### Test&Measurement

Digital Basics

It may seem out of place to jump from RF propagation to a review of digital technology, but todays complex communications systems are a blend of digital and RF. In fact, the RF portion of these new standards is really the last step in a many step process of getting the information transmitted. Before we can begin to understand digital modulation, it is important to understand the form of the information we are modulating.

When we speak of digital we are talking about the use of ones and zeros to represent data and perform actions. For instance, a computer can use a series of ones and zeros to represent a particular letter in the alphabet, which then allows the computer to store the frequency electronically. It is important to understand that computers speak in digital, and even the 'analog' first generation cellular phone system needs the ability to transmit digital information, ones and zeros, in order to relay signaling information.

Usually this is done by using voltage levels to represent the ones and zeros. Thus a high voltage might represent a one, and a low voltage might represent a zero. Often, a bipolar digital system is used, in which a high is +1V, and a low is -1V. This allows some easier mathematical manipulation and is generally used in wireless applications.

You begin to see that ones and zeros could really be represented by any method that has two easily recognizable states--high and low voltage is but one way to perform this task. For instance we can shift frequency over set periods of time--one frequency for a moment for a one, then another if we want to represent a zero. Another way would be to change the amplitude of the signal depending on the bit to send, perhaps turning the amplitude on for one and down for zero, naturally some sort of clocking system would need to be in place for this to work.

We could also shift the phase of a signal--one phase represents a one, while another represents a zero. Additionally we could use relative changes as opposed to absolute positions. For instance a change of -45

^{0}of a phase might represent a one, and a change of +45

^{0}might represent a zero. this is how we will represent ones and zeros on the RF carrier. this form of shifting from one state to another to represent ones and zeros is known as shift keying.

Thus we have frquency shift keying (FSK), phase shift keying (PSK), amplitude shift keying (ASK), and a combination of any two of these systems.

Up to the point were the data is actually modulated on the RF carrier that will be sent in the air, data is manipulated in standard high or true and low or false voltage digital levels. It is important to review some of the basic digital functions in order to understand how digital sequences are manipulated. Several logic gates are common in cellular applications.

AND gate: in the AND gate, two trues at the inputs will result in a true output. Any false on an input will result in a false output.

OR gate: In the OR gate, any true value on an input will result in a true output. The only time it will output false is if both outputs are false.

Exclusive OR (X-OR) gate: The X-OR gate is very important in digital systems, particularly spread spectrum systems. The X-OR gate is used to spread sequences. A true is output when opposite values are input. In other words when the input is false and true the output is true.

Inverter gate: Naturally, the output will be the inverse of the input.

Shift register: A shift register is a logic device used to store a binary value. As it is toggled, it moves its stored value to the output.

another concept that is important to understand is digital correlation. Correlation is the amount of similarity between two items. In this case, it is an actual measurement of the similarity between two digital sequences. By multiplying the bipolar values, we will end up with a digital waveform product. If we integrate, or find the area of) this resulting waveform, we have a correlation measurement. If 100% of the area is present it would indicate perfect correlation, or a one. Perfectly uncorrelated signals woulod be measured as zero. If two digital signals are perfectly uncorrelated, it can be said that they are orthogonal to each other. You may have heard of Walsh codes in code-division multiple access (CDMA). These Walsh codes are actually orthogonal sequences. --Andrew Miceli

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