**4. Digital receiver implementations**

8 Will-be-set-by-IN-TECH

to channel realization, since the autocorrelation process is disturbed by intra-symbol interference. Additionally, if a reference pulse is periodically inserted prior to each modulated pulse, a 3 dB loss occurs. Delay hopping techniques or reference symbol averaging may

It is more than unlikely that analog implementations of differential receivers will have a chance to be applied in low cost products. For the multi-user case with analog multipath combining (next section), we have therefore focused on energy detection combined with

0.1 1 10

normalized frequency *f* · *t*g(0)

As mentioned above, the transmitted pulse *ψ*1(*t*) may be also a chirp or direct-sequence spread spectrum waveform. To realize the filter *g*T(*t*) in the analog domain, SAW-filters (SAW: surface acoustic wave) could be a solution, but only if *ψ*1(*t*) is a fixed, user independent waveform. Therefore, as long as the non-coherent signal processing (or a part of it) takes place

To still enable multiple access (MA) communications, we assume that each symbol to be transmitted is represented by several short pulses which are generated at distinct times according to a user specific TH-code. The decoding will be carried out digitally, i.e., after

Compared to direct sequence MA codes which contain a large number of (non-zero) chips, the sparseness of TH codes facilitates the receiver processing, reduces the complexity and keeps the additional loss due to the non-coherent combining of the code elements within limits. A TH code is determined by two parameters: the number of pulse repetitions *N*s, which is equivalent to the code weight, and the number of hopping positions<sup>3</sup> *N*h. In [12], we have presented a semi-analytical method to assess the multiple access performance. It

<sup>3</sup> One symbol interval can be divided into an integer number *N*<sup>s</sup> of equally spaced intervals (named frames), where

reduce this 3 dB loss, but require even more (and longer) delay lines.

N=5 N=10 N=20

**Figure 7.** Normalized group delay of a Bessel-Thomson all-pass filter of order *N*.

**3.2. Multiple access for analog multipath combining**

in the analog domain, we assume a user independent *ψ*1(*t*).

the non-coherent signal processing took place, cf. Fig. 8.

each frame contains one pulse. Within each frame, *N*<sup>h</sup> positions are possible.

time-hopping (TH) impulse radio.

*t*g(*f*)/*t*g(0)

1.05

0.8

0.85

normalized

 group delay

0.9

0.95

1

Digital receiver implementations according to Fig. 9 have several advantages. First of all, they offer a superior interference rejection capability [11, 16], since user specific filtering can take place prior to the non-coherent signal processing. This restricts the non-coherent combining loss to the multipath arrivals (which exhibit stochastic path weights), whereas that part of the signal energy, which is already spread by a user-specific code at the transmitter, is coherently summed up. Furthermore, digital receiver implementations enable advanced modulations such as Walsh-modulation [15, 17] or advanced NBI-suppression strategies based on soft-limiting [13].

The block diagram of a receiver with a "digital code matched filter" (DCMF) is shown in Fig. 9. The ADC operates with a sampling rate, which is not smaller than the UWB signal bandwidth, where the ADC resolution has been chosen between 1 and 4 bits. Regarding the following results, we have always assumed TH impulse radio transmission.

Fig. 10 shows the *E*b/*N*<sup>0</sup> improvement of a DCMF-based receiver as a function of *N*s, where *N*s depicts the number of non-zero elements of the user-specific code. As the DCMF combines the corresponding pulses coherently, the benefit compared to an analog receiver increases with *N*s.
