Low complexity predistortion and equalization in nonlinear multicarrier satellite communications

Abstract

Aiming to reduce the power/mass requirements in satellite transponders and to reduce mission costs, joint amplification of multiple carriers using a single high-power amplifier (HPA) is being considered. In this scenario, a careful investigation of the resulting power efficiency is essential as amplification is nonlinear, and multicarrier signals exhibit enlarged peak-to-average power ratio. Thus, operating the amplifier close to saturation vastly increases signal distortion resulting in a severe degradation of performance, especially for higher order modulations. This paper proposes a reduced-complexity digital predistortion (DPD) scheme at the transmitter and a corresponding equalizer (EQ) at the receiver to mitigate these nonlinear effects. Scenarios include both the forward as well as the return links. In particular, the paper exploits the MIMO Volterra representation and builds on a basis pursuit approach using a LASSO (least absolute shrinkage and selection operator) algorithm to achieve an efficient basis representation, avoiding large computational complexity, to describe the selection of predistorter/equalizer model. The work further compares and contrasts the two mitigation techniques taking various system aspects into consideration. The gains in performance and amplification efficiency demonstrated by the use of DPD/ EQ motivate their inclusion in next-generation satellite systems.

Multiple‐input multiple‐output symbol rate signal digital predistorter for non‐linear multi‐carrier satellite channels

“…1, data DPD has a MIMO architecture providing a mitigated stream of symbols per carrier. Data DPD operates at the symbol rate, prior to Table 1 Basis functions for the low complexity MIMO data DPD [9] Carrier 1 Carrier 2 Carrier 3…”

“…Several data DPD schemes for MC satellite channels are reported in the literature. E. g., data DPD based on direct learning [8], and low complexity schemes [9]. The latter form the mitigated symbols using a linear combination of the basis indicated in Table 1.…”

“…In the simulations, a target BER of 10 −5 was used. We compare three DPD schemes: MIMO data DPD [9], SC signal DPD [4] and the proposed SRS DPD. All these DPD schemes use a non-linear order p = 3.…”

“…This is because of the dramatic increase in the number of basis functions with an increasing number of carrier signals as shown in Table 5. This results from the MIMO Volterra series expansion considering permutations of all carriers [9]. However, the computational complexity of proposed technique can be further reduced by pruning the MIMO Volterra series, for instance, using memory polynomials that consider solely the diagonal kernels of the series [27] or using basis pursuit approaches [28].…”

“…Noiseless received symbols of the inner carrier a No compensation b MIMO data DPD[9] c SISO signal DPD[4] d Proposed MIMO SRS DPDFig. 5 BER against E s /N o for different DPD mitigation schemes studied, inner carrier in dashed lines, outer carrier in solid lines.…”