CDMA FDMA Techniques

Fundamentals
Title	CDMA FDMA Techniques
Edited by	GMV
Level	Intermediate
Year of Publication	2011

The GNSS concept consists in having Medium Earth Orbit (MEO) satellites transmitting navigation data simultaneously sharing the same communication channel (air). In order to process these signals, the receiver must be able to distinguish among them and this requires a multiple access technique. Although most GNSS nowadays envisage using Code Division Multiple Access (CDMA), GLONASS legacy signals, traditionally, make use a Frequency Division Multiple Access (FDMA) technique. However, over the last decade, modernized GLONASS satellites started to include additional CDMA signals, such as the GLONASS-K1 satellites (launched in 2011, transmitting CDMA signals on L3-band), the GLONASS-M satellites (including CDMA signals on L3-band since 2014) and the GLONASS-K2 satellites (launched in 2018, transmitting CDMA signals also on L1- and L2-bands).

Multiple Access Techniques employed in GNSS

Multiple access techniques are employed in a communication system to enable several users to share the same medium for transmission. In GNSS, satellites transmit their signals over the same physical medium in the L-band by employing direct-sequence spread spectrum (DSSS) techniques . According to this technique:

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, which enables the recovery of the original information signal while significantly reducing interference. Spread spectrum techniques allow for secure communications by keeping low the levels of transmission power and increasing the robustness of signals to interference, jamming and noise. 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 carrier 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-correlation and low cross-correlation properties. In fact, on the one hand, high auto-correlation properties allow the receiver to correctly de-spread the signal and to recover its original version by correlating the incoming signal with the desired PRN code. On the other hand, low cross-correlation properties among different PRN sequences guarantee the recovered signal to be nearly free from interference from other satellites’ signals.

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 common issue in CDMA techniques is the so-called near-far problem, which 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 indoor (or weak signal) environments where signals from different satellites experience different attenuations. Another case affected by the near-far problem is when GNSS space signals are combined with GNSS ground-base pseudolites . In this case, special techniques need to be implemented to cope with this issue.

FDMA

Frequency Division Multiple Access (FDMA) techniques 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 dedicated frequency slot. 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. However, as mentioned, over the last years, GLONASS has been progressively including more and more CDMA signals in its signal plan.

Related articles

• GNSS signal
• Principles of Compatibility among GNSS
• Principles of Interoperability among GNSS
• Correlators
• FDMA vs CDMA
• GLONASS Signal Plan