Tag Archives: modulation

Seg 3: wifi features and frequency band

  1. Features of Wifi
  • Constitution of network is flexible. It can be infrastructure mode, or a self-organizing mode;
  • Mature technology and rich application;
  • Low price devices;
  • Low power, small recover range;
  • Woring at the frequency 2.4GHz, which is the same as IMS (Industrial/Medical/Scientific). Thus, it may be relatively easy to be interfered. Moreover, Microwave ovens, and cordless phone are also working at IMS frequency.
  1. Wireless frequency band

ISM frequency band is reserved for the Industrial/ Scientific / Medical use, which is specialized by FCC. Nowadays, most countries all over the world open this frequency band by following this standard. But the ISM frequency is not unified in these countries. We cannot be allowed to use these frequency bands, i.e., 900MHz, 2.4GHz, 5.7GHz; but the transmitted power must be lower the specialized power (generally lower 1W) and the utilized frequency cannot interfere with other frequencies. At present, most of the wireless products are working at 2.4GHz. And the 802.11 protocol families are shown in Table 1-1.

Table 1-1 Wireless Frequency

802.11b

802.11a

802.11g

802.11n

Release time

Sept. 1999

Sept. 1999

June. 2003

Jan. 2004

frequency

2.4Ghz

5Ghz

2.4Ghz

2.4Ghz5Ghz(2.4Ghz or/and 5Ghz

modulation

CCK/DSSS

OFDM

CCK/OFDM

MIMO OFDM

range

About 100m

About 50m

<100m

Several Km

Max rate

11Mbps

54Mbps

54Mbps

600Mbps

compatible

11g

Not with 11b/g

11b

11b/a/g

 

Seg 2: Relevant Concepts

4.3 Experimental principle

Before presenting the experiment principle, some relevant concepts should be given out firstly.

Digital signal: its means that its amplitude is discrete, and limited into a range, like binary code used widely in computer science. The digital signal has a strong anti-interference ability, and can be easily processed by digital signal processing. Nowadays, there are many digital signals, such as mobile signal, information handling by computer, and so on.

Analog signal: its ware changes continuously. Theoretically, we can get any of the value from the analog signal. Since it is interfered easily by the other signals, it is difficult to handling. Thus, generally, the analog signal should firstly be transformed into the digital signal for the convenient signal processing. The difference could be shown in Figure 4-1.


Figure 4-1 Difference of digital and analog signals

PWM is the abbreviation of Pulse Width Modulation. It means that we can equivalently get the required signal wave on the basis of a series pulse width, as shown in Figure 4-2. We can achieve a sine signal wave by a series of pulse signal by the different width. In fact, this principle can be illustrated by the equivalent area from mathematical integral. For example, the area of the first pulse is equivalent to the area surrounded by sine signal wave. So, we can change the duty cycle to get the voltage signal wave. Please image it. If we want to get a direct current voltage wave, the width of each pulse should be equal. In addition, PWM technique has been widely applied to the motor speed control, valve control, and so on. For example, PWM has been used to electronic cars.


Figure 4-2 Sine wave by PWM