CDMA Technology
5. Auto correlation: It is that phenomena under which the signal gets interfered with its past and future values which might exist due to unpredictable time delays. It can also be termed as a signal’s cross correlation with itself.
Primary requirements for any wireless mode of communication include high quality of service and data secrecy. Realizing that these two factors are to be fulfilled in the most optimized ways without the costs going too high, CDMA, a spread spectrum based technology came into existence. 
Initially restricted to the armed forces, this technology was commercially launched in 1995 by Qualcomm Telecommunications and now, as per Q4 of the year 2011, CDMA has over 8 billion voice and data customers in the 122 countries that it operates.
Initially restricted to the armed forces, this technology was commercially launched in 1995 by Qualcomm Telecommunications and now, as per Q4 of the year 2011, CDMA has over 8 billion voice and data customers in the 122 countries that it operates.
What CDMA Means?
Code: It refers to the string of binary sequence that the transmitter and the receiver share. This code encodes the information into a low frequency signal before it is transmitted over a channel. This same code is used by the receiver to decode the information. The receiver attains the code with the help of the nearest base station.
Division: In CDMA a single channel is divided into numerous slots which can be used by multiple users. This is possible because of the use of unique code.
Multiple Accesses: Due to code based communication, multiple users can communicate and access the same channel simultaneously without any undesirable interference and loses.
Why we need CDMA?
CDMA is regarded as an improvement over GSM technology whose need can be easily understood by taking a simple example. Consider 5 couples that have their respective partners in different rooms. The partners are permitted to communicate only to each other and each is provided with a communication instrument for the purpose. The instruments are aided with a medium that facilitates the communication. The medium can be wired or wireless and is termed as “channel” in communication terminology. Since there is only one channel, the users are allotted some amount of time for which they can utilize the channel. In this case, let it be 5 seconds. So, every user communicates for 5 seconds and then the channel is used by other users.
The channel has a certain limit of allotting time slots and cannot accommodate more users after that. Let’s assume that in this case maximum number of users that the channel can accommodate is 6 i.e. 3 couples can use the channel. Hence, if all the couples want to communicate, 2 couples might have to wait till the channel has an empty slot. Also, there are chances that one couple might interfere in communication of the other due to sharing of same channel. This can be termed as “cross correlation” and is a serious problem in GSM operations. This is how a usual GSM system works. In normal GSM, the channel utilization time or “time burst” is determined by dynamic scheduling where number of users determine the time for which channel is used by a user.
When we go the CDMA way, every user’s voice is converted to a unique code which only the intended recipient instruments can understand. The code here is a “spreading sequence” of digital bits and is common between both the transmitter and the receiver. Since, code is digital, the information to be sent is also required to be converted to a digital format. With this method, there are no time boundations and even if all users are using the channel at the same time, there will be no interference and secrecy will be maintained. In CDMA, since the code is unique for every transmitter, it is determined by the receiver in two steps: Acquisition and tracking.
Under acquisition, the receiver acquires the sent signal and generates the decoding sequence which it receives from the base station. In tracking, it keeps synchronization between the received signal and decoding sequence so that the output is exactly same as the input.
History of CDMA
The idea of using CDMA as commercialized or licensed technology came in the year 1988. In 1993, the first protocol under CDMA IS-95 was introduced and in 1995, it was commercialized. Since that time, lots of changes in CDMA technology have occurred in terms of resource allocation, data usage, bit rate and several such factors.
Interim Standard-95 or TIA-EIA-95 is the first 2G-CDMA based cellular standard by Qualcomm and has been branded as cdmaOne. IS-95 defines forward (Base station to mobile) and reverse (mobile to Base station) link specifications. CDMA2000 is a 3G standard, backward compatible with IS-95. IS-2000 or CDMA2000 1X is the core wireless air interface standard. CDMA 2000 differs from IS-95 is that it includes beam forming; this increases the gain at the mobile and allows better SNR and a larger number of users. Also, CDMA2000 has double the capacity of IS-95.
WCDMA was developed in order to support high bandwidth applications like gaming, multimedia services, etc. The later version of CDMA is WCDMA systems which combine the CDMA air interface with GSM based networks. In contrast to cdmaOne and CDMA2000 (which uses 1.25 MHz wide radio signal), WCDMA uses a 5 MHz wide radio signal and a chip rate of 3.84 Mbps, which is about three times higher than the chip rate of CDMA2000 (1.22 Mbps). Thus WCDMA offers higher capacity and QoS.
Understanding CDMA Terminology:
1. Cross correlation: It can be defined as the measure of alikeness that two signals share. When signals are cross correlated with each other, they suffer interference and efficiency losses. It limits multiple signal transmission and hence should be avoided.
2. Bandwidth: Spectral width over which the frequencies of a system are defined. In usual signals, the bandwidth needed is less but the power required is quite high. However, in CDMA, power requirement is low but bandwidth required is high. Since we use different coding schemes for all transmissions, multiple users can transmit over a single frequency and bandwidth used in excess is not wasted.
3. Chip: It’s a single digital pulse of “spreading sequence” that is superimposed on the information signal to create the low frequency signal.
4. Spread Spectrum: Mode of signal transmission where it takes up more bandwidth but reduces itself in terms of power required for transmission. In usual signals, the bandwidth acquired is less but the power is quite high. However, in CDMA, power is low but excess of bandwidth is required.
5. Auto correlation: It is that phenomena under which the signal gets interfered with its past and future values which might exist due to unpredictable time delays. It can also be termed as a signal’s cross correlation with itself.
6. Spreading Sequences: A spreading sequence is a non-ambiguous identification for a transmitter - receiver pair. Spreading Codes are noise-like and random signals generated at the transmitter. The same signal must be generated at the receiver in synchronization. A spreading sequence consists of C units called chips. The chips are two–valued. The autocorrelation function of a spreading sequence reflects the similarity of these sequences with a replica of itself delayed by a time gap. For a given time gap to be zero the autocorrelation value is one. In any other case than this, the autocorrelation value should be small to minimize the interference among copies of the original signal that are generated and delayed by multi-path propagation. The cross–correlation value of two different spreading sequences represents the interference level for two signals from different wireless terminals with delay τgap. This value should be as small as possible for all τgap such that a maximum number of subscribers are allowed in the cell.
Desirable properties of spreading codes are given below. These properties make sure that randomness is there in spreading sequence generation and efficient encryption takes place.
Randomness here can be referred as the inability to predict or be deterministic about appearance of “0” and “1” in any sequence. The spreading for a signal is fixed in size but the variable occurrence of the bits reduces the chances of two signals following the same pattern. However, the data is encrypted using a known sequence, therefore, it can be referred to as pseudorandom.
. Balance property:
Relative frequencies of “0” and “1” should be ½ .
. Run property:
Run lengths (length of the spreading sequences comprising of “0”s and “1”s) should follow the criteria listed below:
· Half of all run lengths should be unity.
· One - quarter should be of length two.
· One - eighth should be of length three.
· A fraction 1/2n of all run lengths should be of length n for all finite n.
. Autocorrelation property:
If the random sequence is shifted by any nonzero number of elements, the resulting sequence should have an equal number of agreements and disagreements with the original sequence. If auto correlation is less, PN sequences are more random.
Spreading sequences can be divided into orthogonal and pseudo–noise spreading sequences.
Orthogonal Sequence
Orthogonal sequences offer zero cross–correlation for τgap = 0. If all transmitters are synchronized and no multi–path is considered, the multiple access interference can be neglected. For any other value of τgap they have large cross–correlation values. Therefore orthogonal sequences are only applied if perfect synchronism can be guaranteed within the system. The autocorrelation properties of orthogonal sequences are also poor if τgap = 0. This happens if we consider multi–path interference. In such a situation equalization is applied to recover the original signal.
Walsh and orthogonal Gold sequences are some of the commonly used orthogonal codes.
Pseudorandom Sequence
Pseudorandom Sequence (PN) is a deterministically generated binary sequence that nearly satisfies the aforesaid properties. Periodic binary sequences can be conveniently generated using linear feedback shift registers (LFSR). In these LFSRs, the current input is the function of the preceding value of it. If the number of stages in the LFSR is “r” & period of the sequence is “P”, then P £ 2r – 1. If P = 2r – 1, sequence is called Maximal Length sequence or m-sequences. Maximal Length sequences, Gold sequences, and Kasami sequences are few of the popular PN sequences.
Randomness properties of PN sequence are
. Balance property - Of the 2r - 1 terms , 2r-1 are “1” and 2r-1–1 are “0”. Thus the unbalance is 1/P.
. Run length property - Relative frequency of run length “n” (number of “0”s or “1”s) is 1/ 2n for n £ r-1 and 1/(2r - 1) for n = r. One run length each of r-1 zeros and r ones occurs. There are no run lengths for n > r
. Autocorrelation property - The number of disagreements exceeds the number of agreements by unity. Thus again the discrepancy is 1/p
Whenever synchronism cannot be achieved PN sequences should be applied. These sequences are easy to generate and offer reasonable cross–correlation and autocorrelation values. For synchronous systems orthogonal codes should be used. In cellular wireless communication orthogonal codes should be used for the down–link and PN sequences should be used for the up–link.
Working
Code Division Multiple Access is a multiplexing scheme where voice and data are consigned with different codes rather than frequency. This makes sure that large amounts of data can be transmitted wirelessly with a high quality of service.
In Code Division Multiple Access (CDMA) systems all users transmit in the same bandwidth simultaneously. In this technique, the information signal is coded with a code sequence or spreading sequence and the coded signal is then transmitted to the receiver. This process spreads the frequency spectrum of a data-signal and as a result the bandwidth occupancy is much higher than required. The codes used for spreading have low cross-correlation values and are unique to every user. This is the reason that a receiver which has knowledge about the code/ spreading sequence of the intended transmitter is capable of selecting and decoding the desired signal.
Since the bandwidth of the coded signal is much larger than the information bandwidth, the coding process is also called a spread spectrum modulation, while the coded signal is called a spread-spectrum signal. The spreading of the information signal gives the CDMA its multiple access capability. One can have a better detail about CDMA by considering the figure illustrated below.
This particular aspect of spreading the data over the bandwidth depends on the spreading technique used and based on these techniques, CDMA can be classified into:
1. Direct Sequence Spread Spectrum-CDMA
In a Direct Sequence Spread Spectrum (DSSS) transmitter, the information signal i (t) is directly modulated by a spreading sequence. The spreading sequence consists of a number of spreading chips with time duration. The information signal consists of a number of information bits with time duration. Spreading is achieved if multiple chips represent one bit as code created then is encrypted to high standards.
DSSS signals,s(t) can be generated by multiplying information signal, i(t) and spreading sequence, c(t). Figure below shows the generation of a DSSS signal:
At the receiver side, received signal is again multiplied by the same spreading sequence to dispread the signal. Filtering this signal gives us the information signal.
2. Frequency Hopping Spread Spectrum-CDMA
Frequency Hopping Spread Spectrum (FHSS) systems change the carrier frequency of the modulated information signal periodically. The available frequency spectrum is divided into N frequency slices and the set of available frequencies is called a hop–set. The carrier frequency changes (to one of the frequencies in a hop-set) after regular time interval.
While DSSS systems occupy the whole bandwidth, FHSS systems use only one frequency slice at any point in time. FHSS systems are divided into slow and fast frequency hoppers. If one information bit is transmitted over several frequency slices the frequency hopper is referred to be fast. And, if several bits are transmitted over one frequency slice it is referred to be a slow hopper.
The advantage of FHSS systems in contrast to DSSS systems is the less strict synchronization requirement. Spectrum of the information signal, DSSS signal and FHSS signal is shown below:
3. Time Hopping Spread Spectrum-CDMA
In Time Hopping Spread Spectrum (THSS) system, the time axis is divided into frames of the duration τframe. As shown in figure, each frame is divided again into N slots with time duration τslot. A single user uses one slot out of k possible slots within one frame and sends data with k times higher data rate in contrast to the situation where the data is transmitted within the whole frame.
4. Hybrid Spread Spectrum-CDMA
A hybrid system uses combination of the spread spectrum technologies listed above in these formats: DS-FH, DS-TH, FH-TH, and DS-FH-TH.
It is to be noted here that the Spread Spectrum techniques do not attempt to allocate disjoint frequency or time slot resources. Instead, this approach allocates all resources to simultaneous users, controlling the power transmitted by each user to the minimum required to maintain a given Sound to Noise Ratio (SNR). Out of all these three spreading technologies, Direct Sequence Spread Spectrum is the most preferred mode for CDMA operation.
For all these Spread Spectrum Techniques, one needs a spreading sequence. Let us define some basics and desirable features of spread sequences.
Advantages of CDMA
1. Less multi-path interference
CDMA receivers resolve multi–path interference if signals arrive more than one chip apart from each other by using RAKE receivers. A figure below illustrates the basic principle of RAKE receiver.
After spreading and modulation of the information signal on the sender side, the signal suffers from the multi–path channel. The multi–path signal (coming from different propagation paths) is demodulated and passed to the RAKE receiver. The ideal RAKE receiver contains a receiver finger for each multi–path component; the number of fingers is limited by the receiver‘s hardware complexity or costs. In each finger the signals are dispread and time aligned with one of the multi–path channels. After the dispreading process the signals are weighted and combined.
2. Power Control Mechanism
In a DS–CDMA system all transmitters use the same bandwidth at the same time to send their information to the receiver. If multiple transmitters send information to one receiver, the signals are received with different power levels, because of different distances of transmitters to the receiver. Due to the attenuation effect transmitters closer to the receiver have higher power levels than transmitters that are far away. This effect is called the near–far effect. In addition, the signal strength differs dramatically due to the changing propagation conditions (moving obstacles, traffic characteristics, hand over). To overcome the changing signal strength, power control entities are implemented in the transmitters. Transmitter Power Control adjusts transmitter power to ensure that the signals arrive at the receiver with the same power level. Modern CDMA systems use Adaptive power control.
3. Soft Handoff
Process of moving from one cell to another without interrupting the connectivity is called handoff. In GSM hard handoff occurs at the cell boundary. A CDMA system uses soft handoff, a unique feature which allows cell phone to commence Communication with a new BS without interrupting communication with old BS. Soft handoff provides different site selection diversity; process of combining information from multiple transmitted packets to increase the effective SNR of received packets.
4. Miscellaneous
· Low power spectral density.
As the signal is spread over a large frequency band, the Power Spectral Density is small, so less susceptible to interference.
· In all situations the whole frequency-spectrum is used.
· Privacy due to unknown random codes.
· Codes are unknown to a hostile user; therefore, it is hardly possible to detect the message.
· Spread spectrum reduces multi-path effects.
· Good anti-jam performance.
· Bandwidth availability on demand.
Applications of CDMA
CDMA has been to be immensely useful not only for voice calls but also for data transmission. CDMA based data exchange services have been catering not only to mobile phones but are also being used in internet support devices for computer and laptops.
For businesses, CDMA aids in providing high speed push to talk and push to email services. Push to talk gives the mobile an ability to be used as a walky-talky device while push to mail allows one to have email facility every time. These services are often exempted from the service charges by the operator, making CDMA cost effective.
Also, Video conferencing feature allows business representatives from all over the world to communicate altogether without any hassles.
For Consumers, plenty of services are there depending on their connection speed. It covers the basic text messaging service and multimedia messaging service. Also, for medium and high speed connections various services such as monetary transactions, internet surfing, multimedia, gaming, social networking, video calls etc are available at quite affordable prices under CDMA.
CDMA is openly regarded as the highway mode of wireless communications and has been responsible for giving fast and safe modes of data exchange such as 3G and recently it has been merged with GSM to give high speed 4G or LTE internet services. Highly digital, CDMA stands as a power efficient and hence an economic data exchange mode which can connect almost every part of the world without posing risk to the data in terms of secrecy and losses.
Limitations of CDMA
1. High Degree of Synchronization between the sender and the receiver:
In CDMA, use of orthogonal codes is widely present. When these codes are applied, even a slight mismatching between the transmitter and the receiver can decode the whole message in a wrong way and produce lots of errors.
2. Power Variations When Load is Excess:
CDMA is known for high voice clarity and data efficiency. However, whenever the number of users in the channel increases quite above than the expected, there are variable power losses as the coverage area shrinks. This directly affects the efficiency of the system and the SNR.
The same problem in power variations is experienced whenever the user is far from the BS. In this case, user nearer to the BS will receive more efficient results than those at farther locations. This has been termed as the Near-Far Problem in CDMA communication.
3. Usage Restrictions: Qualcomm Telecommunications holds the major patents of CDMA and hence, its usage is limited.
4. Less Popular than GSM: CDMA is still not well spread all over the world as compared to GSM and though more efficient, it will take some time to reach all corners of the world.
5. Practical Limitations: In the case of excessive number of CDMA users, there can be problems in giving each user a unique spreading sequence. This may limit the performance as well as the number of users.
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