What CDMA Technology is all about?
The first proposals for CDMA cellular networks in the USA and Europe (1978-1980) yielded to alternative projects, which later evolved into the GSM and DAMPS standards. However, in the mid 1990s the 2G standard IS-95 was put forward, resting on a fully spread spectrum/ CDMA platform. At a cosmic pace, networks of this standard (later named cdmaOne) gained wide recognition in America, Asia and the former Soviet Union countries. The great success of IS-95, as well as careful analysis and further experiments, had led to acceptance of the spread spectrum/CDMA philosophy as the basic platform for the major 3G mobile radio specifications: UMTS and cdma2000. Both of them are now in the pre-operational stage and undoubtedly will become the main mobile communication instruments for the next decades.
As we know it is all about multiple access, so multiple access is subdivided into:
- Contention- Based Techniques
- Conflict-free technique.
Contention Based Control: This is not in the scope of this project
Conflict Free Control
It involves the division of system resources into fixed channels which are than reserved by transmit/receive pairs of communication. This is beneficial for channels which require regular and continues access to a channel like video and audio.
Code Division Multiple Access
In CDMA systems, channels are defined by the code not by time or frequency. Spread Spectrum relies on pseudo-random waveforms termed spreading codes to create noise-like transmission. If users can be given different codes that have low cross-relation properties, channels can be defined by these codes.
In CDMA channels are defined by Spreading Codes, eg with the direct sequence CDMA two signals can be defined as:
Where a1(t) and a2(t) are spreading codes that define the "channel" of each user signal thus cross relation between a1(t) and a2(t) dictates the performance of CDMA.
Technology behind CDMA
CDMA technology is based on the Spread Spectrum communication technique SPREAD SPECTRUM is a means of transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it. Spread spectrum is accomplished before transmission through the use of a code that is independent of the data sequence (PN).
Introduction to Spread Spectrum Communications 
CDMA is a form of Direct Sequence Spread Spectrum communications. In general, Spread Spectrum communications is distinguished by three key elements:
- The signal occupies a bandwidth much greater than that which is necessary to send the information. This results in many benefits, such as immunity to interference and jamming and multi-user access, which we'll discuss later on.
- The bandwidth is spread by means of a code which is independent of the data. The independence of the code distinguishes this from standard modulation schemes in which the data modulation will always spread the spectrum somewhat.
- The receiver synchronizes to the code to recover the data. The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time.
In order to protect the signal, the code used is pseudo-random. It appears random, but is actually deterministic, so that the receiver can reconstruct the code for synchronous detection. This pseudo-random code is also called pseudo-noise (PN).
Importance of Technology
CDMA support all channel sizes (5 MHz, 10 MHz, etc.) provide circuit and packet data rates up to 2 Mbps, incorporate advanced multimedia capabilities, and include a framework for advanced 3G voice services, including voice over packet and circuit data. The interaction between mobiles in the same channel is the distinguishing characteristic of CDMA.
Features of CDMA Affecting human Life:
1 Primary voice features -.
- Call Forwarding Busy (CFB)/Call Forwarding Busy No Answer (CFNA)/Call Forwarding Busy Unconditional (CFU) CFB, CFNA, and CFU allow a called subscriber to have the system send incoming calls, addressed to the called subscriber's directory number, to another directory number (forward-to number), or to the called subscriber's designated voice mailbox.
- Do Not Disturb (DND) DND prevents a called subscriber from receiving calls. When this feature is active, no incoming calls shall be offered to the subscriber.
Conference Calling (CC) CC provides a subscriber with the ability to conduct a multi-connection call, i.e., a simultaneous communication between three or more parties conferees
2 Short Message Service Features
- Short Message Delivery-Point-to-Point Bearer Service (SMD-PP). SMDPP provides bearer service mechanisms for delivering a short message as a packet of data between two service users, known as short message entities (SMEs). The length of the bearer data may be up to 200 octets.
- Cellular Paging Teleservice (CPT) CPT conveys short textual messages (up to 63 characters) to an SME for display or storage.
3 Data transfer
The best data transfer technology it has to offer is the EVDO technology, allowing for a maximum download speed of about 2mb/s (about 700kbps in practice), which is similar to what a DSL line has to offer. EVDO is not available everywhere yet and requires a cell phone that is EVDO ready.
4 Global Positioning Systems (GPS)
The main advantage of using CDMA cell phone signals for reference clock purposes is that they work better inside buildings, thus often eliminating the need to mount a GPS antenna outside a building.
5 CHEAPER RATES
The cost of Handheld devices which are using CDMA technology are much less in comparison to the GSM handheld devices, so it is quite easy for an individual to afford a CDMA handheld device eg Reliance CDMA phones
The modulation technique used in CDMA (specifically in wireless communication in mobile network) is dual-channel QPSK (hybrid phase shift keying (HPSK) or orthogonal complex quadrature phase shift keying (OCQPSK))
The methods used by dual-channel QPSK to Modulate are-
- Orthogonal spreading Code
- Scrambling code with Walsh Rotator
- Pseudorandom codes (Scrambling)
Orthogonal spreading Code
Spreading transforms each data symbol into multiple data chips. This ratio (number of data chips/symbol) is called the spreading factor (SF). Thus, it increases the signal bandwidth. Data symbols on the I(real, or In-phase data component) and Q(imaginary, or Quadrature-phase data component) branches are combined with the channelization code
In downlink it is used to separate different users within one cell, but in the uplink only to separate the different services of one user
Generation of channelization code
Allocation of code
It is showing the root of code tree. It employs the spreading factors 4 through 512, in which 4 to 256 appears in uplink, and SF 512 is added to the SF catalogue in the downlink direction. It also shows how the codes can be allocated.
Example: If the code C8,2 is allocated, then from its subtree no codes can be used (i.e. C16,4, C16,5, C32,8). These subtree codes would not be orthogonal with their parent code.
Scrambling code with Walsh Rotator
How it works?
Original data chip divided into its I and Q components (1,1) and a complex scrambling signal (-1,1). When complex scrambling takes place, the phases of these signals are added together
(45 + 135 = 180) and the resulting signal constellation is (-1,0).
The distance of I and Q from origin represents the power level of the signal. If the original data signal uses equal power levels for control and data channels, then the constellation points will be [(1,1), (-1,1), (1,-1)]. When they are scrambled using a complex scrambling code, the result always lies on either the I or Q axis; that is, mapped into the constellation points (1,0), (0,1), (-1,0), and (0,-1).
This means that these phase shifts cannot cause zero crossings
Pseudorandom codes (Scrambling)
The orthogonal codes can only be used when the signals applying them are time synchronous. So for asynchronous users in the uplink direction if orthogonal spreading codes alone were used in the uplink, then they could easily cancel each other.
So to overcome this situation pseudorandom codes are used. In this procedure, the signal, which is spreaded (full bandwidth) with an orthogonal spreading code, is further combined (means XOR) with a pseudorandom scrambling code. This scrambling code is either a long code (a Gold code with a 10-ms period) or a short code [S(2) code]
It is used to designing & evaluation of network problems also provides a comprehensive set of coverage and interference analyses of different cell sites.
- It provides view of the network.
- It can evaluate the functionality of Call processing and received base station signal quality
It estimates the performance of the network infrastructure. It can provide a comparison of oue network with the competitor's network
It provides a view of Horizontal and vertical pattern of antenna.
The fault report must contain enough information so that the management system can make the right analysis and react correctly.
Once the number of subscribers increases, new capacity is needed; thus, new equipment must be bought
- Traffic levels within the network, both user data and control signalling
- Verification of the network configuration
- Resource-access measurements
- Resource availability
A roaming agreement is a contract between the home-network operator and the serving-network operator
User equipment management
This is a feature that allows a network operator to trace the particular subscriber within the network.
The main software-management process can contain the following stages:
- Delivery of software from the vendor
- Forwarding of the software to network elements or element managers
- Validation of the software to ensure that it is not corrupted
- Activation of the software to an executable state
ENHANCED SUBSCRIBER AUTHENTICATION ALGORITHM
This method utilizes the Authentication and Key Agreement which provides mutual authentication between a base station and mobile terminal with increased key size. Mutual authentication overcomes the problem of false base station attacks, thereby preventing the voice privacy or private identity information of the subscribers from being compromised. The ESA algorithm also dismantles the cryptographic attacks such as reconstruction attack and list attack, thereby enhancing the security of existing CDMA systems. The simulation results also clearly
Indicates that ESA based systems consume less power and the error rate is also less when compared with CAVE based system. Thus, the Enhanced Subscriber Authentication algorithm enhances the security of the CDMA systems.
Advantages of CDMA
- No frequency management - In CDMA we are not required to use different frequency where as in both TDMA and FDMA the frequency management is always a critical task. Since there is only one channel in CDMA, no frequency management is needed.
- No Guard Time in CDMA. In TDMA uses the concept of guard time so as to avoid interference between simultaneous users whereas in case of CDMA we are not required to add guard time which result in efficient use of bandwidth.
- No Hard Handoff - As we CDMA is based on code sequence and uses the same frequency, the connection to the new cell site can be made without breaking the connection of the current cell. And it also requires less power, which reduces interference and increases capacity.[k1]
- Use of Rake Receiver - In CDMA have rake receiver which is multiple receiver in one which identifies three multi-path signals and combines them to make a very strong signal. Both mobile and cell site use rake receivers.
- Power Adjustment in CDMA- in CDMA handset itself can adjust at which they transmit the signal. This insures that base station is receiving signal at the power which is needed. Both forward and reverse link uses power control techniques.
Whereas in case of GSM handsets it picks the first the first signal it gets. No matter what is the strength of signal?
Where as in case of GSM network handset will transmit at the fix setting whatever the distance between handset and base station is. The base station would face extremely strong signal from the nearer and extremely low from the farer device which result in Near-Far Problem.
Other Advantages are -
Disadvantages of CDMA
- Multi-user interference or multiple access interference- Each user data in CDMA system is spread by a pseudorandom code. Every user transmits on the same frequency band and distinguished at the receiver by the user specific spreading code. All other signal is not de-spread because they use different codes. These signals appear as interference to the desired user because of non zero cross co-relation values between the spreading codes. As the number of user increases performance decreases.
- Connection fixed with the Handset- The main problem with CDMA technology that customer faces it that it does not allow to change handset easily because many functionality is embedded in the handset whereas in case if it uses SIM card that identifies a user and stores the information in the handset. The SIM card can be swapped between handsets, which enable to move all the contacts to the new handset with ease.
- international roaming - In case of international roaming handsets with GSM is far better than CDMA handsets because GSM is used in most the markets across the globe.
- Battery Life- The battery life in case of CDMA is lesser as compared to TDMA because CDMA handsets transmit data all the time but TDMA does not require constant transmission.
CDMA is best suited for implementation
1. For Microcell and in-building Systems: CDMA is probably the best system for microcell and in-building systems. The embedded microcell shares the same frequency and has full connectivity with the overlaying macrocell. The capacity of microcell and macrocell is derived and simulated at various traffic distributions. The microcell capacity is 1.03 to 1.12 times the capacity of a regular cell. The capacity of the combined microcell and macrocell is 2.00 to 2.11 times that of a regular cell. The microcell and macrocell performance is also analyzed in terms of RF reliability, soft hand-off factors, and interference and power levels. The macrocell RF reliability will degrade more seriously than that of the microcell. The radio hand-off factors of the microcell are about 11% higher than that of the macrocell. The average required forward traffic channel power of the microcell is about 10% less than that of the macrocell. Microcell engineering guidelines in a commercial CDMA system are also provided. The results show that embedding the microcell in an existing CDMA network could be a very efficient way to improve hot-spot capacity and dead-spot coverage.[k2]
2. For rural area -
- CDMA is that dropped only when the phone is at least twice as far from the base station. So it will be profitable in constructing the number of tower where in case of GSM it will be more because its coverage area is less.
- CDMA handsets can also work with anolog signal; we can get at rural areas where digital signals cannot be transmitted.
3. For Dense area - CDMA uses spread-spectrum technology and a special coding scheme to allow multiple users to be multiplexed over the same physical channel, coding provides more users for the same amount of available power used in other system.
Problems in Implementation of CDMA
1. Spectrum Shortage - The major problem faced by CDMA technology is lack of available spectrum and this is hindering their subscriber growth. In India CDMA operators are having problem in allocation of spectrum in 1900 MHz band which is common in other regions.
According to B.B. Anand (President for Regulatory Affairs at Reliance Infocomm ) "It is of the utmost importance that the 1900 MHz spectrum be allocated to CDMA operators in India to ensure that the region remains part of the international community of roaming wireless subscribers worldwide,"
In India the band was allocated to Defence Services for their mobile communication usage. However upon the launch of mobile communication services for public, coordination was sought from the Defence department to make the spectrum available for mobile services. The Defence is releasing some spectrum through auction but it is opposed by the GSM operators on the commercial ground for preventing growth of CDMA.
2. Low performance in hilly regions- The towers in CDMA interfere with each other and normally they are installed on much shorter towers and because of this CDMA (IS-95 standard) may not perform well in hilly terrains and India has plenty of such regions in north region.
3. Near Far effect-
In Near Far effect the code transmitted from a transmitter (A) to receiver is interfered by another transmitter (B) which is nearer to the receiver as compared to sender transmitter and this make difficult for identification of the original signals by the receiver .This effect became a major obstacle for successful implementation of CDMA technology.
It is very expensive to solve the near far problem in a CDMA system. The precision power control algorithm are used to solve near far problem which consists of open loop power control and closed loop power control and this make a CDMA transceiver very complicated in both hardware and software implementation.
4. Breathing of Base Stations, where coverage area shrinks under load. In CDMA as number of subscribers using a particular sit increases the range of that site goes down. In other words unlike GSM where number of users are finite but in CDMA users are not finite and in more load the voice quality goes down.
Design and Implementation of CDMA
The CDMA technology works on basic principle of communicating through specific code on available frequency instead of assigning a specific frequency. Since users are specified by code they are able to use same carrier frequency and this eliminates the frequency reuse problem encountered in other technologies.
Steps in generation of CDMA signals
- Analog to Digital Conversion of Voice: CDMA uses Pulse code Modulation (PCM) for converting analog voice or audio to digital signal. This process is also known as digitalization.
- Voice Compression: CDMA uses a special device VOCODER to do voice compression. They are located at the BSC and in the phone. While talking we give space or pause between words and CDMA takes advantage of these pauses in speech activity by using a variable rate vocoder. There are four data rates at which vocoder compresses the voice signal and they are , , 1/8 and Full. Vocoder uses its full rate when a person is talking very fast. It uses the 1/8 rate when the person is nearly silent.
- Encoding and Interleaving: Encoders and interleavers are built into the BTS and the phones. They build redundancy into the signal so that information lost in the transmission can be recovered. CDMA uses convolution encoding to encode audio signals provided by VOCODERS.
- Channelization: The encoded data after encoding and interleaving is again encoded to separate it from other encoded data. After that encoded signals are spreaded over the entire channel.
- Converting digital signal to a Radio Frequency (RF) signal. The channelized data from all calls are combined into one signal and then it is converted into radio frequency signal for transmission.
Interleaving is used to reduce the effects of burst errors and recovering lost bits.
CDMA uses WLASH code to channelize user on the forward or upward link i.e. from BTS to mobile and Pseudorandom noises to channelize user on reverse link or downward link i.e. from mobile to BTS.
A more relevant and self explanatory example is show below of transmission through base station. As explained above each conversation is compressed with a vocoder and the output is doubled by a convolution encoder that adds redundancy for error checking. The encoded bit are then channelized and all calls are combined and modulated onto a carrier frequency.
At receiving site the steps followed are just opposite of sender site
- Conversion of RF signal to digital signal
- Despreading the signal
- Deinterleaving and decoding
- Voice decompression
- Digital to analog voice recovery
RF signal are converted into digital signal by quantization into bits ("chips") by the analog-to-digital converter (ADC). The output is deschannellized and decoded through a decoder know as Viterbi decoder which corrects the errors using the convolutional code. The output goes to the vocoder for decompression and then to digital-to-analog converter (DAC), which decompresses the bits and turns them back into waveforms (sound).
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