Chapter6.Receiver Design Principles

Chapter 6. Receiver Design Principles

Annamalai Annamalai, Sridharan Muthuswamy, Dennis Sweeney, R. Michael Buehrer, Jihad Ibrahim, and Dong S. Ha

This chapter provides a comprehensive review of various UWB receiver architectures designed for different modulation formats and signaling schemes. Rather than being overly concerned with the effect of pulse shaping, antenna design and channel impairments, this chapter focuses on different mechanisms of demodulating the received UWB signal optimally in the sense of minimizing the probability of bit error. In practice, however, receiver complexity and power consumption are important design concerns. Thus, we shall also highlight suboptimal receivers that are good candidates for different UWB applications. Figure 6.1 shows an overview of the UWB receiver architectures discussed in this chapter. We will employ an L-tap multipath channel model for a simplified analysis of various receivers. However, it may be more appropriate to apply the channel model proposed in [1] by the IEEE 802.15.3a working group (which is a modification of the Saleh-Valenzuela channel model described in [2]) that takes into account the clustering phenomenon observed in several UWB channel measurements [3].

Figure 6.1. Overview of Different UWB Receiver Architectures.

In Section 6.1, we describe several receiver structures suitable for I-UWB receivers. I-UWB offers many potential advantages, such as high resolution in multipath reducing fading margins in link budget analysis, allowing for low transmit powers and low complexity implementation. However, ultra-fine time resolution requires long synchronization intervals, and I-UWB may require additional correlators to capture adequate signal energy for demodulation. Further, an all-digital receiver requires highly sophisticated receiver processors. For example, assuming a pulse width of 500 picoseconds, Nyquist sampling of 4 samples/pulse (for both in-phase and quadrature-phase) requires a sampling rate of 8 Giga/samples per second. Using 6 bits per sample, the receiver must process a data stream of 48 Gbps. Processing huge amounts of data requires large memory and high processing speeds, which makes it expensive to implement, and thus the goal of a low-cost, simple, ubiquitous communication system is defeated. A number of methods have been developed for pulse detection and reception using a hybrid analog and digital receiver or, alternatively, a fully analog receiver.

RAKE receiver designs for UWB systems are much different from that of narrowband/wideband systems. Consider the following example that demonstrates some of the challenges of RAKE receivers for I-UWB systems. The delay spread s_T of a realistic indoor channel is approximately 5 to 14 ns [1]. Because the pulse duration is much smaller than the delay spread, the channel is frequency selective. Let us assume a delay spread of 10 ns. If the bandwidth of the transmitted UWB signal is W = 7.26 GHz, the number of possible multipath components is given by L = s_T = 72, where x denotes the closest integer smaller than We will discuss many different forms of I-UWB RAKE receivers with maximum ratio combining and also several multi-user detector-based optimum combining schemes. Further channel estimation is an important task in RAKE reception. Estimating a channel at low energy-to-noise ratio becomes difficult. Imperfect channel estimates further degrade the RAKE receiver performance.

To summarize, I-UWB RAKE receivers suffer from two major drawbacks. First, the energy capture is relatively low for a moderate number of fingers when Gaussian pulses are used. As we have seen, a typical NLOS channel can have up to ~70 resolvable dominant specular components. Even if a RAKE receiver with this many fingers is realizable, it would only be able to capture part of the signal energy [4]. Second, each multipath undergoes a different channel, which causes distortion in the received pulse shape and makes the use of a single LOS path signal suboptimal.

Synchronization and timing are extremely important in I-UWB design. Timing offsets will have a significant impact on the quality of the signal output by all of the receiver techniques (with the exception of the leading edge detector receiver). Particularly, correlation detector-based receivers will see a dramatic drop in the SNR at the receiver output due to imperfect match or correlation with the receiver waveform. Additionally, if PPM is used and the cumulative timing offset becomes large enough, bit errors will occur. In addition to maintaining precise timing at the receiver, acquiring and synchronizing to the transmitted signal is essential for UWB communication systems. Previous studies [5, 6] have shown that the performance of I-UWB is very sensitive to timing jitter and tracking over a range of pulse interference levels.

Section 6.2 explores different variations of UWB receiver architectures for MC-UWB. MC-UWB communication systems use orthogonal UWB pulse trains and multiple subchannels to achieve reliable high bit rate transmission and spectral efficiency [7]. Some of the advantages of MC-UWB systems are better time resolution, which leads to better performance in multipath fading channels; better spectrum utilization, which leads to higher data rates; simple decoupled system design; and more freedom in signal design, which leads to a simple transmitter implementation. MC-UWB systems suffer from interchannel interference (ICI) in a multipath fading channel because the received subcarriers arriving via different paths are no longer orthogonal. Hence synchronization is extremely important [7, 8]. Multicarrier systems also have the problem of the peak-to-average power ratio (PAPR) problem, which must be taken care of in order to maintain the proper linear operating region of the receiver.

Section 6.3 discusses spectrally encoded UWB transceivers systems. In Section 6.4, we present a case study on RAKE receivers for improving the range of communication in UWB systems.