Introduction of High Speed Downlink Packet Access (HSDPA)

HSDPA stands for High Speed Downlink Packet Access. 

It provides faster download speeds. It is an upgrade from the form of UMTS. HSDPA provides a download rate of up to 7.2 Mbps. HSDPA is an enhanced 3G mobile telephone communications protocol. The 3.5G, 3G+ or turbo 3G, which allows networks based on Universal Mobile Telecommunications System to have higher data transfer speeds and capacity.Peak theoretical speeds of 14.4 Mbps.Increased packet data support. Increase maximum user throughput for downlink packet data. Lower packet delay. 

The improved downlink provides up to 14 Mbit/s with significantly reduced latency. Current devices support 7.2 Mbps throughput. In order to support HSDPA features with minimal impact on the existing radio interface protocol architecture, a new MAC sub-layer, MAC-hs, has been introduced. It has many features like fast scheduling, fast link adaptation, Short transmission time interval, and many more. It has Adaptive Modulation and Coding Technology. It has HARQ Technology. It has 16QAM Modulation. 

What does HSPDA offer ?

• Speed –
  Faster downstream throughput.It also supports services requiring instantaneous high data rates in downlink, e.g., Internet browsing. 
  
• Capacity –
  It has 3-4 times improved system capacity at relatively low cost. 
  
• Reduced delay –
  It reduced delay, with HSDPA round trip times can be reduced to below 100 ms. 
  
• Network coverage –
  Short time to market with existing sites, no new sites are needed.Improved end-user quality. 

Methods used by HSPDA :

1. Short transmission time interval (TTI)
2. Higher order modulation
3. Fast scheduling and user diversity
4. Fast link adaptation 

Applications of HSDPA :

• HSDPA helps reduce time delay in data transmission and improves system throughput.
• It optimizes system spectrum efficiency.
• It is also true that experience of using HSDPA is very similar to that of using fixed-line ADSL service.
• It is particularly suitable for uplink and downlink asymmetric traffic and burst data traffic.
• Fast scheduling
• Shared Channel and multicde transmission

How does HSDPA works ?
HSDPA represents an evolution of WCDMA radio interface, which uses few methods to those by EDGE (Enhanced Data Rates for GSM Evolution) technology for GSM radio interface. 
The fundamental characteristics which enable increase in data throughput and capacity with reduced latency are given below:  

• Time and code multiplexing of users.
• Multi-Code transmission.
• Fixed Spreading Factor (SF = 16).
• Shorter TTI = 2ms.
• No DTX (Discontinuous transmission)for data channel.
• No power control.
• No soft handover.

The steps for operation of HSDPA are :  

1. UE reports CQI via HS-DPCCH(High Speed Dedicated Physical Control Channel).
2. Node B determines which UE to be served using HS-SCCH(High Speed Signalling Control Channel).
3. Node B informs UE to be served using HS-SCCH(High Speed Signalling Control Channel).
4. Data delivered via HS-DSCH(High Speed Downlink Shared Channel).
5. UE acknowledges via HS-DPCCH(High Speed Dedicated Physical Control Channel).

Features of HSDPA :

1. Adaptive modulation and coding (AMC)
2. Multi-code operation
3. Hybrid automatic repeat request (HARQ)
4. Higher order modulation
5. Short transmission time interval
6. Fast link adaptation
7. Fast scheduling

Advantages of HSDPA

• It offers gain of radio capacity.
• The network can employ data schedulers that gives higher priority to real-time applications,
• It gives highest data load in downlink direction.
• Employs shorter frame length can react faster to problems in radio channel.
• It provides shorter delays that enables new applications such as interactive networked games.
• Mobile operators can compete with WiFi using HSDPA as they need not require distributed APs.
• It’s best for applications with highly variable bandwidth.

Disadvantages of HSDPA :

• The HSDPA will cause 3G network to run on higher load rate everytime.So this increases noise on existing UMTS and then decreases UMTS capacity.
• Not suitable for applications with low band-width requirements, such as voice.
• There is No soft handover and hence pico-cells overlap in same building and will generate self interference
• HSDPA supports usage of UMTS network only on downlink.