2 CDMA Concept a. Introduction Access network, the network between local exchange and subscriber, in theTelecom Network accounts for a major portion of resources both in terms of capital andmanpower. So far, the subscriber loop has remained in the domain of the copper cableproviding cost effective solution in past. Quick deployment of subscriber loop, coverageof inaccessible and remote locations coupled with modern technology have led to theemergence of new Access Technologies. The various technological options available areas follows : 1. Multi Access Radio Relay 2. Wireless In Local Loop 3. Fibre In the Local Loop2.1.1 Wireless In Local Loop (WILL) Fixed Wireless telephony in the subscriber access network also known asWireless in Local Loop (WLL) is one of the hottest emerging market segments in globaltelecommunications today. WLL is generally used as “the last mile solution” to deliverbasic phone service expeditiously where none has existed before. Flexibility andexpediency are becoming the key driving factors behind the deployment of WILL. WLL shall facilitate cordless telephony for residential as well as commercialcomplexes where people are highly mobile. It is also used in remote areas where it isuneconomical to lay cables and for rapid development of telephone services. Thetechnology employed shall depend upon various radio access techniques, like FDMA,TDMA and CDMA. Different technologies have been developed by the different countries like CT2from France, PHS from Japan, DECT from Europe and DAMPS & CDMA from USA.Let us discuss CDMA technology in WILL application as it has a potential ability to
tolerate a fair amount of interference as compared to other conventional radios. This leadsto a considerable advantage from a system point of view. b. SPREAD SPECTRUM PRINCIPLE Originally Spread spectrum radio technology was developed for military use tocounter the interference by hostile jamming. The broad spectrum of the transmitted signalgives rise to “ Spread Spectrum”. A Spread Spectrum signal is generated by modulatingthe radio frequency (RF) signal with a code consisting of different pseudo random binarysequences, which is inherently resistant to noisy signal environment. A number of Spread spectrum RF signals thus generated share the same frequencyspectrum and thus the entire bandwidth available in the band is used by each of the usersusing same frequency at the same time. Fig : 10.1 CDMA ACCESS – A CONCEPT On the receive side only the signal energy with the selected binary sequence codeis accepted and original information content (data) is recovered. The other users signals,whose codes do not match contribute only to the noise and are not “despread” back inbandwith (Ref Figure 10.1 ) This transmission and reception of signals differentiated by“codes” using the same frequency simultaneously by a number of users is known as CodeDivision Multiple Access (CDMA) Technique as opposed to conventional method ofFrequency Division Multiple Access and Time Division Multiple Access.
In figure 10.1 it has been tried to explain that how the base band signal of 9.6Kbps is spread using a Pseudo-random Noise (PN) source to occupy entire bandwidth of1.25 Mhz. At the receiving end this signal will have interference from signals of otherusers of the same cell, users of different cells and interference from other noise sources.All these signals get combined with the desired signal but using a correct PN code theoriginal data can be reproduced back. CDMA channel in the trans and receive direction isa FDD (Frequency Division Duplexing) channel. The salient features of a typical CDMAsystem are as follows: v Frequency of operation: 824-849Mhz and 869-894 Mhz v Duplexing Mehtod: Frequency Division Duplexing (FDD) v Access Channel per carrier: Maximum 61 Channels v RF Spacing: 1.25 Mhz v Coverage: 5 Km with hand held telephones and approx. 20 Km with fixed units. The different types of codes used for identification of traffic channels and usersidentification etc as follows: c. Different Codes2.1.2 Walsh Code : In CDMA the traffic channels are separated by uinique “Walsh” code. All suchcodes are orthogonal to each other. The individual subscriber can start communicationusing one of these codes. These codes are traffic channel codes and are used fororthogonal spreading of the information in the entire bandwidth. Orthogonality providesnearly perfect isolation between the multiple signals transmitted by the base station. The basic concept behind creation of the code is as follows: (a) Repeat the function right (b) Repeat the function below (c) Invert function (diagonally)
0 ----- 0 0 -------- 0 0 0 0 0 1 0 1 0 1 0 0 1 1 0 1 1 02.1.3 Long code : the long pseudo random noise (PN) sequence is based on 2 42 characteristicpolynomial. With this long code the data in the forward direction (Base to Mobile) isscrabled. The PN codes are generated using linear shift registers. The long code is uniquefor the subscribers and is known as users address mask.2.1.4 Short Code : The short pseudo random noise (PN) sequence is based on 2 15 characteristicpolynomial. This short code differentiates the cells & the sectors in a cell. It also consistsof codes for I & Q channel feeding the modulator. d. Advantages CDMA wireless access provides the following unique advantages:2.1.5 Larger Capacity : let us discuss this issue with the help of Shannon’s Theorem. It states that thechannel capacity is related to product of available band width and S/N ratio. C = W log 2 (1+S/N) Where C = channel capacity W = Band width available S/N = Signal to noise ratio. It is clear that even if we improve S/N to a great extent the advantage that we areexpected to get in terms of channel capacity will not be proportionally increased. Butinstead if we increase the bandwidth (W), we can achieve more channel capacity even ata lower S/N. That forms the basis of CDMA approach, wherein increased channel
capacity is obtained by increasing both W & S/N. The S/N can be increased by devisingproper power control methods.220.127.116.11 Vocoder and variable data rates: As the telephone quality speech is band limited to 4 Khz when it is digitized withPCM its bit rate rises to 64Kb/s Vocoding compress it to a lower bit rate to reducebandwidth. The transmitting vocoder takes voice samples and generates an encodedspeech/packet for transmission to the receiving vocoder. The receiving vocoder decodesthe received speech packet into voice samples. One of the important feature of thevariable rate vocoder is the use of adaptive threshold to determine the required data rate.Vocoders are variable rate vocoders. By operating the vocoder at half rate on some of theframes the capacity of the system can be enhanced without noticeable degradation in thequality of the speech. This phenomenon helps to absorb the occasional heavy requirementof traffic apart from suppression of backgraound noise. Thus the capacity advantagemakes spread spectrum an ideal choice for use in areas where the frequency spectrum iscongested.2.1.6 Less (Optimum) Power per cell: Power Control Methods: As we have already seen that in CDMA the entirebandwidth of 1.25Mhz is used by all the subscribers served in that area. Hence they allwill be transmitting on the same frequency using the entire bandwidth but separated bydifferent codes. At the receiving end the noise contributed by all the subscribers is addedup. To minimize the level of interfering signals in CDMA, very powerful power controlmethods have been devised and are listed below: 1. Reserve link open loop power control 2. Reserve link closed loop power control 3. Forward link power control The objective of open loop power control in the reverse link (Mobile to Base)is that the mobile station should adjust its transmit power according to the changes in itsreceived power from the base. Open loop power control attempts to ensure that the
received signal strength at the base station from different mobile stations, irrespective oftheir distances from the base site, should be same. In Closed loop power control in reverse link, the base satation provides rapidcorrections to the mobile stations’ open loop estimates to maintain optimum transmitpower by the mobile stations. The base station measures the received signal strengthfrom the mobile connected to it and compares it with a threshold value and a decision istaken by the base every 1.25 ms to either increase or decrease the power of the mobile. In forward link power control (Base to Mobile) the cell (base) adjusts its powerin the forward link for each subscriber, in response to measurements provided by themobile station so as to provide more power to the mobile who is relatively far away fromthe base or is in a location experiencing more difficult environment. These power control methods attempt to have an environment which permits highquality communication (good S/N) and at the same time the interference to other mobilestations sharing the same CDMA channel is minimum. Thus more numbers of mobilestation are able to use the system without degradation in the performance. Apart from thecapacity advantage thus gained power control extends the life of the battery used inportables and minimizes the concern of ill effects of RF radiation on the human body.2.1.7 Seamless Hand-off : CDMA provides soft hand-off feature for the mobile crossing from one cell toanother cell by combining the signals from both the cells in the transition areas. Thisimproves the performance of the network at the boundaries of the cells, virtuallyeliminating the dropped calls.2.1.8 No Frequency Planning : A CDMA system requires no frequency planning as the adjacent cells use thesame common frequency. A typical cellular system (with a repetition rate of 7) and aCDMA system is shown in the following figures which clearly indicates that in a CDMAnetwork no frequency planning is required.
Fig : 10.2 CDMA Frequency Fig : 10.3 Frequency Reuse of 7 in GSM2.1.9 High Tolerance to Interference : The primary advantage of spread spectrum is its ability to tolerate a fair amount ofinterfering signals as compared to other conventional systems. This factor provides aconsiderable advantage from a system point of view.2.1.10 Multiple Diversity : Diversity techniques are often employed to counter the effect of fading. Thegreater the number of diversity techniques employed, the better the performance of thesystem in a difficult propagation environment. CDMA has a vastly improved performance as it employs all the three diversitytechniques in the form of the following: A .Frequency Diversity: A wide band RF signal of 1.25 Mhz being used. B. Space Diversity: Employed by way of multipath rake receiver.
C. Time Diversity: Employed by way of symbol interleaving error detection and correction coding. e. Capacity Considerations Let us discuss a typical CDMA wireless in local loop system consisting of asingle base station located at the telephone exchange itself, serving a single “cell”. Inorder to increase the number of subscribers served the cell is further divided into“sectors”. These sectors are served by directional antennas. The capacity of a cellular system is claimed to be 20-40 active lines per sector per1.25 MHz for a single CDMA Radio Channel. In WLL environment assuming an averagebusy hour traffic of 0.1 Erlang, 400 subscribers can be served per sector over a single1.25 MHz channel. Fig : 10.4 A Typical six sectored cell Assuming typically six sectors in a cell the total capacity of a CDMA networkconsisting of 1.25 MHz duplex channels is 2400 (400x6) subscribers. Capacity can further be increased if we use another frequency on the same basestation covering the same geographical area (overlapping cell). Thus in 10 Mhz in thebandwidth we can utilize 5 MHz of bandwidth in the forward link and 5 Mhz in thereverse link. Hence if we have 4 RF carriers in 5 Mhz bandwidth, the network cansupport 12000 (5x400x6) subscribers per cell. A typical CDMA wireless in local loopsystem is depicted in the above figure10.4.
f. Conclusion Hence we see that use of common frequency, multipath rake receiver, powercontrol & variable bit rate vocoding and soft hand-off features of CDMA give us thebenefits of no frequency planning, larger capacity, flexibility alongwith high performancequality. XXXX