System Design Consideration

UWB frequency synthesizer is quite dissimilar to conventional narrow band ones in some aspect: I/Q output phase, data rate, operating bandwidth, co-existence with other standards… and so on. Therefore, the system design consideration should be modified to get the appropriate features of it. Preliminarily, single-ended circuits take the priority for its low power consumption and smaller chip area as compared to differential ones, though it is liable to suffer from higher noise and second-order distortion.

Figure 2.6 Frequency synthesizer in a MB-OFDM UWB transceiver

Fig. 2.6 above [12] illustrates the role of a UWB frequency synthesizer in an MB-OFDM direct conversion transceiver. As in other wireless systems, the frequency synthesizer has the crucial function of generating the local oscillator (LO) signal that drives the down-converter in the receiver path and the up-converter in the transmitter. There are at least two demanding requirements that make a frequency synthesizer for an MB-OFDM UWB radio significantly different from the widely explored synthesizers for narrow-band wireless systems, which are:

1) the range of frequencies to be generated spans several gigahertz and 2) the time to switch between different band frequencies within a band group should be less than 9.47 ns. This requirement prevents the use of a standard PLL-based synthesizer as a solution for this application.

In addition to the frequency switching speed, the synthesizer’s output LO signal must comply with other requirements to ensure proper operation of the MB-OFDM UWB radio.

The specifications outlined here assume the OFDM parameters and bit error rate (BER) requirements described in [3] for a 480-Mb/s data transmission and an additive white Gaussian noise (AWGN) channel. A quadrature phase-shift keying (QPSK) constellation is considered for the individual sub-carriers. For a packet error rate of 8% with a 1024-byte packet, the target BER when using a coding rate

4

= 3

R is 10⁻⁵, which corresponds to an un-coded BER of approximately 10⁻². The complete characteristics over the entire frequency bands and analysis are discussed below:

(1) Phase Noise

The phase noise from the LO in an OFDM receiver has two different effects on the received symbols. It introduces a phase rotation of the same magnitude in all of the sub-carriers and creates inter-carrier interference (ICI) [13]. The first undesired effect is eliminated by introducing pilot carriers with a known phase in addition to the

information carriers. On the other hand, phase noise produces ICI in a similar way as adjacent-channel interference in narrow-band systems. Assuming that the data symbols on the different sub-carriers are independent, the ICI may be treated as Gaussian noise.

The power spectral density (PSD) of a locked PLL can be modeled by a Lorenzian spectrum described by [13]

where β is the 3-dB bandwidth of the PSD, which has a normalized total power of 0 dB.

The degradation ( D in decibels) in the signal-to-noise ratio (SNR) of the received sub-carriers due to the phase noise of the LO in an OFDM system can be approximated as [14] where T is the OFDM symbol length in seconds (without the cyclic extension), β defines the Lorenzian spectrum described above, and

O S

N

E is the desired SNR for the

received symbols (in a linear scale, not in decibels). For this system, 1 =4.125

T MHz

mentioned parameters, β can be computed with (2-4) and is 7.7 kHz. The corresponding Lorenzian spectrum has a power of -86.5 dBc/Hz @ 1 MHz.

(2) In Phase (I) and Quadrature Phase (Q) Matching

In an OFDM system, the amplitude and phase imbalance between the I and Q channels transform the received time-domain vector into a corrupted vector r , which

consists of a scaled version of the original vector r combined with a term proportional to its complex conjugate r . This transformation can be written as [15] ^∗

r_iq =α⋅r+β⋅r^∗

(2-6) where αand β are complex constants, which depend on the amount of IQ imbalance.

This alteration on the received symbols can have a significant impact on the system performance. The effect of a phase mismatch in the quadrature LO signal on the BER versus SNR performance of the receiver was evaluated considering the system

characteristics. Simulation results for uncoded data over an AWGN channel showed that the degradation in the sensitivity is 0.6 dB for 5° of mismatch.

(3) Spurious Content

As in other communication systems, the most harmful spurious components of an LO signal are those at an offset equal to multiples of the frequency spacing between adjacent bands (in this case, 528 MHz) since they directly up/down convert the

transmission of a peer device on top of the signal of interest, as shown in fig 2.7 below.

It was found that, in order to have a negligible degradation in the sensitivity (0.1 dB); the carrier-to-interferer ratio (CIR) at baseband should be at least 24 dB. In other words, to tolerate the presence of other UWB transmissions that arrive with comparable power at the antenna of the receiver, the synthesizer spurs that appear at frequencies

corresponding to other bands must have an aggregate power of less than -24 dBc. A summary of the UWB frequency synthesizer specifications is given in Table 2.1.

Figure 2.7 Effect of unwanted frequency translation of interferers

Table 2.1 The summary of the UWB frequency synthesizer specification

Band spacing 528 MHz Switching time between

adjacent bands

< 9.47ns Phase noise of the LO

signal

< -86.5 dBc/Hz@1 MHz Aggregate power of spurs

at band frequencies

< -24 dBc Phase I/Q mismatch < 5°

There are two types of spurs in this synthesizer. One type of spur is caused by the frequency mixing with 528MHz in mixer1. Hence, the spurs in the first group will decide those in other groups. The other type of spur is due to mixer2. Although the third-order unwanted sidebands have been prevented by the frequency generation scheme in fig 2.3, fig 2.4, first-order spurious tones must be taken into consideration when the groups are

up/down-converted by mixer2. The first-order spurious tones would be generated at the opposite side of the target band, and it would be placed in the 7.5 GHz spectrum of UWB application. Suppressing the first-order spurs and letting it less than -24 dBc should pay close attention to carefully.

2.4 UWB Frequency Synthesizer Architecture and