# Hybrid Satellite-Terrestrial Networks toward 6G: Key Technologies and Open Issues
## 2 Cooperative HSTN
### HSTN Relay Network
#### 2.2.3 Aerial Relays
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **System Model** & **Performance Metrics** & **Other Factors** \\ \hline
[58] & AF-based ground relay and no direct satellite–destination link & SER and SINR & Co-channel interference \\ \hline
[59] & AF-based terrestrial relay & SINR, OP, SNR & Heavy shadowing \\ \hline
[60] & AF terrestrial relay & SINR and BER & CCI and noise \\ \hline
[61] & AF relay-based multi-user HSTN & OP under high and low CCI & CCI \\ \hline
[62] & Multi-antenna multi-user HSTN & OP, EC, and achievable diversity & \\  & relay network & order & Outdated CSI and CCI \\ \hline
[63] & Fixed-gain multi-user AF & EC & CCI \\  & relay network & & CCI \\ \hline
[64] & Two-hop downlink multi-user & Average SEP & Outdated CSI and CCI \\  & HSTN AF relay network & Average SEP & Outdated CSI and CCI \\ \hline
[76] & Fixed-gain AF relaying HSTN & ASER, EC, and SNR & Channel parameters \\ \hline
[78] & Multi-user multi-relay HSTN & OP & High SNR and best \\  & & user-relay selection \\ \hline
[79] & Selective DF relay-based HSTN & Average SER & Satellite elevation angles \\ \hline
[80] & Single antenna multi-relay HSTN & EC & Available LOS between satellite \\  & & and user \\ \hline
[81] & Multiple-relay HSTN with & & \\  & mobile destination & Ergodic channel capacity analysis &
\begin{tabular}{c} Impact of channel conditions and \\ cooperative protocols \\ \end{tabular} \\ \hline
[82] & Selective DF multi-relay HSTN & OP & Known CSI and channel parameters \\ \hline
[83] & AF multi-relay HSTN with & OP & No LOS between \\  & wireless caching & & satellite–destination \\ \hline
[84] & Single-antenna multi-relay HSTN & OP & Multipath and shadowing \\ \hline
[85] & Multi-relay land mobile system & OP & Known CSI and correlated fading \\  & & between relay and user \\ \hline
[86] & single-user multi-relay HSTN & OP and throughput & Hardware impairments \\ \hline
[87] & Two-way multi-relay & OP and throughput &
\begin{tabular}{c} Hardware impairment, number of \\ antennas, and number of relays \\ \end{tabular} \\ \hline
[88] & DF-based multi-relay HSTN & OP & Hardware impairments \\ \hline
[89] & Uplink land-mobile satellite system & & \\  & with multiple relays & & \\  & & & \\ \hline
[90] & Dual-hop mmWave multi-user AF & & \\  & multi-relay HSTN & & \\ \hline
[91] & AF CDMA-based HSTN & OP &
\begin{tabular}{c} Increased number of relays and \\ LOS link \\ \end{tabular} \\ \hline
[92] & Cooperative HSTN with multiple & &
\begin{tabular}{c} ASEP and OP \\ mobile relays \\ \end{tabular} & Known CSI and slow fading \\ \hline
[93] & Multi-user integrated & Outage analysis and OP & High SNR \\  & satellite—UAV aerial relay & & \\ \hline
[94] & Dual-hop satellite relay network & OP & Rain attenuation \\ \hline
[95] & Multi-antenna 3D UAV & OP &
\begin{tabular}{c} Additive white Gaussian noise \\ (AWGN) at all receiving nodes \\ \end{tabular} \\ \hline
[96] & ARMA-based SAGIN & Link congestion & Network traffic \\ \hline
[97] & 3D UAV relaying HSTN & OP & Receiving nodes corrupted \\  & & by AWGN \\ \hline \hline \end{tabular}
\end{table} 
Table 4: List of major contributions to HSTN relay networks.