This model assumes a suburban mobile radio environment and takes into account the effective gain introduced by differing heights of transmit and receive antennas.
Software Defined Radio for 3G
Equation 3. Assuming a frequency of 1. However this will increase by The loss is the sum of the free space loss, Lfs; the diffraction loss from rooftop to the street, Lrts; and the reduction due to multiple screen diffraction past rows of buildings, Lmsd. Figure 3. The link budget for the simple system shown in Figure 3. The received power, Pr, at the input to the receiver is equal to the transmitted power Pt, plus the antenna gains Gt and Gr, minus the cable losses lt and lr, minus the propagation loss lp.
The cable loss, lt and lr, is con- Figure 3. The output of the transmit subsystem at reference point B drives the transmit transmission line with power Pt. The transmit subsystem consists of two cascaded frequency upconverters followed by a power amplifier with gains of Gtx1, Gtx2, and Gtx3, Figure 3. RF System Design 39 respectively. The receive subsystem consists of a low-noise amplifier followed by two cascaded frequency downconversion stages with gains of Grx1, Grx2, and Grx3, respectively. Equations 3. Designing the receive subsystem cannot be made on the basis of gain selection alone.
One must consider the cumulative noise effect, as introduced in Section 3. A useful performance parameter is noise figure. The noise figure, F, of the device is simply the input signal-to-noise ratio divided by the output signal-to-noise ratio, as expressed by 3. Therefore, F will always be larger than one.
It has been shown  that the total noise figure, Ft , for a cascaded system is calculated by 3. However, calculation of transmitter noise figure is not usually a concern for the cellular mobile RF system engineer. This is because the thermal noise contributions from the transmit subsystem will be attenuated by an amount equal to the propagation loss after arriving at the receive antenna. The ambient thermal noise seen by the receive antenna is usually much greater than the attenuated transmitter-borne noise and is the dominant noise contribution affecting signal detection performance and bit error rate.
This analysis assumes that the transmit subsystem uses a similar number of processing stages to the receive subsystem and has a similar noise figure. If we were to design the receiver subsystem for Figure 3. Using 3. The following paragraphs cover the key RF parameters included in these standards. The 3G specifications define these as follows.
The receiver is required to fulfill a specified BER requirement for a specified sensitivity degradation of the required signal in the presence of an interfering additive white Gaussian noise AWGN signal in the same channel. For a BER of less than 0. Intermodulation rejection is a measure of the capability of the receiver to receive a wanted signal on its assigned channel in the presence of two or more interfering signals that have a specific frequency relationship to the wanted signal. In other words, only 0. Out-of-band emissions exclude spurious emissions and result from the modulation process and nonlinearity in the transmitter.
The out-of-band emission limit is specified in terms of a spectrum emission mask and adjacent channel leakage power ratio for the transmitter. Part of the UMTS base station specification states that for a transmitter with power output of greater than 43 dBm, at an offset of 7. For a UMTS base station the requirement applies at frequencies that are more than Part of the requirement states that in the kHz to 30 MHz region spurious emissions shall be less than —36 dBm in a kHz measurement bandwidth.
For a UMTS base station the intermodulation level is the power of the intermodulation products when a WCDMA modulated interference signal is injected into an antenna connector at a level of 30 dB lower than that of the wanted signal.
The frequency of the interference signal can be 5 MHz, 10 MHz, or 15 MHz offset below the first or above the last carrier frequency used in the transmitter. The transmit intermodulation level must not exceed the out-of-band emission or the spurious emission requirements. However, it is possibly the 3G SDR base station with its multicarrier highpower amplifier that poses the biggest design challenge and offers the biggest system improvement. A major benefit of multicarrier SDR is that an array of single carrier amplifiers and power combiners can be replaced by a single multicarrier unit that takes up less space, consumes less power, and costs less to own and operate over its lifetime.
This is because it is easier to operate power transistors over a smaller dynamic range and maintain linearity. QPSK has better spectral efficiency but also a higher peak to average power ratio; this is because the signal constellation can transition through zero and cause large changes in instantaneous amplitude. For a multicarrier system, adding two or more GMSK carriers at different frequencies destroys the constant modulus property, thereby increasing dynamic range and the need for better linearity.
Table 3. The UMTS scenario exhibits higher peak to average values because of its wider band nature i. In general, 3G multicarrier amplifiers must have better linearity performance than 2G multicarrier amplifiers, and 2G and 3G amplifiers must have better linearity performance than 1G power amplifiers. RF System Design 45 In the past, single channel power amplifier design was a relatively simple design task. Linearity requirements could be handled by increasing the dc biasing of the final power transistors.
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Classes A, AB, B, and C alter the transistor biasing, which changes the utilization of the power curve. Class A is often coupled with selective use of power back-off. This is equivalent to selecting transistors with much higher power rating than required in an attempt to gain more linearity over a larger amplitude range.
A disadvantage of back-off is that the technique is limited to lower powers and the cost of transistors is increased, sometimes to the point where the technique is not commercially viable. Many complicated techniques for improving linearity have been developed over the years as modulation schemes have become more complex and multicarrier systems more prevalent. We will now discuss one popular technique for improving linearity.
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As shown in Figure 3. The input low-power Figure 3. The high-power amplifier with gain Gt in the main signal path drives through a directional coupler; this device maintains good directivity and splits off a small amount of post amplifier power to the subtractor in the error signal path. The output of the subtractor is the difference between a delayed version of the input and more linear signal and the less linear amplified signal from the main signal path.
With perfect components this difference signal only contains the distortion components added by the main power amplifier. The error amplifier inverts the difference signal and adds it to a delayed version of the main signal via the directional coupler at the output. Considering realistic system components, the final process will subtract more of the distortion components than the original signal, resulting in an output signal that more closely matches the input signal; this is equivalent to making the system more linear.
The feed-forward design and other similar approaches can be implemented purely with analog components transmission lines, amplifiers, and directional couplers or by a mix of digital and analog components. More complex but better performing linearizers are possible using digital signal processing in the error signal path.
Multicarrier combining prior to the power amplifier eliminates the need for high-loss transmitter combiners prior to the antenna.
The SDR design goal should be to use a wideband power amplifier that has better efficiency than the equivalent combination of single channel amplifiers and transmit combiners. Source: WSI, A method for calculating the link budget has been provided; this implies that SNR can be maintained when base station and terminal separation is increased providing that the appropriate power is transmitted by both ends of the link. This is because the signal bandwidth is shared in time only.
However, for CDMA systems the signal bandwidth is shared in power. The addition of a user to a CDMA cell will result in all other users increasing their power to equalize the apparent interference produced by the additional user. Prior to a mobile terminal being switched on, neither the terminal nor the base station knows how far apart each is.
The initial searching algorithms following switch-on need to search over a cell distance or time equal to the largest required distance. This searching process is performed by the baseband digital signal processing subsystem. The larger the distance the more computations that need to be performed and the greater the signal processing resources consumed. There will be iteration between each of the stages as detail is fleshed out and assumptions refined.
RF System Design Figure 3. Conclusion We have built upon the basic architecture of Chapter 2 and concentrated on RF issues for the 3G software radio designer. Significant RF development effort has been invested in key enabling technologies for software radio over recent years e.
The RF design is tightly coupled with the radio system design and the architectural partition between hardware and software. It is likely that the design of a generic platform for more than one air-interface will require careful system design and iteration to ensure the best outcome. TE  Shanmugam, K. The organization of this book follows the software radio functions from the air-interface through to baseband.
The previous chapter dealt with the analog functions from the antenna to the start of the digital domain. In this chapter we cover the important process of digital conversion, from analog to digital and digital to analog. Possibly the weakest link in the SDR signal processing chain is the process of analog-to-digital conversion and, to a lesser extent, the process of digital-to-analog conversion.
Unfortunately, digital conversion techniques face more fundamental challenges; in many respects their progress has not kept pace with the computing devices that follow them in the signal processing chain.
Software Defined Radio: Baseband Technologies for 3G Handsets and Basestations
Digital conversion resolution improvements of approximately 1. We are now seeing suitable wideband bit ADCs with enough performance for GSM, and better bit converters are on the horizon. Conversion of an analog signal into the digital domain by Nyquist sampling is also known as lowpass sampling. This is because all the frequency components from 0 Hz to Fs are recovered.
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