The designing and standardizing of the cellular systems involve the selection of the multiple access method. Basically, there are three types of multiple access schemes that are TDMA, FDMA and CDMA. In this article, we are going to differentiate between TDMA and CDMA. The significant difference between TDMA and CDMA is that TDMA permits the user to share the channel based on the time slots whereas CDMA distribute the channel by code.
All this is related to the signal multiplexing, in which multiple signals are combined and transmitted over the single channel. Then, what is multiple access? Multiple access can be considered an application of the multiplexing as multiplexing allows the multiple users to access a frequency bandwidth.
Content: TDMA Vs CDMA
|Basis for comparison||TDMA||CDMA|
|Expands to||Time Division Multiple Access||Code division Multiple Access|
|Technique||Sharing of time||Sharing of bandwidth and time|
|Data transmission rate||Medium||High|
|Cost||Low||Installation cost is high maintenance cost is lowest.|
|Code words||Does not use any code words.||Code words are utilized.|
Definition of TDMA
TDMA expands to Time Division Multiple Access which is a technique for voice-oriented networks such as cellular and PCS (Personal Communication Services). It is used for allocating the resources on the predetermined basis to the users for a particular interval. The time is an important entity in this technique because the same frequency band distributed between the several users on the basis of time.
The TDMA technique is founded on the TDM (Time Division Multiplexing) commonly used in the trunks of the telephone system, we have discussed TDM in the previous article, the difference between TDM and FDM. Now, how does it works? The first operation is performed by the time controller where the time slots are allocated to the users until the user releases it. At the receiver end, the TDM synchronizes the signal frame and retrieves the designated slots from the TDM. Synchronization is an important part of the TDMA.
In the older technologies, FDMA is more popular and used in the 2G networks, which is later replaced by the TDMA technology. TDMA carries out hard handoff where the call of the user moving from one cell to other, drops and immediately connected to the network the is user travelling.
Definition of CDMA
CDMA (Code Division Multiple Access) is a combined technique of the TDMA and FDMA where the resources are allocated on the basis of frequency as well as time. In FDMA the frequency band is divided among the more than one user for the whole session. While TDMA allows each user to utilize the entire band of frequency but for a fixed session. However, CDMA uses the features of both the techniques, where numerous users simultaneously use the same frequency band that is distinguished by a code.
These codes are chosen, so that if the code is used second time the user knowing the code can detect that specific user among the various users. This technique is highly susceptible to noise where each user is a source of noise for the receiver which makes it limited for a certain number of users. Because as the number of the user increases the amount of noise also increases; as a result, the system can be collapsed.
There are two variations of CDMA, if the forward and reverse channel frequencies are different then the technique is CDMA/TDD and if the frequencies are same then the technique known as CDMA/FDD. The CDMA is mainly used in the 3GPP2 standards.
Key Differences Between TDMA and CDMA
- In TDMA technique user acquire the whole frequency but for the allotted time period. Conversely, CDMA modulates the data bits with the help of the orthogonal sequence of bits to disperse the signals over the large frequency band and split the users space according to the code.
- CDMA offers higher transmission rates as compared to TDMA.
- The overall cost of CDMA is higher than the TDMA. However, the operational cost is lowest in CDMA.
- TDMA performs hard handoff while CDMA carries out soft handoff.
- The area covered by CDMA is larger than the TDMA.
- CDMA is more flexible relative to TDMA.
- The transmitting and receiving end must be synchronized in TDMA. In contrast, CDMA does not need synchronization.
Advantages of TDMA
- It can easily accommodate data and voice transmission.
- TDMA is immune to interference to some extent generated by concurrent transmission.
- It can offer data rates from 64 Kbps to 120 Mbps.
- Cost effective technology for digital systems.
- It also saves battery life.
Advantages of CDMA
- The dropouts only occur when the user is located at the double distance from the base station.
- It provides a very high spectral capacity to accommodate large number of users in MHz of bandwidth.
- To lessen the background noise an EVRC vocoder is used.
- Provides more secure transmission.
Disadvantages of TDMA
- It is prone to multipath distortion.
- The implementation of the time slot scheme is proved to be adverse sometimes, as the user is allotted a predefined slot, the user will not be connected to call if all the time slots of the current and another cell in which user is entering are occupied.
Disadvantages of CDMA
- Channel pollution is one of the major issue present in the CDMA, where more than one cell sites can exist in the user’s phone but neither of them is powerful.
- The absence of international roaming capabilities.
- Restricted variety of the handset.
The bandwidth efficiency offered by CDMA is extremely higher than the TDMA. CDMA is more secure, robust, and cover a more extensive area, but when it comes to battery life of the devices the TDMA provides higher battery life because it does not transmit data continuously while CDMA constantly transmits the data.