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CDMA FDMA Techniques
| Fundamentals | |
|---|---|
| Title | CDMA FDMA Techniques |
| Author(s) | GMV |
| Level | Intermediate |
| Year of Publication | 2011 |
The GNSS concept consists in having Medium Earth Orbit (MEO) satellites transmitting navigation data simultaneously over the same channel (air). In order to process these signals, the receiver must be able to distinguish them and therefore requires a multiple access technique. Although most GNSS nowadays envisage using Code Division Multiple Access (CDMA), GLONASS legacy signals still use a Frequency Division Multiple Access (FDMA) technique, mainly for historical reasons.
Multiple Access Techniques employed in GNSS
Multiple access techniques are employed in a communication system whenever several users need to share the same medium for transmission. In GNSS, satellites transmit their signals over the same physical medium in the L-band.
GNSS employ spread spectrum signals:
- Each GNSS satellite transmits a Pseudo-Random Noise (PRN) code which is independent from the transmitted data
- The transmitted signal occupies a bandwidth which is wider than the necessary to send the navigation data information
At the receiver, the incoming signal is correlated with a local replica of that same PRN code, allowing recovering the original signal. Spread spectrum techniques allow keeping low levels of transmission power and they render the signals robust to interference and jamming. Two types of multiple access techniques are currently used in GNSS: Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA). In FDMA, all satellites transmit the same PRN code but in different (dedicated) carrier frequencies, whereas in CDMA all satellites transmit in the same carrier frequency but with a different (dedicated) PRN code, that is assigned to each satellite beforehand.
CDMA
Code Division Multiple Access (CDMA) technique allows several signals to be transmitted simultaneously over the same frequency. For that purpose, each satellite is assigned with a dedicated Pseudo-Random Noise (PRN) code chosen for its low cross-correlation properties with the PRN codes transmitted by the other satellites. Although there are different families of PRN codes, the main objective when designing such codes is to guarantee high auto-correlations and low cross-correlations properties. In fact, the receiver correlates the incoming signal with the desired PRN code in order to de-spread the signal and to recover its original version. Low cross-correlation properties guarantee that the recovered signal has not suffered interference from the signals transmitted by the other satellites.
In CDMA techniques, the number of users is limited by the selection of the PRN codes and their low cross-correlation properties – this fact does not affect GNSS which include only a few tens of satellites. Another aspect in CDMA techniques refers to the near far effect, that can be felt when the receiver is much closer to an emitter than to another. In general, this does not affect terrestrial GNSS receivers since all satellites are far by the same order of magnitude, and hence this problem is mostly felt in indoors (or weak signal) environments where signals from different satellites suffer different attenuations. Another case of near far effect is when GNSS space signals are combined with GNSS ground-base pseudolites. In this case, special techniques need to be implemented to resolve the near-far effects.
FDMA
Frequency Division Multiple Access (FDMA) techniques consist in assigning each satellite with a specific carrier frequency. The main advantage of FDMA when compared to CDMA is that it guarantees signal separation since each signal is transmitted in a different frequency. On the other hand, it requires a higher complexity (and cost) regarding antenna and receiver design, related to the implementation of the different band-pass filters and calibration. FDMA is used only by the legacy GLONASS signals, mainly for historical reasons. Although the legacy GLONASS uses FDMA, there are already plans to include a CDMA signal in the modernized GLONASS. In fact, the first reception of GLONASS CDMA signal was announced in 2011 [1].