Determining Data Rate and bandwidth for an RF Communication System

The speed at which a data can be transferred from a transmitter device to a receiver device is called data rate. The range of frequencies used for RF communication is called the bandwidth. These two parameters are most important considerations in an RF communication system after the range of the RF link. The major concern in any wireless projects is the range that can be achieved between a transmitter and receiver. The transmitted power, receiver sensitivity and the losses occurred in the medium determines the range of an RF link. There are also other parameters that influence the range of an RF system. The Data rate and the Bandwidth are two parameters which have significant impact on the receiver sensitivity, which in turn has effects the range of the RF link. There is a theoretical limit for the maximum data rate that can be used for a given bandwidth and as the data rate increases the range that can be achieved decreases. So there need to be a trade off between the speed at which the data can be transferred and the range that can be achieved. In this tutorial, it will be examined that how these major design parameters for RF communication are interrelated and how to select the optimum values of data rate and bandwidth for a particular range requirement.

Let’s take two configurable RF transceivers. The data transferred during this experiment will be digital data and modulation technique used will be Frequency Shift Keying (FSK). In this experiment two RF Pico boards housing Si-1062 wireless microcontroller are taken with an additional LED indicator circuit interfaced to the boards on breadboards. The LED indicator circuits along with the RF Pico boards are supplied 3V voltage from two 1.5 V batteries connected in series at both the transceivers. Both the transceivers operate within ISM band of frequencies. As setup in the previous tutorial, the frequency set for the RF link will be 868 MHz. The LED indicator circuit built on the breadboard is assembled according to the following schematics – 

Both the programmable wireless transceivers are kept on a table with antennas removed, so that they can be operated in a comfortable space. A measurement tape is also placed on the table for measuring the range that can be achieved with different RF configurations.

Let’s first see how theoretically, the data rate, bandwidth and range of RF system are interlinked. In a binary digital transmission a high frequency carrier is shifted to a higher frequency or to a lower frequency based on the information bit (logical 0 or 1) need to be transmitted. The FSK modulated signal on a time axis looks as shown in the graph below –  

If the amplitude of frequencies in a modulated FSK signals is plotted against their signal strength, the graph is plotted as shown in the figure below – 

This is the Spectrum of FSK. From the Spectrum of the FSK modulated signal, it can be concluded that after modulating an information on to a signal, the resultant signal varies its frequency in a range of frequencies which is actually the Bandwidth of the system. In the above images the ‘f0’ and ‘f1’ are the frequencies for either of the binary bits and ‘Nb’ is the data rate and the amount of frequency spread is given as ‘Nb/2’. This implies that as the data rate increases the spreading of the modulated signal in the frequency axis.

The bandwidth is the difference between the minimum and the maximum frequency that can be transmitted through a wireless link. The significance of Bandwidth is that, for an error free communication the wireless link should be capable of transmitting all the frequencies that ranges in a modulated signal.

Bw = f1 – f0 + Nb

The wireless engineers can choose the data rate but the bandwidth is always limited due to the device limitations. There is no device that can operate in infinite bandwidth but they can be designed to have a fixed bandwidth where it can transmit or receive signal with constant signal strength. Hence one has to decide the Bandwidth to be used and then select the appropriate data rate.

The maximum data rate that can be used in a given bandwidth is limited by the Nyquist’s formulae –

Nb_max = 2 * Bw

Suppose a bandwidth of 5 kHz has been selected, then the maximum data rate can be 10 kbps only, any data rate above that will result in reception error.

As discussed in the previous tutorial, the range of an RF communication system is given by the following formulae –

R = (10^{(Pt + Gt + Gr – Pr – Lf + 60 ) / 20} ) / 41.88 * F

Where,

Pt = Transmission Power

Gt = Transmitting Antenna Gain

Gr = Receiver Antenna Gain

Pr = Receiver Sensitivity

Lf = Loss factor

F = frequency of the signal

From the range equation as the value for receiver sensitivity increases the range decreases. It is important to know that the Receiver sensitivity ‘Pr’ is dependent on the Data rate ‘Nb’ and the Bandwidth (Bw) according to the following equation –

Pr = Nr* (Eb / No)*(Nb/Bw)

Where Nr – Noise power introduced by the receiver itself

          Eb / No – Signal to noise ratio per bit

So as the Data rate ‘Nb’ increases, the Receiver sensitivity ‘Pr’ decreases. The smaller the value for Receiver sensitivity better will be the receiver performance. From the range equation it can be found that as the value for ‘Pr’ increases the range decreases. So it can be concluded that as the data rate increases, the range decreases.

Let check the effect of data rate on range of the system practically. When using a configurable RF transceiver having a wireless microcontroller, the design factors mentioned above can be programmatically controlled. The values for the various design factors can be set by the Wireless Development Suite for Si-1062 wireless microcontroller based RF Pico board. In this experiment, all the design factors will be kept constant except the data rate. So the values of the design factors for both the transceivers will be set according to the following table –

Keeping all the above parameters same the data rate is varied to find out the range that can be achieved for each data rate. In this experiment the Bandwidth selected is 150 kHz. Starting with a data rate of 10 kbps, Nr* (Eb / No) = 1500, and a loss factor (Lf) of 82 for the devices not having antenna connected but have the matching circuitry and antenna connector; the range equation gives a value 97 cm on substituting the values. Since there is no antenna connected the Antenna Gains can be taken as 0. On testing for 10 kbps Data rate, a range of 64 cm is derived.

If the data rate is taken 1 Kbps, the range is found to be 77 cm. 

If the data rate is instead increased to 100 Kbps, the range is reduced to 40 cm. 

From this experiment it can be concluded that the actual range can be achieved in real world situations if the antenna is connected on both the devices. When Antenna is connected, the Loss Factor reduces to a value less than 3 and the Antenna Gain for the 868 Mhz frequency antennas (Gt and Gr) can be substituted by 5 dBi (as per the specifications of antenna used for testing actual range). The actual range in open space LOS is found to be around 600 meters with the given RF parameters for maximum transmission power of 13 dBm. The range achieved with different data rates keeping all other parameters same without use of antenna are as follow –

So it can be concluded that as the Data rate (Nb) increases the wireless range (R) decreases. 

Project Source Code

Project Source Code

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