CDMA codes [Under UMTS]
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Orthogonal codes (Walsh codes) are disrable due to their orthogonality. But orthogonal codes need synchronised operation. This is certainly possible in downlink in a cell as common channel can be used for synchronisation. For uplink, due to varying distances of users from base station, orthogonal codes will not be help. Rather we need codes which even if not orthogonal, would help in extracting information, similar to PN codes (refer CDMA article). Similar is the challenge when UE is able to receive transmissions from multiple base stations (base stations are not synchronised).
UMTS specify:
- OVSF (Orthogonal Variable Spreading Factor) codes for
channelisation codes,
- Scrambling codes for separating out UEs in uplink direction and
base stations in downlink direction.
In downlink, synchronisation which channelisation codes require is achieved by using CPICH (Common Pilot Channel).
In uplink, the same is achieved by having user specific (dedicated) pilot channel DPCCH (Dedicated Physical Control Channel).
Easy to see that in downlink, same channelisation code can not be allocated to two UEs whereas in uplink, channelisation codes can be used by multiple UEs. Result is: in downlink, limitation is brought out *more* by availabillity of OVSF codes than by interference and in uplink, it would be signal power/ interference ratio that need special attention.
We saw in CCTrCh article that PHY has responsibility of sending CCTrCh over one or several physical channels. Note that chip rate is to be constant at 3.84 Mcps; so idea is to use appropriate spreading factors to realise required data rates. RRC - based on TFC - decides the appropriate SF to be used and configures PHY accordingly.
References: UMTS by Sanchez and Thioune, and WCDMA for UMTS by Holma and Toskala.
| Copyright © Samir Amberkar 2010-11 | § |
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