Gain more throughput per cell and higher bit rate per
user
HSDPA introduces new technologies in the UMTS world and consequently a new way to manage PS traffic.
The most important impacts of it are on:
Modulation
Retransmission schemes
Scheduling
Usage of Power and Code
HSDPA introduces new technologies in the UMTS world and consequently a new way to manage PS traffic.
The most important impacts of it are on:
Modulation
Retransmission schemes
Scheduling
Usage of Power and Code
HSDPA Basic Principles
1 - Shared Channel Transmission
Shared-channel transmission implies that a certain amount of radio resources of a cell (codes and power) is seen as a common resource that is dynamically shared between users.
The idea is that a part of the total downlink code resource is dynamically shared between a set of packet-data users, primarily in the time domain.
The codes are allocated to a user only when they are actually to be used for transmission, leading to efficient code and power utilization.
For P4 only 5 codes (SF = 16) will be available for the HSDPA feature and they will be shared on a time base.
HSDPA data channel is called HS-DSCH
So a set of radio resources dynamically shared among multiple users
-In the time domain
-In the code domain
The Shared-channel transmission allows:
-Higher peak bit rate: all the resource can be allocated to a single user in case of low load.
-Better application performance being closer to the model TCP has being designed for.
-More efficient utilization of available code resources compared to the use of a dedicated channel, i.e. reduced risk for code-limited downlink.
The Shared-channel transmission impacts:
Scheduling become more complex
In case of shared channel transmission for packet data, a part of the total downlink code resource is seen as a shared resource that is dynamically shared between a set of packet-data users, primarily in the time domain. Thus, in case of shared channel transmission, codes are allocated to a user only when they are actually to be used for transmission.
Shared-channel transmission is possible already in Release 99 of the WCDMA specification by means of the Downlink Shared Channel (DSCH) transport channel. The main benefit with DSCH transmission is to reduce/avoid the risk for code-limited capacity. However, in general, shared-channel transmission offers the possibility for several other benefits that are not supported by DSCH but that will be supportedin case of HSDPA.
Shared-channel transmission implies that a certain amount of radio resources of a cell (code space and power in case of CDMA) is seen as a common resource that is dynamically shared between users, primarily in the time domain.
Transmission by means of the WCDMA Downlink Shared Channel (DSCH) is one example of shared-channel transmission. The main benefit with DSCH transmission is more efficient utilization of available code resources compared to the use of a dedicated channel, i.e. reduced risk for code-limited downlink. However, with the introduction of HS-DSCH, several other benefits of shared-channel transmission can be exploited, as described below.
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2 - Short 2 ms TTI
-The Transmission Time Interval becomes extremely short in HSDPA; 2 ms compared to the 10 ms used by R99 high bit rate radio bearer.
-The HS channels are organised in sub-frame of 3 slots each; this means that the slot time 2/3 ms/slot is the same as for R99 slots (10/15 ms/slot).
-The scheduling and the link adaptation algorithms work at this frequency
The shorter TTI allows:
-Reduced air-interface delay: this is required by the the TCP at high data rates to Improved end-user performance
The shorter TTI is necessary to benefit from other HSDPA features:
-Fast Link Adaptation
-Fast hybrid ARQ with soft combining
-Fast Channel-dependent Scheduling
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3 - HSDPA – Power Allocation
-HS-DSCH allocated power is decided by the RNC, prioritizing the DCH channel
-HS-DSCH adjusts the data rate to match the instantaneous radio conditions and the available transmission power in the RBS
No closed loop power control is specified for HS-DSCH, unlike the DCH channel
The system adjusts the data rate by
-varying the effective code rate
-changing the modulation scheme
This leads to a higher efficiency in the usage of power.
There are two main alternatives for the allocation of power to HSDPA transmission:
-Static power allocation, where a fixed amount of power is allocated for HS-DSCH transmission
-Dynamic power allocation, where HS-DSCH is allocated the remaining power after power has been allocated to other channels.
The two alternatives are illustrated on the next page. Dynamic power allocation is more efficient as it allows for full use of the overall available cell power.
Typical power allocation for HS-DSCH can be in the range 30% to 80% of the overall base station power. In the higher case, the cell basically carries only HS-DSCH traffic, in addition to necessary control channels.
4 - Fast Link Adaptation
The target for the link adaptation is to select a TFRC (Transport Format and Resource Combination) resulting in transmitting an as large transport block as possible with a reasonable error probability.
-Adjust transmission parameters to match instantaneous channel conditions
-HSDPA: Adapt on 2 ms TTI basis the Rate (“constant” power)
-Adaptive coding
-Adaptive modulation (QPSK or 16QAM,64QAM)
Link adaptation is implemented by allowing the MAC-hs to set the TFRC (Transport Format and Resource Combination) independently for each 2 ms HS-DSCH TTI
-In order to estimate current channel conditions, an estimate of the Channel Quality is reported by the UE to RBS (CQI).
-Based on the channel conditions and the available power, the network will select the Transport Format to have the maximum throughput achievable
5 - Fast Channel-dependent Scheduling
Scheduling = which UE to transmit to at a given time instant
-There is a main tradeoff to choose between:
1-fairness :Every user has the same “rights to access the resource”
2-cell throughput :The user with better radio condition transmit more
Examples of scheduling algorithms
-Round Robin (RR)
Cyclically assign the channel to users
Channel quality variance unexplored
-Proportional Fair (PF)
Assign the channel to the user with the best relative channel quality
Improved cell throughput
-Max C/I Ratio
Assign the channel to the user with the best channel quality
High system throughput but not fair
6 - Fast Hybrid ARQ with Soft Combining
HSDPA introduces a new retransmission level “under” the RLC scheme in the RNC.
-This new “level” allows rapid retransmissions of erroneous data:
-Hybrid ARQ protocol terminated in RBS
- short RTT (typical example: 12 ms)
-Soft combining in UE of multiple transmission attempts
- reduced error rates for retransmissions
-A fundamental difference between conventional ARQ (used in RLC) and HARQ is that:
- in the latter case received data blocks that cannot be correctly decoded are not discarded but buffered
T-hey are soft combined with later received retransmissions of the same set of information bits.
-Finally, decoding is applied to the combined signal.
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7 - UE capabilities
The UE capabilities are divided into a number of parameters:
-Total RLC AM and MAC-hs buffer size
-Maximum number of HS-DSCH transport channel bits received within a HS-DSCH TTI
-Support of HS-PDSCH Yes/No
-Maximum number of HS-DSCH codes received
-Total number of soft channel bits in HS-DSCH
-Minimum inter-TTI interval in HS-DSCH
-Supporting 16QAM & 64 QAM
These physical layer UE capabilities can be translated in a limit on the requirements for 3 different UE resources:
-the de-spreading resource (codes decoded in parallel)
-the soft buffer memory used by the hybrid ARQ functionality
-the turbo decoding speed (the maximum number of transport channel bits received within an HS-DSCH TTI and the minimum inter-TTI interval).