Fund manager Aberdeen Asset Management has rated mortgage backed securities the "standout" asset class in the Australian fixed income market, saying RMBS represented "outstanding value".Aberdeen's head of fixed income in Australia, Victor Rodriguez, said the group's local portfolios were long RMBS and had held those positions when other parts of the portfolio were "de-risked" earlier this year.Speaking at a briefing on the fixed income market yesterday, Rodriguez said: "If you are looking at the risk of principal loss RMBS is bullet proof. You are getting 130 basis points over bills."Despite the positive outlook Aberdeen is not looking to buy much new issuance.Rodriguez said: "We went into it big last year. This year, as we have de-risked the portfolio, we sold some financial debt but held our RMBS positions."We are still long. Our flagship Australian Fixed Interest Fund has a 10 per cent allocation to RMBS and our Cash Enhanced Fund has a 25 per cent allocation. But we have not been buying new deals."Rodriguez said issuance in the RMBS market would continue to recover slowly. "We are still adjusting to the fact that the SIV bid no longer exists. It is not coming back. The only demand is from real money managers."Other managers are probably in a similar position to us. They do not need to be aggressively buying because they are already holding long positions."