlink and one
for the uplink. This does however make FDMA less efficient since most data applications are
downlink intensive.
FDMA channels also suffer since they cannot be close together due to the energy from one
transmission affecting the adjacent/neighboring channels. To combat this, additional guard
bands between channels are required, which also reduces the system’s spectral efficiency.
Time Division Multiple Access
In TDMA systems the channel bandwidth is shared in the time domain. It shows how each device is allocated a time on the channel, known as a “timeslot”. These are then grouped into a TDMA frame. The number of timeslots in a TDMA frame is dependent on the system, for example GSM utilizes
8 timeslots.
Devices must be allocated a timeslot; therefore it is usual to have one or more timeslots
reserved for common control and system access.TDMA systems are normally digital and therefore offer additional features such as ciphering and integrity. In addition, they can employ enhanced error detection and correction schemes including FEC (Forward Error Correction). This enables the system to be more resilient to noise and interference and therefore they have a greater spectral efficiency when compared to FDMA systems.
Code Division Multiple Access
The concept of CDMA is slightly different to that of FDMA and TDMA. Instead of sharing
resources in the time or frequency domain, the devices are able to use the system at the same
time and using the same frequency/bandwidth. This is possible due to the fact that each
transmission is separated using a unique code.
There are two main types of CDMA, FHSS (Frequency Hopping Spread Spectrum) and DSSS
(Direct Sequence Spread Spectrum), with all the current cellular systems utilizing DSSS.
The basic concept of CDMA. The narrowband signals are spread with a wideband code and then transmitted. The receivers are designed to extract the encoded signal (with the correct code) and reject everything else as noise.
UMTS, cdmaOne and CDMA2000 all use CDMA. However the implementation of the codes
and the bandwidths used is different. For example UMTS utilizes a 5MHz channel bandwidth,
whereas cdmaOne uses only 1.25MHz.
Orthogonal Frequency Division Multiple Access
OFDMA (Orthogonal Frequency Division Multiple Access) is the latest addition to cellular
systems. It provides a multiple access technique based on OFDM (Orthogonal Frequency
Division Multiplexing).
It can be seen that the bandwidth is broken down to smaller units known as “subcarriers”. These are grouped together and allocated as a resource to a device. It can also be seen that a device can be allocated different resources in both the time and frequency domain.
In wireless cellular systems, mobiles have to share a common medium for transmission. There
are various categories of assignment, the main four include:
FDMA (Frequency Division Multiple Access),
TDMA (Time Division Multiple Access),
CDMA (Code Division Multiple Access)
OFDMA (Orthogonal Frequency Division Multiple Access).
Frequency Division Multiple Access
In order to accommodate various devices on the same wireless network, FDMA divides the
available spectrum into sub-bands or channels. The concept of FDMA is Using this technique a dedicated channel can be allocated to a user, whilst other users occupy other channels, i.e. requencies.
In a cellular system mobiles typically occupy multiple channels; one for the downlink and one
for the uplink. This does however make FDMA less efficient since most data applications are
downlink intensive.
FDMA channels also suffer since they cannot be close together due to the energy from one
transmission affecting the adjacent/neighboring channels. To combat this, additional guard
bands between channels are required, which also reduces the system’s spectral efficiency.
Time Division Multiple Access
In TDMA systems the channel bandwidth is shared in the time domain. It shows how each device is allocated a time on the channel, known as a “timeslot”. These are then grouped into a TDMA frame. The number of timeslots in a TDMA frame is dependent on the system, for example GSM utilizes
8 timeslots.
Devices must be allocated a timeslot; therefore it is usual to have one or more timeslots
reserved for common control and system access.TDMA systems are normally digital and therefore offer additional features such as ciphering and integrity. In addition, they can employ enhanced error detection and correction schemes including FEC (Forward Error Correction). This enables the system to be more resilient to noise and interference and therefore they have a greater spectral efficiency when compared to FDMA systems.
Code Division Multiple Access
The concept of CDMA is slightly different to that of FDMA and TDMA. Instead of sharing
resources in the time or frequency domain, the devices are able to use the system at the same
time and using the same frequency/bandwidth. This is possible due to the fact that each
transmission is separated using a unique code.
There are two main types of CDMA, FHSS (Frequency Hopping Spread Spectrum) and DSSS
(Direct Sequence Spread Spectrum), with all the current cellular systems utilizing DSSS.
The basic concept of CDMA. The narrowband signals are spread with a wideband code and then transmitted. The receivers are designed to extract the encoded signal (with the correct code) and reject everything else as noise.
UMTS, cdmaOne and CDMA2000 all use CDMA. However the implementation of the codes
and the bandwidths used is different. For example UMTS utilizes a 5MHz channel bandwidth,
whereas cdmaOne uses only 1.25MHz.
Orthogonal Frequency Division Multiple Access
OFDMA (Orthogonal Frequency Division Multiple Access) is the latest addition to cellular
systems. It provides a multiple access technique based on OFDM (Orthogonal Frequency
Division Multiplexing).
It can be seen that the bandwidth is broken down to smaller units known as “subcarriers”. These are grouped together and allocated as a resource to a device. It can also be seen that a device can be allocated different resources in both the time and frequency domain.
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