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In the field of mobile communication, the user requirements and demands are ever increasing. 3G mobile system has been partially able to fulfill the requirement in terms of higher bandwidth, seamless connectivity, and the most important of all quality of service. So, to cope up with these limitations, various researches are being carried on and 4G technology has been proposed. 4G is all about the next generation of mobile communication technology where the present available wireless communication technology will be integrated to provide seamless connectivity to the users with the benefit of higher bandwidth and quality assured services. This paper discusses in brief about the generations of communication technology implemented till date and evaluates on the failure of third generation of mobile communication technology to fulfill today's technological requirements .This paper specially focuses on the medium access technology used in fourth generation of mobile communication technology to overcome the limitations of 3G and serve the purpose of higher bandwidth and quality of service to the users.
3G, 4G, OFDM, CDMA, MC-CDMA, DS-CDMA
TODAY'S period is the world of communication and people demand their information sharing to be prominent and qualitative to the most. 3G is the latest technology serving till date with its highest possible capabilities. But it is not enough to fulfill current demands like higher bandwidth in data transmission rate and QOS (Quality of Service) in terms of HQ videos and HQ voices. Also the present generation demands seamless connectivity so that they are not interrupted in the middle of communication and QOS is ever maintained. All these issues have forced the researchers to come up with the technology to meet up the demands of users in terms of areas like high speed in data transfer, quality of service and uninterrupted connectivity. So, the engineers have proposed 4G as the next generation of promising technology with characteristics like higher bandwidth, quality of service, seamless connectivity among any available wireless communication technology. 4G is not a completely new technology but the seamless integration of current available terminals, networks and applications.
This journal is arranged in the following manner. Section II tells in brief about generations of mobile technology. Section III and IV describes about the characteristics and technology implementation of 4G mobile technology. Section IV is especially focused on various medium access technology proposed in 4G to serve higher bandwidth, security and less error in data transmission. Section V shows tabular description of various technology and features of all the mobile generations. Section VI gives the conclusion and section VII finally concludes the journal by notifying tasks that are needed to be implemented in future to enable 4G to successfully put up into operation in the market.
II. Generations on mobile communication
i. First Generation:
1G was designed during 1970'a and based on the analog system. This generation introduced the basic framework of mobile communication like basic architrcture, frequency multiplexing, roaming concept etc. This generation saw two key improvements: the invention of the microprocessor and the digitization of the control link between the mobile phone and the cell site.1G is based on TDMA and FDMA technology in which voice is the main traffic but calls could be made only within a country. The data transmission is synchronous.
ii. Second Generation:
2G digital cellular systems were developed at the end of 1980's. This technology prioritized on voice transmission and also implemented the digital system in this generation. The digital system added feature of SMS (Short Message Service). This generation was also marked revolutionary as it shifted the mobile generation from analog to digital. The new system provided better quality and higher capacity at lower cost to consumers and also allowed to make international roaming. GSM (Global System for Mobile Communication) was first commercially operated digital cellular system in this generation which is based on TDMA. GSM introduced the concept of SIM (Subscribers Identification Module) cards.
iii. 2.5 Generation:
This generation of mobile stood between 2G and 3G and was introduced at late 1990 in the market. 2.5 G is mostly based on 2G technology but also has added feature of packet switched domain in addition to circuit switched domain of the 2G. Due to this, technology such as GPRS has been implemented in this generation and has enabled internet to exist on mobile device. There is no official or formal declaration as 2.5G in the market but it is specified for marketing purpose.
iv. Third Generation:
The implementation of 3G in the early 2000 has broadened the area for multimedia service and higher bandwidth data transfer in mobile communication technology. With the implementation of this generation, mobile users are now able to make video call, watch video broadcast and surf internet with high speed. 3G is the combination of circuit switching and packet switching domain. CDMA and W-CDMA is the medium access technology used.
Imperfection of 3G
Why was the need of technology beyond 3G required? The answer to this question lists the imperfection of 3G.
Although 3G provides multimedia service and internet service with the bandwidth upto 2Mbps, the demands of consumers are still growing and the service limit of 3G is unable to fulfill that demand. Today's demand is the need of high speed bandwidth and quality of service which enables to broadcast HQ video and voices over the mobile devices. Also people want their same service provider to let them use all the available wireless services under one technology. In term of technical sense, the service demands a single ip to integrate with various network like GPRS, GSM, WiFi without even knowledge to the user so that seamless connectivity could be offered.
But the current technology used in 3G isn't capable to fulfill all these demands. So, all these reasons have led 3G as imperfect to accomplish the present user demands. The technical limitations of 3G are listed below:
a) Difficulty of CDMA to provide higher data rates
b) Limitation of spectrum and its allocation
c) Inability to roam between different services in different frequency bands.
d) Inability to provide seamless transport end-to-end mechanism.
v. Fourth Generation:
The 4G network of mobile communication is targeted to be implemented by 2010 but still lot more standardization based on various features of this technology is yet not conceived. 4G is not a completely new technology but the integration of the wireless communication technology developed till date to provide seamless wireless service under a single ip. Mobile ip technology will be implemented to shift service from one network to the other. High speed data transfer and QOS is the main targeted feature of 4G.
Superiority of 4G over 3G
The following features to be implemented as standards on 4G makes 4G superior to 3G.
a) 4G will be able to provide bandwidth from 100Mbps to 1Gbps based on the external or internal mobility which is at any condition 50 times faster than 3G. This enables 4G to serve users with HQ video streaming.
b) 4G is based on mobile IP technology. Any of the heterogeneous wireless technology each having their own superior technology in comparison to other can be integrated by a single 4G technology. Also global standards in 4G in contrast to multiple standard technology in 3G makes possible for 4G to interoperate and roam across various networks.
c) With 4G users can use the same network for various wireless services like using cell phones for voice, video broadcasting multimedia applications, internet; accessing internet through laptops; using broadband internet in home desktop and so on. Also users can also pay charges for all their services under one bill. Hence, this gives absolute freedom to the users.
d) The IP core level of 4G is scalable. So, this feature enables 4G to handle ever increasing users and services.
III. Access Technology Proposed in 4G to overcome the limitations of 3G.
3G uses CDMA and WCDMA as medium access technology but it is prone to fast fading due to which it is difficult to receive error free user bits in data transmissions. Also WCDMA has less flexibility in spectrum utilization as it requires use the whole spectrum when sharing spectrum with service providers. So when multiple users are in the same spectrum then there is higher probability if MAI(Medium Access Interface). Hence, to avoid the problem of MAI and ISI(Inter Signal Interface), engineers have proposed various techniques for 4G. Some of the highlighted techniques are listed below. But this paper is focused on DS-CDMA
(Direct Sequence Code Division Multiple Access) and MC-CDMA(Multi Carrier Code Division Multiple Access) as a solution to avoid limitations of MAI and ISI in 3G and to achieve higher bandwidth and minimal bit error in data transmission.
a) Software Defined Radio (SDR): It is a radio communication system where components that have typically been implemented in hardware (i.e. mixers, filters, amplifiers, modulators/demodulators, detectors etc.) are instead implemented using software on a personal computer or other embedded computing devices.
b) Orthogonal Frequency Division Multiplexing (OFDM): It is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method.
c) Multiple Input and Multiple Output (MIMO): It is the use of multiple antennas at both the transmitter and receiver to improve communication performance.
d) Universal Mobile Telecommunications System (UMTS)
e) Time Division Synchronous Code Division Multiple Access (TD-CDMA): It is a 3G mobile telecommunications standard, being pursued in the People's Republic of China by the Chinese Academy of Telecommunications Technology. All of these delivery methods are typified by high rates of data transmission and packet-switched transmission protocols. 3G technologies, by contrast are a mix of packet and circuit-switched networks.
f) MC CDMA: It is the combination of OFDM and CDMA technology and promising technology for 4G.
g) DC CDMA: It is a spread spectrum technique that transmits the signal of all users simultaneously over the specified bandwidth.
IV. Mccdma and dscdma AS candidate for multiple access technique for higher bandwidth and minimal bit error rate.
Although various multiple access techniques has been proposed for multiple access with negligible error in data transmission, DSCDMA and MCCDMA has stood as a strong candidate with superior features of each. But still the simplicity in implementation technique of MCCDMA has led to be almost sure technique to be implemented in 4g in comparison to the complex technique of DSCDMA.
MCCDMA is the combination of OFDM and CDMA and the combination of specific features from both the OFDM and CDMA technique makes possible for MCCDMA to avoid MAI (Multiple Access Interface) and ISI(Inter Signal Interface) in data transmission. Also the implementation of adaptive modulation technique where the current modulation scheme is changed dynamically based on current channel estimates facilitates 4G for best spectral efficiency.
OFDM is a multicarrier transmission technique, which converts a stream of signals into parallel form and modulates them into different orthogonal frequencies and transmits them in blocks. OFDM enables the channels spacing to be much closer by applying orthogonality to one another to prevent interference between closely spaced carriers. This helps to utilize spectrum much effectively. There is no bandwidth expansion in OFDM. The available limited bandwidth is utilized most efficiently. Each carrier has an integer number of cycles over a symbol period to accomplish the orthogonality of the carriers. As a result, the spectrum of each carrier has a null at the centre frequency to other carriers in the system. This result prevents interference between the carriers, allowing them to be spaced as close as theoretically possible. This solves the problem of the overhead of carrier spacing in FDMA . OFDM is not a multiple access scheme by design. Thus to achieve multiple access it has to be combined with another multiple access techniques to make multiple access possible. Thus, OFDM is combined with CDMA in MCCDMA to achieve multiple access.
CDMA is a digital modulation and radio access system that employs signature codes (rather than time slots or frequency bands) to each data transmitted to arrange simultaneous and continuous access to a radio network by multiple users. Cell sectorization and voice activity used in CDMA radio schemes provide additional capacity compared to FDMA and TDMA. However, CDMA still has a few drawbacks, the main one being that capacity (number of active users at any instant of time) is limited by the access interference. Furthermore, Near-far effect requires an accurate and fast power control scheme. 
In MC-CDMA, a user specific spreading code is mapped onto a separate subcarrier, so that the chips of the spreading code and the multiple replicas of data symbol are transmitted in parallel on multiple subcarriers. While it may offer better spectrum efficiency, MC-CDMA is also less complicated compared to the DS-CDMA. MC-CDMA applies spreading sequences in frequency domain. The original information becomes spreaded in frequency domain directly.MC-CDMA considers well on spreading signal bandwidth without increasing the undesirable effect of the delay spread. With MC-CDMA, the data symbol is transmitted over N narrow band subcarriers. Each subcarrier is encoded with a - π or π phase offset. The output of the encoding enables the signal to have a PN (pseudo random) coded structure in the frequency domain. The MC-CDMA signal will be free from inter chip interference and inter symbol interference (ISI) as it is composed of N narrow band subcarrier signals with symbol duration much larger than the delay spread. In addition, multiple accesses may be achieved by allowing different users to transmit using the same subcarriers, but with a different PN code. These codes of the different users must be orthogonal to each other.
MCCDMA Transmitter and Receiver
Figure 1 shows the configuration of an MC-CDMA transmitter for user j. It takes the input data stream and converts in to parallel data sequences each parallel data sequence is multiplied with the spreading code. A guard interval in inserted between the symbols to eliminate ISI caused by multi-path fading.
In MC-CDMA receiver the received data is first coherently detected and then multiplied with the gain to combine the energy of the received signal scattered in the frequency domain.
Adaptive modulation in MCCDMA:
The above figure shows system model for adaptive MC-CDMA. There is a channel estimator which estimates the characteristics of the channel using pilot symbols (QPSK symbols) and updates the modulation selector which in turn selects the optimal modulation scheme. An OFDM transmitter accepts data from an IP network, converting and encoding the data prior to modulation. An IFFT (Inverse Fast Fourier Transform) transforms the OFDM signal into an IF analog signal, which is sent to the RF transceiver. The receiver circuit reconstructs the data by reversing this process. With orthogonal subcarriers, the receiver can separate and process each subcarrier without interference from other sub carriers. More impervious to fading and multipath delays than other wireless transmission techniques, ODFM provides better link and communication quality.
Direct Sequence Code Division Multiple Access (DS-CDMA) was proposed as medium access technique before MCCDMA but the complexity of its technique has made MCCDMA as the best candidate to fulfill the requirements of 4G. DSCDMA is one of the approaches tospread spectrum modulationfor digital signal transmission over the airwaves. In direct sequence spread spectrum, the stream of information to be transmitted is divided into small pieces, each of which is allocated across to a frequency channel across the spectrum. A data signal at the point of transmission is combined with a higher data-rate bit sequence (also known as achipping code) that divides the data according to a spreading ratio. The redundant chipping code helps the signal resist interference and also enables the original data to be recovered if data bits are damaged during transmission.  The duration of a data in the chipping code is called the chip time. The ratio between the user symbol time and the chip time is called the spread factor. The transmit signal occupies a bandwidth that equals the spread factor times the bandwidth of the user data. The receiver end receives the transmitted data by multiplying the received signal with the same (synchronized) code.
A CDMA receiver can retrieve the wanted signal by multiplying the receive signal with the same code as the one used during transmission. We find:
where c1 is the code sequence used by user 1, Tc is the chip duration, td is a common time offset, shared between transmitter and receiver and N is the length of the code sequence.. Note that the receive code must be perfectly time aligned with the transmit code.
Drawback in DSCDMA
A major difficulty in Direct Sequence transmission is the Near-Far effect. If more than one user is active, the incoming interference power is suppressed by the cross correlation between the code of the reference user and the code of the interferer. In the event that the interferer is closer to the receiver then the reference user, the interference components may not be sufficiently attenuated by the dispreading process. In cellular CDMA systems, (adaptive) power control is needed to avoid this problem.
C. Eminence of MCCDMA over DSCDMA
a. The main difference between MC-CDMA and DS-CDMA is that the codes that identify different users are modulated in the frequency domain instead of in the time domain. The introduction of the codes in the frequency domain does not add additional complexities in receiver design as it does not require any Rake receivers. Therefore, this method greatly simplifies the receiver design, but yet achieves a comparable result.
b. MC-CDMA does not need to use complicated equalizers at the receiving end because the bit period is the same as the user's information bit. Therefore, the spreading codes employed will not cause the bit to experience frequency selective fading .
c. The receiver design is much easier to employ compared to the receiver in DS-CDMA system. At first the received signal is separated to different subcarriers and to recover the user information bit, the specific users' spreading codes is multiplied with the received signal. Then applying into a parallel to serial converter and feeding into the detector, the user information bit is recovered.
d. Information bit has a period that is much longer than the channel coherence time as the information bit was not modulated to form smaller chips and thereby, causing the transmitted signal to suffer only flat fading.
e. The overall bandwidth of MC-CDMA is exactly the same as in DS-CDMA, although MCCDMA system transmits the signal over different subcarriers. Therefore, it does not employ any extra cost in term of bandwidth expansion between both the systems.
f. One of the distinguishable benefits of MC-CDMA system is the flexibility in spectrum allocation. If spectrum sharing is an option to a service provider, MC-CDMA provides the capability of achieving better spectrum efficiency compared to the DS-CDMA system. This is because in a DS-CDMA system if the service provider wants to borrow frequency spectrum from other service providers, they will have to borrow the entire frequency band which they might not need. This not only wastes the additional capacity that is feasible within the borrowed frequency spectrum, but also makes spectrum sharing inefficient.
V. Comparison between various generation of mobile communication technology.
Higher Capacity, Packetized data
Higher Capacity, Broad-band data upto 2Mbps
Completely IP based, speed from 100Mbps to 1Gbps
NMT, AMPS, Hicap, CDPD, TACS, ETACS
GSM, iDEN, D-MPS
W-CDMA, CDMA 2000
PSTN, Packet network
Table1: Information on different generation of mobile networks 
Hence, the technique implemented in MCCDMA acts qualitative in comparison to other wireless technology in achieving higher bandwidth and to reduce bit error rate in data transmission. Thus, 4G seems to be a promising technology to bring a new dimension in information communication and multimedia access in terms of QOS and reliability where any wireless technology could be accessed on anytime anywhere basis, all with the sense of freedom.
VII. Further Works
There are several issues yet to be considered and resolved in order to fully implement 4G in the market. Some of the vital issues are listed below.
a) Selection among various wireless systems.
Every wireless system has its unique characteristics and roles. The proliferation of wireless technologies complicates the selection of most suitable technology for a particular service at a particular place and time.
Heterogeneity of wireless networks complicates the security issue. Dynamic reconfigurable, adaptive and lightweight security mechanisms should be developed.
c) Network infrastructure and QoS support
Integrating the existing non-IP and IP-based systems and providing QoS guarantee for end-to-end services that involve different systems is also a big challenge.
d) Charging/ billing
It is troublesome to collect, manage and store the customers' accounts information from multiple service providers. Similarly, billing customers with simple but information is not an easy task.