* FTSEurofirst and Euro STOXX 50 both steady
* Altice rises on Cablevision deal
* Fed rate decision due at 1800 GMT
* Europe bourses in 2015: link.reuters.com/pap87v
* Asset performance in 2015: link.reuters.com/gap87v
By Sudip Kar-Gupta
LONDON, Sept 17 (Reuters) - European shares held steady on Thursday, supported by new signs of bid activity which lifted the shares of telecoms company Altice and insurer Phoenix.
However, many investors were expected to refrain from buying up big positions before the U.S. Federal Reserve's interest rate decision, due at 1800 GMT.
The pan-European FTSEurofirst 300 index and the euro zone's blue-chip Euro STOXX 50 index made little progress but held their ground to trade flat.
Altice shares initially surged more than 10 percent before then settling back after the company agreed to buy U.S. cable TV operator Cablevision in a deal worth $17.7 billion, including debt.
"Cablevision is ripe for taking out. It's the fourth-largest cable operator in the U.S. and would give Altice access to the highly attractive North East corridor," said Gary Paulin, co-founding partner at brokerage Aviate Global.
Phoenix Group also rose 2.8 percent after confirming talks to buy Guardian Financial Services, with the Altice and Phoenix planned deals coming a day after brewer ABI approached rival SAB Miller.
Asian stocks edged up to a three-week high on Thursday while the dollar drifted lower against other currencies as investors consolidated positions before the Fed's decision.
Observers of the Fed see the outcome as a toss-up. Fed Chair Janet Yellen's consistently stated desire to see workers reap more benefits from the recovery, has contrasted with weak price rises and economic risks, such as a slowdown in China.
Harry Shann, investment manager at Logic Investments, remained upbeat on European stock markets in spite of the risks posed by any move by the Fed to raise rates on Thursday.
Shann pointed to economic stimulus measures from the European Central Bank (ECB) as helping to underpin European equities, along with the prospect that China itself could also undertake similar steps to prevent any slowdown in its economy.
However, OANDA senior market analyst Craig Erlam said equity markets could fall back sharply if the Fed increased rates.
Higher rates are typically bad for equities as they boost the appeal of bonds and cash by raising returns on those assets, and an interest rate hike can also lead to higher debt costs for companies listed on stock markets.
Today's European research round-up (Editing by Hugh Lawson)