Table of Contents
Comparison of FDAM TDMA CDMA and SDMA
Comparison of FDAM TDMA CDMA and SDMA: The four multiple-access techniques that are used to increase efficiency in sharing the radio spectrum.
These are as under:
- FDMA, in which users share the spectrum by dividing it into different frequency channels.
- TDMA, in which users time-share the spectrum.
- CDMA, in which all users use the same spectrum simultaneously, but the number of users is limited by their multiple-access interference. (iv)
- SDMA, in which users share the spectrum in angular direction with the use of smart antennas.
The basic Comparison of FDAM TDMA CDMA and SDMA is based on the modulation technique which is used for the modulation of the signals.
Comparison of FDAM TDMA CDMA and SDMA in Table form
|Parameters of comparison||FDMA||TDMA||CDMA||SDMA|
|Modulation||Relies on band-width efficient modulation||Relies on band-width efficient modulation||Simple modulation||Transparent|
|Forward error correction||Increases power efficiency at expense of bandwidth efficiency||Increases power efficiency at expense of bandwidth efficiency||Can be implemented without affecting bandwidth efficiency||Transparent|
|Source coding||Improves efficiency||Improves efficiency||Improves efficiency voice activation advantage||Transparent|
|Diversity||Requires multiple transmitters or receivers||Requires multiple transmitters or receivers can be frequency hopped||Includes frequency diversity when implemented with a RAKE receiver||Simple antenna reduces space diversity orthogonal coding improves diversity with multiple transmit antenna|
|User terminal complexity||Simple||Medium complexity||More complex||Requires smart antennas|
|System complexity||Large number of simple components||Reduced number of channel units||Large number of complex interacting components||Additional complexity related to antennas|
|Multiple-access interference||Limited by system planning||Limited by system planning||Dynamic power control||Limited by resolution of antennas|
|Fading||Flat-fading no diversity simple to track||May need frequency selective may need equalizer||frequency selective diversity via RACK receiver||Reduced multi-path|
|Bandwidth efficiency||Hard limits based on modulation and channel spacing||Hard limits based on modulation and channel spacing||Soft limits||Depends on antenna resolution|
|Synchronization||Low resolution||Mid-resolution||High resolution||Requires terminal location|
|Flexibility||Fixed data rate||Data rate variable in discrete steps||Can provide a variety of data rates without affecting signal in space||Transparent|
|Voice and data integration||Possible, but may require revisions to system||Straightforward using multiple slots||Multi-code transmission, which may decrease efficiency of mobile terminal||Transparent|
|Evolution||Bandwidth to fit application||Requires medium initial bandwidth||Requires large initial bandwidth||Flexible, can be added as needed does not affect mobile|
Although one approach may have a significant technical advantage over another, there may be other factors. Such as economic considerations, that prevent the use of the basic strategy of interest. For example, in a single-user system, the use of CDMA would be difficult to justify, since it requires a very large amount of bandwidth.
In the above table, various properties of the multiple-access techniques have been compared; and some clarification of this comparison with the following comments:
TDMA and FDMA depend on the choice of a modulation scheme to maximize spectral efficiency. To achieve a higher throughput in the same bandwidth, we must use higher order modulation schemes. With CDMA, the simple method of BPSK modulation is required, although on for practical symmetry considerations, QPSK is often used. In fact, the choice of modulation strategy and the use of SDMA are independent.
Forward Error-Correction (FEC) Coding
All multiple-access techniques are affected by the distortions offered by the wireless channel. With FDMA and TDMA, the redundancy introduced by FEC coding requires a higher transmission rate, and thus a greater bandwidth, if the same basic throughput is to be maintained. This is the classic tade-off between bandswidth and power efficiency. With CDMA, FEC coding can be added without increasing the system bandwidth or harming the processing gain. The inclusion of FEC is transparent to SDMA. If transmit diversity is implemented, then there can be increased bandwidth with SDMA.
The use of source coding improves the bandwidth efficiency of all multiple-access techniques. However, CDMA is in a position to take greater advantage of voice activation than are other techniques, since its bandwidth efficiency is determined by average interference.
To obtain diversity with FDMA, we require either multiple transmitter or multiple receivers, both, which is an added hardware expense. The same is applied to TDMA, except when it is used as part of a TDMA/FDMA hybrid. In that case, frequency-hopped TDMA can provide some diversity advantage. The large bandwidth of CDMA naturally provides some frequency diversity, and this can be used advantageously with a RAKE receiver. The implementation cost of a RAKE receiver is less than the dual-receiver cost of an FDMA system with frequency diversity.
User Terminal Complexity
With the progression from FDMA through TDMA to CDMA comes an evolution of terminal complexity. SDMA systems introduce a different and additional form of complexity – one related to the antennas – that is not present in any of the other systems.
With their single-receiver terminals, both FDMA and TDMA are somewhat handicapped when they must switch between base stations at a cell boundary. With CDMA, since the same frequencies are used in adjacent cells, it is easier to implement a “dual receiver” and provide a soft handover capability.
With an FDMA system, users can operate quite independently. With TDMA, the level of cooperation among users must increase to share slots. With CDMA, the system must delegate spreading codes, power control information, and synchronization information.
Multiple-Access Interference (MAI)
Because FDMA and TDMA tend to be limited by worst-case interference, interference is often limited in the system planning stage by the fixed assignment of frequency groups to specific cells. With CDMA, the same bandwidth is used everywhere, and performance is limited by average interference levels. However, CDMA relies heavily on accurate power control to eliminate the near-far problem.
FDMA systems are typically narrowband and therefore suffer from flat fading. If the fading is not severe, then simple channel estimation and forward error correction can often compensate for its effects. TDMA systems are typically medium-bandwidth solutions. Because of this, they observe some frequency selectivity. This requires the implementation of an equalizer. In fact, the implementation of a robust tracking equalizer in wireless channels is of utmost importance. Because of their large bandwidth, CDMA systems face frequency-selective channels, but take advantage of this natural diversity with a RAKE receiver.
For single-cell systems, FDMA and TDMA systems are generally more bandwidth efficient than CDMA systems, because they do not have to cope with multiple-access interference (MAI). However, once their frequency plan is made and the modulation selected, the maximum throughput is fixed. CDMA holds an advantage because it can reuse frequencies everywhere, while FDMA and TDMA have much lower frequency reuse rates because they are limited by peak interference levels. CDMA can often add a user at the expense of a small degradation of e3tisting users.
Wireless systems using FDMA, TDMA, and CDMA show a progression in synchronization resolution and a corresponding progression in complexity. The main concern of FDMA is symbol timing. In fact, TDMA terminals must contend with both symbol timing and slot timing, and CDMA terminals must contend with chip timing.
FDMA is the least flexible of the techniques. Once the service is designed, any change requires a redesign. With TDMA, higher data rates can be provided by assigning more slots per user, usually with very little change to the hardware. With CDMA, deferent data rates can be provided by trading off the spreading rate (processing gain), making it very flexible. However of these techniques are transparent to SDMA.
Voice and Data Integration
The comments regarding flexibility also apply to the integration of voice and data over the same terminal. With TDMA, it is possible as well to make use of periods of voice inactivity to transmit data, thus making the system more efficient. CDMA can easily integrate voice and data, but usually it leads to multicode transmissions, which may reduce the efficiency of the user-terminal power amplifier.
Evolving from a small system to a large system is the easiest with the FDMA approach. We can easily start with a single-user system and remain relatively efficient at each step. With TDMA, start-up efficiency is related to the transmission rate; the system can evolve easily through the addition of more TDMA channels using an FDMA overlay. With CDMA, therein is a large start-up cost, because a large bandwidth to serve perhaps only a few initial user terminals is needed.