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Last two decade has shown a huge growth in the field of cellular communication. Millions and billions of people all around the globe are connected with the use of cellular phones. It allows an individual to either make a call or receive it. Also, the conversation continues even when a person is moving from one region to another. The essential infrastructure that supports this is known as cellular network.
This technology of cellular network has went through three generations, starting with the analog generation to digital to the third generation where there was an integration of cellular phones with the internet. The use of digital technology helped in having more data security in transmission with the help of encryption using various coding techniques. The third generation gives high data packets transmission and reception rate.
Cellular network consists of mobile stations (MSs), which provides a wireless means to access public switched telephone network (PSTN). The total service coverage region of cellular network is further divided into smaller areas, called as cells, where each of such cells is served by base station (BS). BS in turn is connected to mobile telephone switching office (MTSO), also called as mobile switching center. MTSO handles a whole cluster of BSs, thus connected to PSTN. Since BC and MS are connected through wireless connection, MSs i.e. cell phones can be used to communicate even with the wire-line phones present in the PSTN. Also a transceiver is provided to BSs and MSs, to have both way communications.
The total available spectrum is efficiently used using the concept of frequency reuse. In this method, each cell is allocated with fixed number of frequency bands, which would be used by a user in that particular cell. Similarly, a neighboring cell would be allocated some different set of frequency bands. Now, this helps to avoid the co-channel interference. But two cells separated with a sufficient distance such that the co-channel interference is under a tolerable limit, can be allocated with the same set of frequency bands. Typically, a cluster is formed with seven neighbor cells, where the total available frequency spectrum is divided among these seven cells cluster and such cluster is repeated.
In another technique, the capacity is increased by using smaller sized cells. This is done by spilling the original cell into four parts, thus forming four times the original number of cells. This is followed by reducing the height of antenna as well as the transmitting power. In reality, while practically applying these concepts, the whole region is divided into multiple cells, of both smaller and larger size. This helps the high-speed users to use cells that are bigger in size, which trim down the total number of required hand-offs.
Another technique which is used is the sectoring method, which also increases the total network handling capability. In this method, size of cell is maintained same, but every cell is further divided in different sectors with the use of corresponding number of directional antennas in the same position as that of BS replacing the single antenna of Omni-direction type. Generally, each cell is further sub-divided in three sectors of 120â-¦ each or six sectors of 60â-¦ each. With this method, the interference level can be brought down as cell is divided sectors. Thus, there is an increase in the network capacity.
The total capacity of the network can even be increased by using digital technology. The process of transmitting digitized voice signal goes through three steps, only after which it is actually transmitted via the radio channel. These are speech coding, then channel coding, and finally modulation. The first step of speech coding helps in compressing the voice signal. For example, parameter values can be used to represent the various short segments of the total voice signal. Now, it is not safe to transmit this compressed data directly, as there are high chances of alternation in these values, when they reach the receiving end. And the total voice data can be changed to a great extent even with a small change in the values. Thus, it becomes essential to arrange the data accordingly, with redundancy introduced to reduce the probability of a fluctuation in the data values. This process of arranging the values or data in such a way is known as channel coding. After this, the final data obtained from channel coding are then modulated for the transmission. Applying a speech coding technique along with a corresponding channel-coding technique will reduce the total bandwidth required by each individual user and thus would increase the effective capacity at the same voice quality.
The frequency bands or channels are used for voice transmission as well as some as assigned for control purpose. The channels used for the actual voice transmission is called as the voice channel, while those used for the setting up the conversation is called as the control channel. These are again further divided as either downlink or forward channel and uplink or reverse channel. The function of a downlink channel is to carry the signal or the traffic to the MS from BS, while that of the uplink is to carry the traffic to BS from MS. Methods of multiple access are employed in order to have a sharing of channels in each of the cell.
MULTIPLE ACCESS METHODS
There are many MS's in a cell covered by BS, which have to communicate with it. In order that no MSs within cell interfere with each other, Mobile stations should share air interfaces properly. The way in which MSs Share air interfaces in orderly manner is called as Multiple Access Methods. Some of well-known multiple access methods are DDMA, TDMA & CDMA.
Frequency spectrum associated to particular BS is divided into many frequency band referred as Channels by FDMA. Channels don't interfere with each other because of the well maintained separation between them.MS may use exclusive assigned channels. Advance Mobile Phone System uses FDMA. AMPS utilize 40MHz in a 800MHz spectrum. Every channel of AMPS have approximately 30 kHz Bandwidth, 1332 channels are translated by 40 MHz bandwidth. Each cellular company from US provides 666 channels. After frequency division multiplexing, out of 66 channels half (333) of channels are there for BS to MSs Communications& remaining are for Mobile to base station communications.
TDMA allows many MSs to share same channel. Time is divided by TDMA. Only MSs are allowed for using shared channel in every time division to transmit/ receive & MS share the channel one at a time. Hence possibility of interference is not there. Although each MS using channel one at a time makes it difficult to deliver voice signal. Fortunately, Tolerance to a delay of 20 milliseconds is acceptable for a human hearing system.
CDMA allows multiple MSs sharing same spectrum. Each MSs is given a particular sequence code using which the signal is spread over entire bandwidth. At receiving end same unique sequence code is provided to get the signal. Due the unique sequence code, no interference is caused although the spectrum is shared.
Location management deals with tracking MS. In cellular network it is one of the important tasks to keep track of MS (Location Management) since it's crucial to know exact location of an MS when Call is ongoing. Location management helps in tracking active MS is in call or not i.e. MS is powered on or not. When MS gets an incoming call it's important to know exact cell in which MS is located for cellular network.
There are 2 extreme cases in Location Management:-
One of extreme case knew as "Never Update" case in which MS never responds its cellular network its location while roaming. In such a case when MS gets incoming call, cellular network pages all cells in service area to get to know MS's current position. Then the incoming call is routed via BS of concerned cell. But this solution is very much expensive.
Second Extreme case is called "Always update" in which MS is supposed to update its location while it enters any new cell. So when Incoming call is received the call is routed to latest updated cell. Like the 1st case this case also involves great deal of cost.
Location Update & paging are basic operations involved in location management. Both these operations work in synchronization & are interdependent. As in, paging function cellular network will page all possible cells for finding exact location of MS for whom incoming call is arriving. The number of possible cells which are paged depends upon the frequency at which each MS updates its location. Both functions use wireless bandwidth in following manner:
Paging consume forward control channel
Location Update consume reverse control channel
Out of the wire line costing portion & wireless costing portion just the wireless portion is taken into consideration, as Radio frequency bandwidth is limited. As the functions Location update &paging are dependable thus the effective cost gets balanced. For example, if location update is done frequently by MS the costing for location update will be higher, but the network will know MS location better so paging cost will be reduced &tradeoff between update cost & paging cost is observed.
HAND-OFF STRATEGIES AND CHANNEL ASSIGNMENT
The previous section described how to track the movement of an MS when it is not in a call. This section deals with the movement of an MS when it is in a call. When an MS is in a call, it has acquired two channels (one for transmitting and the other for receiving) from the current cell for communication with the BS. When the MS moves out of the current cell and enters a neighboring cell, the MS needs to acquire two channels from the neighboring cell in order for the call to continue. The process of transferring a call from the current cell to a neighboring cell is called hand-off. To a cell, a hand-off call is a call that is in progress in a neighboring cell and needs to be continued in the cell because of the movement of an MS. In contrast, a new call is a call that is started in the cell. As mentioned earlier, the number of channels assigned to each cell is limited. Channel assignment deals with how to assign available channels to new calls as well as hand-off calls.
The simplest channel assignment scheme is the fully shared scheme (FSS), in which all available channels are shared by hand-off calls and new calls. No distinction is made between a hand-off call and a new call. FSS is widely used in the current cellular networks because of its simplicity. In addition, FSS has the advantage of maximizing the utilization of wireless channels. The disadvantage is the increased dropping rate of hand-off calls. In general, it is less desirable to drop a hand-off call than to block a new call. The dropping probability of hand-off calls is considered as one of major metrics that measure the quality of service of calls.
Recently intensive research on channel-assignment schemes has been conducted to decrease the dropping probability of hand-off calls. One such scheme is the hand-off queuing scheme (HQS; Tekinay&Jabbari, 1991). When an MS detects that the received signal strength from the current BS is below a certain level, called the hand-off threshold, a hand-off operation is initiated. The hand-off operation first identifies the new BS into which the call is moving. If the new BS has unused channels, the call will be transferred to the new BS. If there is no unused channel available, the hand-off call will be queued until a channel is released by another call. The HQS scheme is feasible because there is a difference between the signal strength at the hand-off threshold and the minimum acceptable signal strength for voice communication. This gives an MS some time to wait for a channel at the new BS to become available. A new call will be blocked in the new cell until all the hand-off calls in the queue are served. Therefore, the HQS scheme decreases the dropping probability of hand-off calls while increasing the blocking probability of new calls because the scheme gives higher priority to hand-off calls.
The scheme proposed by Li, Shroff, and Chong (1999) goes one step further. When a hand-off call is not able to acquire the necessary channels in the new cell, the call is allowed to carry the channels in the current cell to the new cell, a concept the authors called channel carrying. However, carrying the channels from the current cell to the new cell may reduce the reuse distance of the channels and violate the minimum reuse distance requirement. To ensure the minimum reuse distance requirement is not violated, an (r + 1)-channel assignment scheme is used. That is, the same channels are reused exactly (r + 1) cells apart. Here r is the minimum reuse distance. In this scheme, a channel can only be carried from the assigned cell to a neighboring cell, and the carried channel will be returned as soon as a local channel is available. By using channel carrying, a hand-off call can continue, even if there is no channel available in the new cell. Therefore, the dropping probability of hand-off calls will be reduced. However, the capacity of the cellular network is reduced because the (r + 1)-channel assignment scheme is used instead of r-channel assignment.
AUTHENTICATION AND ENCRYPTION
Since communication via cellular medium is carried through the use of air interface, the chances of eavesdropping and various frauds increase. It is therefore essential to authenticate the users and encrypt the data or say have the proper data encryption. Authentication confirms whether the user is the same as he or she has claimed. Encryption is done by using key which is used to scramble any message in such a way that a third party would not be able to read the message unless and until that person has the right key. The private key encryption and public key encryption are two widely used encryption techniques. Both these encryption methods can be used for the authentication as well.
Each of a GSM mobile subscriber is provided with a SIM (subscriber identity module) card which is to be inserted inside the mobile phone (Black, 1999; Mouly&Pautet, 1992). Any phone with SIM card inserted into it can be used only after activating it, by entering a four digit number called PIN (personal identification number), specific to that SIM. The SIM keeps identity as well as security-related key information like IMSI (international mobile subscriber identity), authentication key (Ki), A3 authentication algorithm, and also A8 ciphering key generating algorithm. The authentication system in GSM runs and checks out if SIM is valid, and this is done using a method known as challenge and response. A random 128-bit number is sent to MS by network system as a challenge. MS inputs this challenge and authentication key (Ki) in to A3 algorithm for signed response (SRES) to be generated, which is then sent to network. Then network compares this received SRES and SRES provided by authentication center. Mobile user can be said to be authenticated, if this matches; otherwise, not. Once the user gets authenticated, A8 algorithm, with the help of random number along with authentication key (Ki), generates ciphering key (Kc) on both the ends of air interface. This key is used by A5 algorithm, to encrypt and decrypt the data.
For the purpose of making GSM a secure technology, all specifications like A8 and A3 algorithms are intentionally kept secret for security reasons. Even with such high level of security, the system is to a certain extent prone to risk, where an attacker is able to figure out the encryption algorithm by sending multiple challenges and collecting the corresponding responses and understanding the pattern. But still, as of now, GSM is the most secure system of public wireless communication.
HISTORY OF GENERATIONS
1G was meant for analog Phones & hence was based on analog technology. It was introduced in early 1980's. This generation introduced basic framework for mobile communication like architecture, Frequency multiplexing, concept of roaming, etc..
2G was revolutionary in itself as it launched Digital Mobile communication. Introduced in late 1980's, 2G presented GSM which was one of main attractive aspects. Also the Subscriber Identity Module (SIM) card was introduced in 2G.GSM & CDMA were main access technologies.
Extension of 2G wireless networks. 2.5 Generation made it possible to connect internet with mobile communication. The phenomenon of connect internet with mobile communication was known as Hybrid Communication.
The basic concept of 3G is to utilize new systems with new services instead of just providing higher bandwidth with higher data rates. Supporting multimedia transmission was distinguishing aspect of 3G, which applied circuits switching as well as packet switching strategies. Main access technologies for 3G are CDMA (Code Division Multiple Access), WCDMA (Wideband CDMA) & TSCDMA (Time Division Synchronous CDMA).
4G WIRELESS NETWORKS
Wireless world is searching for a new generation technology to get over with set-backs from third generation. Following limitations of 3G are expected to be overcome by 4G:
Issues in providing higher data rates to CDMA
Continuous increasing data rate as well as bandwidth to fulfill multimedia requirements
To establish new better system, which will reduce cost
Better efficiency of services while roaming
Though there are some challenges that are yet to be resolved in 4th Generation. Some of them are:
Terminal Mobility: Biggest issue, as it involves difficulty in 2 aspects, Location Management & Hand off management. Terminal mobility is expected to allow user to use different technologies while he is roaming in different locations. Unlike traditional mobile systems, 4G is supposed to do horizontal & vertical hand off.
[Horizontal hand off- Mobile move from one cell to other
Vertical hand off- Mobile move from one wireless system to another]
Multimode User Terminals: The user terminals should be adaptive to configure themselves as per the service or different kind of technology.
Selection of Wireless system: This suggests that in 4G there should be a feature which will enable user terminal to discover & select desired wireless network i.e. LAN/ GSM/ GPS, etc.
Billing Mechanism: In 4G wireless network the user might switch between different service providers. So the operators are expected to come up with a system that will provide the user with a single bill for consuming different services from different service providers.
Security & Privacy: 4G systems needs large security measures, but the existing measures are inadequate. The existing systems for security are developed for limited and particular services. To achieve greater flexibility, there is a need to introduce new systems of security.
Personal mobility: It deals with user's mobility instead of that of user terminals. Basic principle underlying this is that, user irrespective of the location where he is and irrespective of the device that is being used by him, he must be able to receive or send his messages.
Fault tolerance: Structure of existing system has topology of a tree-structure. Thus with a damage in even one of component can result in the breakdown of whole system. Such cases are undesirable, especially in the 4G system. Thus designing a full proof or system with high fault tolerance is an important issue in 4G.
It is expected that the wireless system of 4G would be designed based on network that would be IP based for the purpose of global routing. Also it would have much high customized LAN (local area network), which would help in supporting the dynamic hand-off execution along with Ad-Hoc routing.
The following figure 1 depicts the various requirements of a 4G network or architecture. Some of the requirements are higher bandwidth, high integration and security, multimedia application supportive system, and heterogeneous network.
In 4G systems, every terminal is to be assigned with a particular home agent, only those having a permanent IP address (home IP address). When the terminal is on a move and enters another location, a new but temporary address is assigned to it which is called as care-of address. It updates its care-of address regularly with its home address. In another case, when user is present at home, other device from some other location is able to communicate with this user using its home address. On the other hand, a different procedure is employed in cases where user moves out to other different location. In cases, when the host wants to get connected with user to communicate his message, he sends the message of setup to home agent of user. Home agent has the knowledge about care-of address of user, so forwards setup message back to user terminal. Home agent too, forwards this care-of address to host, which enables it to sent other messages directly to user in the future.
Fig 1: Architecture of 4G network
MULTIPLE ACCESS TECHNIQUES
3G Wireless multiple access techniques were mostly build on CDMA & WCDMA which created issues like MAI i.e. [Multiple Access Interference] & ISI i.e. Inter Symbol Interference. Hence 4G network found its best option in MC-CDMA as it not only helped to overcome above issues but also improved bit error rate.MC-CDMA is hybrid blend of OFDM [Orthogonal Frequency Division Multiplexing] & CDMA. MC- CDMA along with Adaptive modulation helps 4G with lower BER [Bit Error rate] & higher data rate.
OFDM: OFDM can cancel multi path distortion in spectrally efficient manner. Multi path & Doppler spread cause rapid variation in channel characteristics. These varying channels are patterned by good SNR [signal to noise ratio] but also worse SNR. Fixed modulation method doesn't consider the worst case scenario, thus can't achieve best spectral efficiency. Hence adaptive modulation, as it takes benefit of time varying channel trait & adjust transmission power, data rate & modulation scheme for achieving best spectral efficiency.
MC-CDMA: To maintain sense of orthogonality between users to remove MAI is the basic concept in CDMA, which is done by making use of spreading codes to spread data sequence. MC-CDMA defines spreading codes in frequency domain.
MULTIMEDIA - VIDEO SERVICES
4G wireless systems are expected to hand over multimedia services at efficient data rate. Bursting & streaming video services are two types of video services.
Streaming video: this is carried out for user requiring real time video service. In streaming video server transfers data at playback rate continuously. Its memory necessity is very less compared to bursting video.
Bursting Video: this is like file downloading using buffer. Unlike streaming video, bursting video transmits data at highest data rate, taking benefit of whole available bandwidth. The drawback with bursting video is large memory requirement & chances of overusing available bandwidth of system.
APPLICATIONS OF 4G
1. Tele geoprocessing applications - blend of GIS [Geographical Information System] & GPS [Global Positioning System]
2. Crisis Management - 4G is expected to fix & restart the work in natural calamities within few hours unlike earlier generation take which take days to overcome such issues.
3. Mobile marketing & Advertising
4. E-Reader Apps, etc