1 Introduction to WLL
Version 1 Revision 0 CDMA Concept
2CDMA Concept
2.1Introduction
Access network, the network between local exchange and subscriber, in the Telecom Network accounts for a major portion of resources both in terms of capital and manpower. So far, the subscriber loop has remained in the domain of the copper cable providing cost effective solution in past. Quick deployment of subscriber loop, coverage of inaccessible and remote locations coupled with modern technology have led to the emergence of new Access Technologies. The various technological options available are as follows :
1.Multi Access Radio Relay
2.Wireless In Local Loop
3.Fibre In the Local Loop
2.1.1Wireless In Local Loop (WILL)
Fixed Wireless telephony in the subscriber access network also known as Wireless in Local Loop (WLL) is one of the hottest emerging market segments in global telecommunications today. WLL is generally used as “the last mile solution” to deliver basic phone service expeditiously where none has existed before. Flexibility and expediency are becoming the key driving factors behind the deployment of WILL.
WLL shall facilitate cordless telephony for residential as well as commercial complexes where people are highly mobile. It is also used in remote areas where it is uneconomical to lay cables and for rapid development of telephone services. The technology employed shall depend upon various radio access techniques, like FDMA, TDMA and CDMA.
Different technologies have been developed by the different countries like CT2 from 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 leads to a considerable advantage from a system point of view.
2.2SPREAD SPECTRUM PRINCIPLE
Originally Spread spectrum radio technology was developed for military use to counter the interference by hostile jamming. The broad spectrum of the transmitted signal gives rise to “ Spread Spectrum”. A Spread Spectrum signal is generated by modulating the radio frequency (RF) signal with a code consisting of different pseudo random binary sequences, which is inherently resistant to noisy signal environment.
A number of Spread spectrum RF signals thus generated share the same frequency spectrum and thus the entire bandwidth available in the band is used by each of the users using same frequency at the same time.
Fig : 10.1CDMA ACCESS – A CONCEPT
On the receive side only the signal energy with the selected binary sequence code is 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 in bandwith (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 Code Division Multiple Access (CDMA) Technique as opposed to conventional method of Frequency 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 9.6 Kbps is spread using a Pseudo-random Noise (PN) source to occupy entire bandwidth of 1.25 Mhz. At the receiving end this signal will have interference from signals of other users 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 the original data can be reproduced back. CDMA channel in the trans and receive direction is a FDD (Frequency Division Duplexing) channel. The salient features of a typical CDMA system are as follows:
s Frequency of operation: 824-849Mhz and 869-894 Mhz
s Duplexing Mehtod: Frequency Division Duplexing (FDD)
s Access Channel per carrier: Maximum 61 Channels
s RF Spacing: 1.25 Mhz
s 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 users identification etc as follows:
2.3Different Codes
2.1.2 Walsh Code :
In CDMA the traffic channels are separated by uinique “Walsh” code. All such codes are orthogonal to each other. The individual subscriber can start communication using one of these codes. These codes are traffic channel codes and are used for orthogonal spreading of the information in the entire bandwidth. Orthogonality provides nearly 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 ----- 00 ------0000
0 10101
0011
0110
2.1.3 Long code :
the long pseudo random noise (PN) sequence is based on 242 characteristic polynomial. With this long code the data in the forward direction (Base to Mobile) is scrabled. The PN codes are generated using linear shift registers. The long code is unique for the subscribers and is known as users address mask.
2.1.4 Short Code :
The short pseudo random noise (PN) sequence is based on 215 characteristic polynomial. This short code differentiates the cells & the sectors in a cell. It also consists of codes for I & Q channel feeding the modulator.
2.4Advantages
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 the channel 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 are expected to get in terms of channel capacity will not be proportionally increased. But instead if we increase the bandwidth (W), we can achieve more channel capacity even at a 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 devising proper power control methods.
2.1.5.1 Vocoder and variable data rates:
As the telephone quality speech is band limited to 4 Khz when it is digitized with PCM its bit rate rises to 64Kb/s Vocoding compress it to a lower bit rate to reduce bandwidth. The transmitting vocoder takes voice samples and generates an encoded speech/packet for transmission to the receiving vocoder. The receiving vocoder decodes the received speech packet into voice samples. One of the important feature of the variable 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 the frames the capacity of the system can be enhanced without noticeable degradation in the quality of the speech. This phenomenon helps to absorb the occasional heavy requirement of traffic apart from suppression of backgraound noise. Thus the capacity advantage makes spread spectrum an ideal choice for use in areas where the frequency spectrum is congested.
2.1.6 Less (Optimum) Power per cell:
Power Control Methods: As we have already seen that in CDMA the entire bandwidth of 1.25Mhz is used by all the subscribers served in that area. Hence they all will be transmitting on the same frequency using the entire bandwidth but separated by different codes. At the receiving end the noise contributed by all the subscribers is added up. To minimize the level of interfering signals in CDMA, very powerful power control methods 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 its received 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 of their distances from the base site, should be same.
In Closed loop power control in reverse link, the base satation provides rapid corrections to the mobile stations’ open loop estimates to maintain optimum transmit power by the mobile stations. The base station measures the received signal strength from the mobile connected to it and compares it with a threshold value and a decision is taken 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 power in the forward link for each subscriber, in response to measurements provided by the mobile station so as to provide more power to the mobile who is relatively far away from the base or is in a location experiencing more difficult environment.
These power control methods attempt to have an environment which permits high quality communication (good S/N) and at the same time the interference to other mobile stations sharing the same CDMA channel is minimum. Thus more numbers of mobile station are able to use the system without degradation in the performance. Apart from the capacity advantage thus gained power control extends the life of the battery used in portables 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 to another cell by combining the signals from both the cells in the transition areas. This improves the performance of the network at the boundaries of the cells, virtually eliminating the dropped calls.
2.1.8 No Frequency Planning :
A CDMA system requires no frequency planning as the adjacent cells use the same common frequency. A typical cellular system (with a repetition rate of 7) and a CDMA system is shown in the following figures which clearly indicates that in a CDMA network no frequency planning is required.
Fig : 10.2CDMA Frequency
Fig : 10.3Frequency Reuse of 7 in GSM
2.1.9 High Tolerance to Interference :
The primary advantage of spread spectrum is its ability to tolerate a fair amount of interfering signals as compared to other conventional systems. This factor provides a considerable advantage from a system point of view.
2.1.10 Multiple Diversity :
Diversity techniques are often employed to counter the effect of fading. The greater the number of diversity techniques employed, the better the performance of the system in a difficult propagation environment.
CDMA has a vastly improved performance as it employs all the three diversity techniques 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.
2.5Capacity Considerations
Let us discuss a typical CDMA wireless in local loop system consisting of a single base station located at the telephone exchange itself, serving a single “cell”. In order 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 per 1.25 MHz for a single CDMA Radio Channel. In WLL environment assuming an average busy hour traffic of 0.1 Erlang, 400 subscribers can be served per sector over a single 1.25 MHz channel.
Fig : 10.4A Typical six sectored cell
Assuming typically six sectors in a cell the total capacity of a CDMA network consisting of 1.25 MHz duplex channels is 2400 (400x6) subscribers.
Capacity can further be increased if we use another frequency on the same base station covering the same geographical area (overlapping cell). Thus in 10 Mhz in the bandwidth we can utilize 5 MHz of bandwidth in the forward link and 5 Mhz in the reverse link. Hence if we have 4 RF carriers in 5 Mhz bandwidth, the network can support 12000 (5x400x6) subscribers per cell. A typical CDMA wireless in local loop system is depicted in the above figure10.4.
2.6Conclusion
Hence we see that use of common frequency, multipath rake receiver, power control & variable bit rate vocoding and soft hand-off features of CDMA give us the benefits of no frequency planning, larger capacity, flexibility alongwith high performance quality.
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BRBRAITT Jabalpur, Issued in April-2008