I. INTRODUCTIONMobile personal communication systems use Radio Frequency (RF) spectrum that are operating in some predefined or pre-allocated frequency bands (bandwidth) according to the international standards. These frequency bands later will be used for transmission and reception purposes. In order to share the available bandwidth among multiple users, there are three major access techniques used in a wireless communication system:
- Frequency Division Multiple Access (FDMA)
- Time Division Multiple Access (TDMA)
- Code Division Multiple Access (CDMA)
FDMA divides the available bandwidth into individual channels (frequency band) and assigns them to individual users. Thus, each user has unique channel which could not be shared with others. This is considered to be waste of resource since not-in-use FDMA channel will sit idle and cannot be used by other users to increase or share the capacity. Moreover, FDMA was used exclusively in the first generation cellular systems (1G)
TDMA on the other hand, shares the bandwidth by chopping it into time slots and in each slot only one user is allowed to either transmit or receive. In other words, each user takes turns to transmit and receive signals.
Unlike FDMA and TDMA, CDMA enables user to use the same carrier frequency and transmit simultaneously. CDMA does it by spreading signals over the channel in form of pseudo random codewords. Each user has unique codeword. In this scenario, the receiver has to know the codeword used by the transmitter in order to detect the transmitter’s message signals. Thus, each user will communicate independently with each other with no knowledge of the other users.
Furthermore, TDMA and CDMA became the most popular standards of the second generation cellular systems (2G). Particularly, TDMA with its Global System Mobile (GSM) standard, while CDMA with its Interim Standard 95 Code Division Multiple Access (IS-95) standard which also known as cdmaOne.
II. COMPARISONIn this section, we will compare GSM and CDMA in several ways, so we could see them in different points of view.
A. Does It Use FDD or TDD?
As mentioned above, GSM uses TDMA technique in order to share the available bandwidth among multiple users by dividing it into several time slots. In addition, GSM combines TDMA technique with Frequency Division Duplex (FDD); the combination is referred to as TDMA/ FDD. Having this combination, TDMA will use different frequency for the forward and reverse transmission links, so that several time slots of delay will be induced between the forward and reverse time slots for a particular user. Thus, duplexers will not be required in the subscriber unit. Instead, a switch will be required in subscriber unit to switch between transmitter and receiver.
As mentioned in the introduction, CDMA enables users to use the same carrier frequency and transmit simultaneously. Thus, either FDD or TDD (Time Division Duplex) may be used. Since signals are simultaneously sent by users to the receiver, there will be some problem called as near-far problem if signals powers are not controlled. Near-far problem occurs when stronger signals (signals from places which are closer to the base station) decreases the probability of the weaker signals (signals from far away subscribers) to be received. Therefore, a power control is needed on each base station in order to assure that each mobile within base station coverage area provides the same signal level to the base station receiver.
B. How Does It Affect The Battery Life?
Data transmission in GSM occurs in burst, but the time interval between these bursts are so small, so that we will never know that the speech of the other person is discontinuous; hence it will result in low battery consumption and higher talk time. However, high synchronization overhead will be required since the receiver need to be synchronized for each data burst; hence it will reduce the efficiency.
CDMA extends battery life by using effective power control.
C. Number of Users
Numbers of users are limited since there are limited numbers of time slots available. Despite of that, GSM could allocate different numbers of time slots per frame to different users, in other words, it could concatenates or reassigns time slots based on users’ demand.
Nowadays, GSM is already used world wide over 450 million subscribers, almost 8 times in number than CDMA users worldwide.
CDMA can accommodate more users per MHz of bandwidth than any other technology. It has no absolute limit on the number of users. However, system performance degrades for all users as number of users increases (i.e. more user = more noise). In addition, this makes CDMA sounds quality poorer than GSM.
Currently, CDMA covers a smaller portion of the world as compared to GSM which has more subscribers and is in more countries overall worldwide.
Basically, GSM uses 2 levels of encryption, one is for the frequency (prevent unauthorized person from detecting the frequency) and the other one is for the content (if the frequency is detected, it’s still cannot be modulated to voice). However, it is still possible for someone to put fake base station and crack the encryption algorithms.
Moreover, GSM is clone-able. This means somebody can recreate your SIM (Subscriber Identity Module) card codes and can make and receive calls on your behalf. This kind of action is a serious threat for the m-commerce development.
Since CDMA uses the entire spectrum, base station snooping is not possible (too many codes for the hacker to crack, approximately number of trial needs to be done is ‘1 followed by 33 zeros’ times). Thus, it results in CDMA having better security than GSM.
E. Coverage Area (Cell Site Range)
GSM has a fixed maximum cell site range of 35 km, which is imposed by technical limitations. These technical limitations caused by several factors, such as the antenna system, the type of terrain, and the timing advance.
CDMA has a clear advantage at this point. CDMA uses the entire frequency spectrum and hence can have the broadcast happen at very high signal strength spread over bigger radius (approximately 500 feet) than GSM cell radius.
F. Handset Manufacturers
Dominant players in GSM handset manufacture are Nokia and Ericsson, while some examples of dominant CDMA handset manufacturers are Motorola, Lucent, Nextel, and Samsung. Moreover, GSM phones usually come with many new facilities with lesser price than CDMA phones. This happens since manufacturers will likely to invest more in GSM because GSM has 8 times more users than CDMA.
G. Dominant Area
GSM is dominant in much of Western Europe and Asia, while CDMA is dominant in mostly in North America and in some parts of Asia.
GSM came to the market 2 years earlier than CDMA. Thus, GSM has the advantage in access to the European and Asian market since it was introduced earlier. Therefore, GSM has taken the worldwide lead as the technology of choice since the majority of wireless users are in Europe and Asia.
Compared to CDMA, GSM is more mature since its development was started in the mid-80s. This maturity results in a more stable network with robust features, while CDMA is still building its network.
I. Handoff Technique Used
While not listening or broadcasting on its own time slot, the GSM mobile can carry out measurements on the network, detecting surrounding transmitter on different frequencies. This allows safe inter frequency handovers.
CDMA on the other hand, uses Soft Handoff, reducing the likelihood of dropped calls by attempting to make a new connection before breaking the previous connection as you move trough the terrain. However, it can also be used to move a call from one base station that is approaching congestion to another with better capacity.
GSM covers virtually all parts of world so international roaming is not a problem. International roaming permits subscribers to use one phone throughout the entire Europe, Asia except Korea, Africa, Middle East and Australia.
GSM also has more flexibility as to what and where you can use your service. Since GSM uses SIM card, you can remove it from one device such as a regular phone and just insert it in PDA, then you will immediately have the service.
CDMA on the other hand, cannot offer international roaming.
K. Voice & Data Communication
In voice communication, GSM has better sounds quality rather than CDMA. This happens due to CDMA background noises which are actually unrecognized codewords received by the receiver.
In data communication, CDMA is a better choice since channel data rates are very high in CDMA systems. CDMA also has a well-defined path to higher data rates. Thus, CDMA is the stronger candidate to be 3G chosen technology which is broadband access and the use of big multimedia messages.
L. Any Other Consideration
Please note that GSM is a widely spread standard, while CDMA is a patented technology of Qualcomm. Thus, you must pay a royalty for using CDMA.
Note also that in GSM, intellectual property is concentrated among a few industry participants, creating barriers to entry for new entrants and limiting competition among phone manufacturers.
III. CONCLUSIONIn general, we conclude that both GSM and CDMA are frontier technologies and both have advantages and disadvantages.
Furthermore, GSM technology is recommended, if you:
- Use more voice communication than data communication since GSM has better sounds quality and provides higher talk time.
- Heavily mobilized from one region to another region (where your initial wireless service provider does not provide coverage). By using GSM roaming features or by changing to another SIM card, you will immediately have the service.
- Would likely to switch network or handset quiet frequently.
- Prefer to have new facilities which are readily available on certain GSM phones.
Whereas, CDMA is recommended, if you:
- Work more on data communication.
- Mobilized only in certain region under your service provider’s coverage area.
- Prefer to have better security.
Finally, it’s yours to decide which technology best suites your needs.