By Paul Sandle LONDON (Reuters) - Phone buyers looking for innovation at next week's big industry show in Barcelona will need to look beyond the handsets themselves to additional gadgets like 360-degree cameras which can be hooked up to their computing power, as manufacturers struggle to differentiate their core products. The need to invent has never been greater as phone buyers are less keen to buy devices only marginally better than the ones they already own, leaving manufacturers struggling to eclipse rivals with only faster processors, higher resolution screens or better cameras - all at ever lower prices. Only two of the top five vendors, Samsung and Huawei, increased sales to end-users in the last quarter, while Apple suffered its first quarterly decline, with iPhone sales down 4.4 percent, according to market research firm Gartner.

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