This paper introduces the basic design of SDR, use of VLSI chips in mobiles and their working principles.
1. VLSI - Very large scale integration
2. SDR - Soft ware defined Radio
3. R.F - Radio Frequency
4. I.F -intermediate frequency
Tele communication industry is one of the highly developed segment which is dependent on VLSI technology. SDR also plays a prominent role in the mobile communication VLSI helps in reducing the size and price of the mobile where as SDR increases the flexibility of the mobile.
Existing networks in telecommunication can be classified into two major types -
a) GSM (Global System for Mobile communication)
b) CDMA (Code Division Multiple Access)
These networks differ in their accessing frequencies. The problem encountered while using these network is, both these networks can not be accessed from the same hand set. Now a days dual SIM card phones have been developed using SDR.
Soft ware radio provides solution by using super heterodyne radio frequency front end.
Soft ware defined radios have significant utility for the military and cell phone services both of which must serve a wide variety of changing radio protocols in real time.
The following segments introduce the use of VLSI and SDR, their working principles, advantages and disadvantages
VLSI technology :- Most of the student are exposed to ICs at a very basic level involving SSI and MSI circuits like multiplexers, encoders, decoders etc. VLSI is the next stage of SSI and MSI. This field involves packing more and more logic devices into smaller and smaller areas. Particularly in this era of Nano technology simplicity plays a very prominent role for any device. This is possible by using VLSI design. But this design involves a lot of expertise in many fronts like system architecture, logic and circuit design way for fabrication etc. A lot of knowledge is required for the actual implementation and design of VLSI.
Digital VLSI circuits are predominantly CMOS based. The way normal blocks like latches and gates are implemented is different from what students have seen so far, but the behaviour remains the same. All the miniaturization involves new things to consider. A lot of thought has to go into actual implementations as well as design. Let us look at some of the factors involved.
1. Circuit Delays. Large complicated circuits running at very high frequencies have one big problem to tackle - the problem of delays in propagation of signals through gates and wires ... even for areas a few micrometers across! The operation speed is so large that as the delays add up, they can actually become comparable to the clock speeds.
2. Power. Another effect of high operation frequencies is increased consumption of power. This has two-fold effect - devices consume batteries faster, and heat dissipation increases. Coupled with the fact that surface areas have decreased, heat poses a major threat to the stability of the circuit itself.
3. Layout. Laying out the circuit components is task common to all branches of electronics. What’s so special in our case is that there are many possible ways to do this; there can be multiple layers of different materials on the same silicon, there can be different arrangements of the smaller parts for the same component and so on.
The power dissipation and speed in a circuit present a trade-off; if we try to optimize on one, the other is affected. The choice between the two is determined by the way we chose the layout the circuit components. Layout can also affect the fabrication of VLSI chips, making it either easy or difficult to implement the components on the silicon.
Most of the today’s VLSI designs are classified into 3 categories -
Analog :- Small transistor counts circuits such as amplifiers, data converters ,sensors etc.,
Application Specific integrated circuits :- Progress in the fabrication of ICs has enabled us to create fast and powerful circuits in smaller and smaller devices. This also means we can pack a lot more of functionality in the same area. This is key for design of ASIC`s .
Systems on chip :- These are highly complex mixed signal circuits (digital & analog on the same chip).
Mobiles developed by using all the above VLSI designs will be simple as they contain a large number of transistors on one chip, more ever they become cheap. Thus VLSI makes the mobiles compact, affordable and energy efficient.
Role of SDR in mobiles:- Frequency is an important term in the operation any networks. Cell phones are categorized in to CDMA and GSM based on this principle. CDMA operate with in a frequency range of 1 MHz to 800 MHz. GSM operates for a frequency range of 900 MHz to 1900 MHz. Thus both these networks can not be access for a single hand set. To solve this problem soft ware defined radio is developed. SDR can tune to any frequency band and relieve any modification across a large frequency spectrum Operating principles of SDR:- There are two concepts in the working of SDR, one is ideal and other is practical.
Ideal concept:- The receiver has an analog attached to digital converter and this is attached to the antenna. A digital signal processor would read the converter and its soft ware would transform the stream of data form the converter to any other form it requires. An ideal transmitter is of the similar type. A digital signal processor would generate a stream of numbers which will be sent to a digital to analog converter connected to the radio antenna. But this ideal stream is not completely realizable.
Practical concept :- Current digital electronics are too slow to receive tropical radio signal over approximately 40 MHz directly. An ideal soft ware radio has to collect and process samples at more than twice the maximum frequency at which it is to operate for frequencies below 40 MHz a direct conversion hard ware solution is required. In this hard ware solution an ADC converter is directly connected to antenna. The output stream of digital data obtained from analog to digital converter is then passed to a soft ware defined processing stage for frequencies above 40 MHz . The actual analog to digital converter does not perform with sufficient speed, so direct conversion is not possible. To solve this problem a super heterodyne RF front end is adopted.
Super heterodyne:- It consist of frequency mixer and reference oscillator to heterodyne the radio signals to lower frequencies. The mixer changes the frequency of the signal. The Super heterodyne RF front end lower the frequency of the received to intermediate frequency values under 40 MHz convertible limit. This intermediate frequency is then treated by ADC. Thus by using same mobile both the frequencies corresponding to GSM & CDMA networks can be accessed.
1. Lower package count.
2. Low board space.
3. Fewer board level connections.
4. Higher performance.
5. Reliability and lower cost due to the lower chip count
1. Long design.
2. Long fabrication time.
3. Higher risk project.
4. Spiking problem.
5. Leakage of power.
However, CMOS transistor can reduce most of the problems.
CONCLUSION :- If we take geographical conditions into account, some networks will be advantageous in one part of the world and the other at some other places. Using different phones for this purpose will be some what inconvenient. So having ability to use all the networks will be a welcome always. With the advent of SDR one needs just one set to access different networks, thereby, providing flexibility. Dual Simcards phones developed based on SDR technology have gained good response. In countries like India, price place an important role in determining the demand for a particular product, and also interest towards compact and simple devices is increasing day by day. In this context a lot of progress has been made in the circuit design. As VLSI has succeeded in reducing the cost and also making the product efficient it has gained a lot of craze. Most of the companies are producing the product based on single chip design.