Question 1
There are three main multiple access technique which are the FDMA, TDMA and CDMA. List out and describe the advantages and disadvantages of the 3 multiple access techniques.
Advantages TDMA
• Permits flexible bit rates by assigning more slots per frame to a certain user.
• Can support bursts or variable bit rate traffic.
• No guard bands required for wideband system.
• No narrowband filters required for wideband system.
Disadvantages of TDMA
• Each user has a predefined time slot. However, user roaming from one cell to another are not allotted time slot. Thus, if all the time slot on the next cell is already occupied, a call might disconnect and user will not receive a dial tone.
• Subjected to multi-path distortion. However, a time limit can be used on the system to solve the problem. Once the time limit is expired the signal is ignored.
• Higher synchronization overhead because of burst mode operation.
• Complexity equalization necessary for high data rates of wideband systems.
• Frequency/slot allocation complexity
• Pulsating power envelop: interference with other devices
• Electronics operating at high bit rate increase poser consumption.
Advantages CDMA
• CDMA can support many users in the same channel, example: high capacity.
• Lower mobile transmit power, example: longer battery life and better power control.
• Improved performance in multipart environments, RAKE receivers can be used to improve signal reception by exploiting diversity.
• Soft handoffs can be used. Mobile can switch base station without switching carries. Two base stations receive the mobile signal and the mobile is receiving from two base stations.
• High peak data rates can be accommodated.
Disadvantages of CDMA
• One major problem in CDMA technology is channel pollution, where signals from too many cell sites are present in the subscriber’s phone but none of them is dominant. When this situation arises the quality of the audio degrades.
• Another disadvantage in this technology when compared to GSM is the lack of international roaming capabilities. The ability to upgrade or change to another handset is not easy with this technology because the network service information for the phone is put in the actual phone unlike GSM which uses SIM card for this.
• One another disadvantage is the limited variety of the handset, because at present the major mobile companies use GSM technology.
• The code length has to be careful selected because a large code length can induce delay or even cause interference.
• CDMA also requires tight power control as it suffers for far-near effect.
• Soft handoff increases use of radio resources and hence can reduce capacity.
Advantages FDMA
• Reducing the information bit rate and using efficient digital codes can increase the capacity.
• As FDMA systems use low bit rates (large symbol time) compared to average delay spread, they reduce the cost, and there is low Inter Symbol Interference (ISI).
• There is hardly any equalization required.
• Technological advances required for implementation are simple. A system can be configured so that improvements in terms of speec coder bit-rate reduction could be readily incorporated.
• Since the transmission is continuous, less number of bits are needed for synchronization and framing.
Disadvantages of FDMA
• It does not differ significantly from analog systems, capacity improvement depends on reducing signal to interference ratio, or signal to noise ratio (SNR).
• The maximum bit rate per channel is fixed and small.
• The guard band between each two users results in wastage of capacity.
• Hardware involves narrow band filters, which cannot be realized in VLSI and thus increase the cost.
• Law call capacity
• Lack of security where a call could be overhead with considerable ease and one could steal a phone’s serial code and make illegal calls on the systems.
Question 2
In 3G cellular network, the two popular standards are WCDMA and CDMA-2000. List out of all the similarities and differences of the two technology.
CDMA is a complete cellular system which was employed for 2G and now for 3G networks. CDMA-2000 is the version of CDMA system for 3G. WCDMA is the physical layer technology adopted for 3G UMTS system. The difference between CDMA and WCDMA in term of physical layer spreading is that in CDMA system signal is spread up to 1.25 MHz while in WCDMA signal is spread up to 5MHz band. Hence extra "W" meaning "wide" is used and hence called as WCDMA.
The biggest difference between WCDMA and CDMA-2000 is the origin. WCDMA is an evolution of GSM, while CDMA-2000 is an evolution of CDMA. WCDMA uses a broader spectrum and the signalling is based on GSM. CDMA's signalling is somewhat related to TDMA signalling. Both use Code Division to get the packets in sequence and both offer similar data speed and voice capabilities.
