text: The city in western Germany is putting 2,500 police officers on the streets for the week-long event.
  Germany was shocked by the New Year assaults, largely blamed on migrants. More than 100 women were victims, but the full scale only emerged later.
  Separately, police have arrested three suspected Islamist militants in raids.
  Cologne sex attacks: Women describe 'terrible' assaults
  The raids on flats and offices took place on Thursday in Berlin, Lower Saxony and North Rhine-Westphalia.
  The two men and one woman detained are among four suspected of links to the so-called Islamic State group.
  One of those held was being sought by Algerian authorities, and they may have been planning an attack in Germany, police said.
  The New Year unrest in Cologne fuelled German unease about a huge influx of asylum seekers.
  Two men - a Tunisian and a Moroccan - have been charged over the Cologne offences.
  But in total there were 945 complaints to Cologne police after the New Year trouble, 559 of them allegations of sexual assault. Thirty-five suspects are being investigated.
  German authorities spoke of a new type of crime, in which gangs of drunken men - described as North African - targeted women.
  Cologne resident Miriam was attacked as she and a friend made her way home on New Year's Eve.
  She said she was going to the carnival celebrations "but with really mixed feelings".
  "I'm wondering if something like that could happen again."
  Miriam's mother, Jozi, said the incident changed the way she sees migrants.
  "We live in a multicultural area with lots of Moroccans, I've never had a second thought before - we're friends, we greet each other, talk to each other. It was all fine. But now I'm looking at it with different eyes."
  Organisers have not said how many people have attended this year's carnival, but correspondents say the numbers are much lower than in previous years.
  Police will be present at a new "Security Point" for women near Cologne Cathedral - the area where most of the assaults took place during New Year celebrations.
  Police have been trialling "body cams" which can film suspects during incidents.
  German authorities have also brought in British face recognition experts to help identify those involve in the attacks on New Year's Eve.
  Known as "super-recognisers", the officers from the London Metropolitan Police can recognise up to 95% of the faces they have seen - compared to about 20% for most people.
  Migration to Germany from outside the EU soared to a record 1.1 million last year.
  The Cologne assaults contributed to a slump in the approval rating of Chancellor Angela Merkel.
  She has been criticised by conservative allies, as well as by many opposition politicians, for having welcomed so many asylum seekers to Germany.
  The many Syrian and Iraqi refugees generally get asylum, but many economic migrants from outside the EU are also trying to settle in Germany.
target: Cologne has beefed up security for the city's annual carnival, after many women suffered sexual assaults and robberies there on New Year's Eve.