The telecommunication industry

INTRODUCTION

The telecommunication industry is expanding day by days. It need to upgrade their technology from time to time, so as to cover a wide range of people, be it from urban areas and rural areas. In today's world the two common communication technologies are CDMA and GSM. Before selecting which is a better one, let's analyze both the technologies:

  • CDMA: stands forCode Division Multiple Access. It sends signal in a wide range of frequency, allowing multiple users to use the same frequency.
  • GSM: stands forGlobal System Mobile. GSM is an older technology when compared to CDMA. GSM is the most used cell phone technology in the world, with 73% of the worldwide market. It has a very strong presence in Europe.

So, seeing definition of both technologies we decide we choose CDMA technology.

EVOLVEMENT OF CDMA

CDMA refers to Code Division Multiple Access, is a cell phone service technology that has provided a competition to the GSM (Global System for Mobile) which is widely used. CDMA is a technology which was developed English military allies during World War II. The main purpose of using the technology was to prevent the German military from jamming their transmission. For this purpose the English allies transmitted data over different frequencies, so that it becomes difficult to catch the whole signal. [1]

CDMA uses multiple access scheme for communication purpose, this means that various subscriber uses a particular frequency band for communication. Because of multiple users transmitting on the same frequency band, there must be certain protocols so as to establish a flow in communication. There are essentially three ways to do this.

  1. FREQUENCY DIVISION MULTIPLE ACCESS: In this method, the service provider divides the available bandwidth into various sub bands of frequency. These sub bands are allocated to different user whenever they need it. There are guard bands between two adjacent sub bands so as to avoid any interference between users. This technique is generally used in AM or FM stations where each station has got its own frequency bands.
  2. TIME DIVISION MULTIPLE ACCESS: In this technique, the frequency is divided among various users, for a short duration of time. The duration of time for which the bandwidth is allocated is known as time slot. There are guard times between two time slots. This is done to reduce interference between users which arises due to system imperfection.
  3. CODE DIVISION MULTIPLE ACCESS: In both the above methods; i.e. TDMA and FDMA; multiple user cannot easily use the channel at a particular time. For other call to happen, the original call is needed to be ended. Voice data being burst in nature; so if no transmission happens for a particular time it tends to lower system capacity. To overcome the above drawbacks a new technique was introduced, Code Division Multiple Access (CDMA).

The above drawbacks are overcome in this third technique in which the users are spread across both frequency and time in the same. CDMA is a hybrid of TDMA and FDMA.

IMPORTANCE OF CDMA

CDMA changes the nature of the subscriber station from a predominately analog device to a predominately digital device. In CDMA technology multiple users can occupy the same frequency band ensuring frequency reusability. This is the reason why CDMA is the technology of choice for next generation digital wireless communications products and services. Some of the major importances of CDMA technologies are:

  • It is very useful for military purpose. Since, these signals are transmitted at different frequencies at that too in a very large bandwidth, so it becomes hard to either interpret or jam or cause interference to the transmission.
  • CDMA is more secured than other wireless communication service such as GSM. This is because CDMA uses a pseudo random code, which is integrated with the data and then removed at the receiver end to recollect the original data.
  • CDMA signals provide greatest coverage in the wireless industry. This requires to built fewer cell sites reducing expenses both on providers and customer side.
  • Because of CDMA's soft handoff policy, there are fewer call drops. This is because at any instant of time two or three cell are monitoring a call, and passing calls between cells is sharply in case of CDMA.

MODULATION TECHNIQUE

Modulation can be referred to as the process of addition of information to an electronic signal. CDMA, as we all know is a wireless technology, which uses the concept of spread spectrum for its modulation. After the modulation of the signal, certain multiplexing concepts are used to combined various signals into one unit and transmit them. For the purpose of security, the signal is coupled with a certain code which is required when we need to send or receive the frequency. [2]

CDMA works on spread spectrum. Spread spectrum communication has the following key features:

  1. The signal uses a bandwidth which is much greater than the bandwidth which is required to send the information. This increased bandwidth is very much helpful, as this extra bandwidth protects the signal from interference, jamming and multi user access. [11]
  2. For increasing the bandwidth, we use a special code which is totally independent of the data which is to be transferred. So as to enhance the security of the signal, the code which is integrated is pseudo-random. The generated code may appear to be random, but it is generated through a sequential manner so that the receiver can again regenerate the code and extract the original signal. This pseudo random code is also known as pseudo noise code. [11]
  3. The receiver then regenerates the code, and synchronizes the code to reconstruct the original data. [3]

In all, there are three general ways to spread the bandwidth of a signal:

Frequency Hoping: In this technique, the signal is switched between different frequencies within the bandwidth. This switching is not pre-determined, it just happens pseudo-randomly. The receiver also knows before hand where to find any signal at a given instant of time. [11]

Time Hoping: In this technique, the signal is transmitted after certain time. The time interval is not pre-determined, it happens pseudo-randomly. The receiver before hand knows the time interval of each signal and synchronizes itself according to it. [11]

Direct Sequence: In this technique, the digital data is integrated with the code at a very high frequency. The code is generated pseudo-randomly and the receiver also knows how to generate the same code, so that it could synchronizes the original signal back. In CDMA technology, Direct Sequence is generally used for the spreading spectrum. [11]

CDMA basically works on 64 kbits/sec digital signals. These signals may include digitized voice, ISDN channel, modem data etc. The steps involved in transmission of a signal are as follows: [3]

  • A Pseudo-random code is generated. The code which is generated is different for each channel and each transmission.
  • The code is then integrated with the signal, and this is known as spreading of data.
  • The resulting signal modulates a carrier.
  • Then, the modulated carrier is amplified and broadcasted.

The steps involved in the receiver's end are as follows: [3]

  • The carrier is received and amplified.
  • The received signal is then mixed with a local carrier, so as to recover the spread signal.
  • A pseudo-random code is then generated, for the received signal.
  • The received signal is then co-related with the generated code, so as to extract the original information.

As, we know CDMA works on the concept of spread spectrum. Also for the security purpose a pseudo-code is added to the transmitting signal. The generation of the code has got certain procedures.

CDMA follows a coding theory, in which it integrated certain code to the data which is to be sent. For this each of station which needs o send data is assigned a code, which is basically a series of numbers called chips. Let's suppose four station A, B, C and D needs to send some data, then each one of them is assigned a sequence of numbers or chips. Let the assignment of chips for the stations are as follows:

  1. (+1, +1, +1, +1)
  2. (+1, -1, +1, -1)
  3. (+1, +1, -1, -1)
  4. (+1, -1, -1, +1)

Now when a station needs to send the data, it has to follow certain rules for encoding:

  • If a station needs to send a 0 bit, it sends a -1.
  • If a station needs to send a 1 bit, it sends +1.
  • When a station is idle, it sends no signal, it is represented by 0.

Following the above example, we shall how the above four stations shares a link. Let's assume Station A and B are sending 0 bit; Station D is sending 1 bit whereas Station C is idle. Then, the steps which are followed by the multiplexer at the sender end are as follows:

  • The multiplexer receives signal from each of the station which is (-1, -1, 0, +1).
  • The multiplexer multiplies the encoded number sent by each station by its corresponding pseudo generated code. As, on multiplying station A encoded code with its pseudo generated code, we get the result as (-1, -1, -1, -1). Similarly the results for other stations are as follows:
    • Station A: (-1, -1, -1, -1)
    • Station B: (-1, +1, -1, +1)
    • Station C: (0, 0, 0, 0)
    • Station D: (+1, -1, -1, +1)
  • The result of all the above stations are added, which forms a new result. The final result for our example is (-1, -1, -3, +1)
  • The above result sequence is transmitted through a link.

Now when the final result is received at the receiver's end, then it is the job of the de-multiplexer to extract out the original data. The steps followed by the de-multiplexer are as follows:

  1. The de-multiplexer receives the result sequence sent by the sender.
  2. Then the de-multiplexer multiplies the result sequence, with the pseudo-code of each station. The result of the above multiplication for station A would be (-1, -1, -3, +1). Similarly, the result for other stations would be as follows:
    • Station A: (-1, -1, -3, +1)
    • Station B: (-1, +1, -3, -1)
    • Station C: (-1, -1, +3, -1)
    • Station D: (-1, +1, +3, +1)
  3. The chips in each sequence are then added. The result of these additions is always +4, -4 or 0.
  4. The result obtained from step 3 for each station is then divided by 4.
  5. The result obtained from step 4 is then decoded, as 0 for silent; -1 for sending 0 bit and +1 for sending 1 bit.

The generation of code is not done at random. We follow certain procedures to generate a pseudo-number, which is done through a Walsh table. Walsh Table is basically a two dimensional table having same numbers of row as well as column. In Walsh table each row is a sequence of chips such as Walsh table having 1 row and 1 column generates one chip sequence. Walsh table only contains two values +1 and -1, where +1 signifies sending of a bit and -1 signifies sending of no bit. According to Walsh, if we know the table for N sequences WN, we can create the table for 2N sequences W2N,

The above figure and below figure illustrates the Walsh table, where we assign +1 to every WN whereas we assign -1 to the complement of every WN. [4]

The sequences of chip generated by the Walsh code are orthogonal in nature. The properties of orthogonal sequences are as follows:

  1. If we multiply every element of a sequence by -1, then the element gets complemented.
  2. If we multiply two sequences, element by element and add the result, we get a number called inner product. The value of the inner product is equal to N, if we multiply two same sequences, where N is the total number of elements in each sequences. Similarly, on multiplying two different sequences we get the result as 0, and on multiplying two sequences A and A', i.e. where one is the complement of other, we get the result as -N, where N is the number of elements in sequence.

SECURITY ISSUES OF CDMA

Security has been a major area of concern when it comes to wireless communications. CDMA was introduced by the military so as to secure their transmission from the enemy. Service providers are also concerned with the security to prevent various fraud issues faced by the customers such as cloning fraud while the subscriber are mainly concerned with the privacy issues. CDMA due to its design and implementation provides various security measures:

  1. Communications from network to phone, scrambled using"Long Code" 42 Bit PIN Pseudo Random Code. From network to mobile it's scrambled at a rate of 19.2 Kilo symbols per second. From the mobile to the network it's scrambled at 1.2288 Mega chips per second. An unintended listener would need the PIN and to time their listening on both ends to the exact same communication timing (extremely difficult). This I believe relates to the spread spectrumtransmission over several frequencies andat different timing.[5]
  2. CDMA secures its network by the use of a 64 bit encryption key, which is actually installed on the phone. This key along with the phone ESN (Electronic Serial Number) generates various keys for additional service. A key generates various sub keys for voice privacy and message encryption. [5] [6]
  3. CDMA uses another technology known as CAVE (Cellular Authentication and Voice Encryption) algorithm to generate a 128 bit sub-key, which is known as Shared Secret Data (SSD). The input to generate SSD key are ESN, A key and network supplied random binary numbers. The SSD key has two parts, SSD_A and SSD_B. each being 64 bits long. SSD_A is used for creating authentication signatures, while SSD_B is used for generating key to encrypt voice and signaling messages. The SSD can be shared with roaming service provider for local authentication. A fresh SSD is generated when a mobile return to a home network or roams at a different system. [6] [5]

IMPLEMENTATION OF CDMA

CDMA has been a great boom to various industries, from the time of its emergence. It has catered the various industries according to its needs. Mobile companies have been benefited the most from this technology. Some of the major benefits for the mobile companies are as follows:

  1. MOBILE COMMUNICATION: Through CDMA's application in cellular telephony is relatively new, but it is not a new technology. CDMA has been used in much military application, such as anti jamming, ranging and secure communication. The use of CDMA for civilian radio application is novel. Commercial application became possible because of following evolutionary developments.
    • Availability of very low cost, highly dense digital integrated circuits, which reduce the size, weight and cost of the subscriber station to an acceptably low level.
    • Realization that optimal multiple access communication requires that all user station regulate their transmission power to the lowest that will achieve adequate signal quality. [7]
  2. CELLULAR RADIO STATION: Cellular services are now being used every day by millions of people worldwide. The number of customers requiring such services is increasing exponentially, and there is a demand for integration of a variety of multimedia services. The range of services includes short messaging, voice, data and video. Consequently, the bit rate required for the services varies widely from just 1.2 kbps for paging up to several Mbps for video transmissions. Furthermore, supporting such a wide range of data rates with flexible mobility management increase network complexity dramatically. In a cellular network, each base station assigns separate directional sector antennas or separate outputs of a phased array to cover this is joint cell sector in both the transmitting and receiving modes. Typically there are three sectors, and 2?/3 radians span each sector. In cellular network, the frequency- hopping pattern can be chosen so that at any given instant of time, the frequency of the users within a cell s
  3. ector are all different, and hence, the received signals are all orthogonal if the mobile transmission are properly synchronized. Exact synchronization on the forward link is possible because a common timing is available .the switching time or guard time between frequency- hopping pulses must be large enough to ensure that neither a small synchronization error nor multi-path signals can subvert the orthogonality. [7]

ADVANTAGES OF CDMA

With the implementation of CDMA technology, both the service providers as well as the subscribers have been highly benefited. Some of the advantages of CDMA technology to the subscriber are:

  • Increase system capacity: around 4 to 5 times that of GSM. This increase in capacity means easier and faster access to cellular networks with fewer dropped calls.
  • Improved voice quality: The reason for this improvement is that the voice is converted into digital signal, which provides clarity. Also, inclusion of new vocoders is also responsible for it.
  • Soft Handoff: This, means there would not be any degradation in the call quality, when moving from one site to another, as there would be signals coming from two or more stations. Soft handoff also improves data communication.
  • Improved privacy: This is possible, as CDMA technology provides in built privacy by the use of pseudo-random codes.
  • Longer battery Life: The use of CDMA technology helps in increasing mobile's battery life. This is because mobiles using CDMA technology requires less RF power, ultimately resulting in high battery life.

Some of the advantages of CDMA technology to the service providers are:

  • Increased System Capacity: In CDMA technology, various phone calls are transmitted simultaneously at the same frequency by assigning unique secure code to each call.
  • Interference Immunity: The CDMA technology is built in such a way, that it is not affected by seasonal change in weather, atmosphere etc.
  • Secured Network: By its nature of using spread spectrum, CDMA transmissions are more difficult to decipher making the network secured. [8]

DISADVANTAGES OF CDMA

Though CDMA is very beneficial both for subscriber as well as service providers but it has got its own set of flaws. Some of the disadvantages of CDMA are:

  • Due to its proprietary nature, all of CDMA's flaws are not known to the engineering community.
  • CDMA is relatively new, and the network is not as mature as GSM. [12]
  • CDMA cannot offer international roaming, a large GSM advantage. [12][13]
  • Channel Pollution: This is another major problem in CDMA technology. There are situations when there are signals from too many cell sites but none of them are dominant. If such situation arises, then the audio quality if the signal is degraded. [9]

PROBLEM IN IMPLEMENTATION OF CDMA

As we know CDMA is one method implementing multiple access communication. In multiple access, may subscriber or service provider can access the same channel simultaneously. Some of the problems faced in implementing this technology are:

  1. MULTI-USER INTERFERENCE: In CDMA many user send data in same range of frequency, where each user's data is separated through a unique pseudo-random code. The receiver only interprets its respective message as it cannot despread the pseudo-random code of other user's data. So, ultimately the other signals appear as interference. When the number of user's in a particular frequency band increases, the signal to interference ratio decreases ultimately resulting in poor performance. [7]
  2. NEAR FAR PROBLEM: Near far problem occurs when two signals are received by a single receiver, where one signal is transmitted from far station where other from nearer station. Due to the distance of the transmitter, the signal strength of nearest signal is more than that of far away transmitter. There may be a situation, if the nearer transmitter sends the signal at a power more than the signal of the farther transmitter, then the Signal to Noise Ratio (SNR) of farther transmitter may become so high that it could not be detected, and there may be chances that the farther transmitter may not transmit. This jams the communication channel. [10]

CONCLUSION

Though CDMA is a new technology in the world of wireless communication, but it has achieved worldwide acceptance. This is visible by the increase in the strength of CDMA users. Te reason for such an increase is the use of spread spectrum technology. This modulation technique was a boon to both the subscriber as well as the service provider. It is useful to the subscriber as it improves the voice quality, there are less dropped calls in case of CDMA technology, more security is provided in this technology and other things. While for the service provider, it's a boon as it enable them to allow multiple user using a specified frequency band, it is immune to various interference, so thus enabling the service provider to provide a high quality of network.

REFERENCES

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