Signal design criteria and parametric analysis

Current global navigation satellite systems (GNSSs) [1] are based on signals lying within the L-band of the radio-navigation satellite service (RNSS) spectrum. Since the need for more systems and signals is emerging, new alternative frequency resources are needed. In particular, the C-band frequency portion is envisioned as an option for future GNSSs.

Recent studies [2], focusing on comparing C-Band navigation performance with L-Band performance for global Earth coverage, have shown that C-band signals might be an interesting option in combination with L-band signals, considering forthcoming technology development. Focusing on the C-band reserved for Galileo, this is partitioned into the uplink service band (5000-5010 MHz) and the RNSS band (5010 and 5030 MHz), the latter currently unoccupied.

In addition, new aeronautical mobile (R) service (AM(R)S) are being proposed in the 5000-5010 MHz and 5010-5030 MHz bands. The analysis of compatibility issues of such AM(R)S and RNSS services calls for more specifi c defi nition of system parameters of a potential GNSS System in C-Band, especially considering the upcoming technology development in space, ground and on the receiver market envisaging in the next 10 to 15 years.

In the current phase of defi ning new navigation signals for the C-band case, it is fundamental to identify trade-off criteria and issues to deal with in the new allocated band, and then to span the possible signal options that can be adopted to exploit the available bandwidth.

With respect to signal design, in the recent past, several signal options have been introduced, ranging from new chip waveforms [3]-[5], to combining existing signals, like multiplexed binary offset carrier (MBOC) [6]. These solutions resulted in general in an improvement of the performance of the L-band services and could be used as a starting point for the defi nition of a C-Band signal.

The objective of this study is thus to compare some possible signal options on the basis of identifi ed criteria responding not just to performance improvement but also taking into account generation and emission constraints.

This paper is not meant to contain a complete and exhaustive analysis of the identifi ed options, rather it intends to anticipate possible issues to be further analysed, and/or to establish criteria to guide the selection.

In particular, it will be shown that the existing regulatory constraints of 20- Mhz band-limitation, limits considerably the possibility to improve positioning accuracy, namely, maximization of meansquared bandwidth (Gabor bandwidth, GB) of the radio signal. The idea of band-limited signals is also taken into consideration, given the strong Out of Band emission constraint in the adjacent Radio Astronomy band. As an example, in [2] a simple raised cosine (RC) pulse shape is compared with those of timelimited signals under similar conditions.

Figure 1: Signal PSD. Case QBLSC (8.5, 2).

Figure 2: Constellation plot. Case QBLSC (8.5, 2).

C-band constraints and comparison criteria for signal design

When designing signals for satellite positioning, user positioning accuracy is the main driving factor, that is directly related to time delay estimation (TDE). By using conventional parameter estimation techniques [7], such as the Cramér- Rao bound (CRB), the key factor for optimizing signals in an additive white Gaussian noise (AWGN) channel has been identifi ed in the maximization of the GB [6]. That is, the more the signal PSD is concentrated at the edge of the band, the more the accuracy is enhanced. Nevertheless, several works [1]–[6], have also shown that multipath (MP) is one of the dominant sources to the error budget, due to its random nature. As a consequence, MP mitigation has become another relevant design driver for new signal-in-space (SIS) formats. Recent studies [5] have shown that maximizing the GB bandwidth with a constraint on the autocorrelation sidelobe levels represents an effective optimization criterion for designing spreading signals with higher robustness against MP. It follows that, generally speaking, the two main issues in signal design are fulfi lled by the maximization of the GB.

When focusing on the C-band portion, a certain number of specifi c issues arises and they need to be taken into account. First of all, an intrinsic constraint is the strict allocated bandwidth (20 MHz), less than half the bandwidth available for the Galileo L-band E5 signal, that automatically limits the achievable accuracy. Moreover, differently from

the L-band portion, a new impairment becomes relevant: the level of the emissions in the adjacent band portions. To this regard, ITU [8] specifi es the maximum tolerable absolute values of power fl ux density (PFD) in the out of band regions. The requirement for the microwave landing system interference (5030-5150 MHz) allows a maximum PFD of –124,5dBW/m2 in a band of 150 kHz, while the requirement for the radioastronomy interference (4990-5000 MHz), choosing a worst case of 10 interfering satellites, allows a maximum PFD of –196.5 dBW/m2 in a bandwidth of 10 MHz. It follows that, while the requirement in the upper bandwidth is not so strict, the one in the radio-astronomy band is so stringent that the need for low out of band emissions (OOB) becomes a driving factor in defi ning new SIS for Cband. It is worth to be noticed that, to the best of our knowledge, the requirement for the C-band uplink region has not been fixed yet, but we believe it should represent a driving criterion as well.

Finally, we prefi gure that the impact of non-linearities will be another important trade-off criterion. In fact, a C-band signal needs to be transmitted at an increased power than an equivalent Lband signal due to the increased signal attenuation in the C-band region. In order to be conservative, we envision the use of a travelling wave tube amplifi ers (TWTA) that guarantees a larger gain. The drawback is the enhanced effect of non-linearities, that needs to be taken into account from the very beginning of the signal design process.

Concluding, the identifi ed criteria lead to a trade-off between two different trends in SIS design: on one hand, it would be desirable a signal with a power concentrated at the edge of the band (following the GB principle it can better perform in terms of TDE accuracy and MP robustness) and, on the other hand, it is requested a signal that has its power concentrated at the center of the band (it guarantees lower out of band emissions). This could be achieved either with bandlimited or time-limited signals, examples of which are analysed in the following.

Figure 3: Signal PSD. Case QBOC (8, 2).

Figure 4: Constellation plot. Case QBLSC (8.5, 2).