HSDPA & HSUPA Overview

7/27/2019 HSDPA & HSUPA Overview

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HSDPA/HSUPA Overview

Tech Support AB, 2007 (1.0) 35

Figure 5-10 E-DCH dedicated physical control channel

E-TFCI

The E-DCH transport format combination indicator is a 7-bit field informing the base station about the

transport block size and the number of parallel code channels as well as their spreading factor. If the base

station is unable to decode this information, the whole radio frame (or sub-frame) is lost. If a 10 ms TTI

is used, the information is repeated 5 times for increase reliability and reduced power.

Retransmission sequence number, RSN

This 2-bit field indicates the sequence number of the retransmission by setting the initial transmission to

RSN=0 and then the first retransmission to RSN=1 and so on.

Happy bit

This bit indicates if the terminal is happy or not with the current data rate or whether it could use a higher

output power (increase data rate).

5.3.3 E-DCH HARQ Indicator Channel, E-HICHThis channel is used to send acknowledgements (Ack) or negative acknowledgements (Nack) in response to

uplink data transmission. The decision about the correctness is taken in relation to the appended 24-bit cyclic

redundancy code (CRC). The base station will compare the appended CRC with the one calculated over thereceived data block.

As the E-DCH can be in soft handover, there is a possibility that one of the cells receive the data correctly

while others do not. There is always one serving E-DCH cell that decides about the capacity in uplink. The

serving cell may send an Ack and a Nack while other cells in the active set only send Ack. The terminal will

repeat the transmitted data until it receives an Ack from any cell. Nack will not be sent by other cell than the

serving cell in order to save transmission resources in downlink. Figure 5-11 shows the main principle

Figure 5-11 Main principle of E-HICH

E-DPCCH, SF256 (15 kbit/s)

E-DPDCH with data

E-TFCI, transport format (7 bits) Retransmission sequence number (2 bits) Happy bi t (1 bit)

ACK or NACK

Serving

E-DCH cel lOther cell i n

active set

ACK but no NACK

Data on E-DCH Data on E-DCH

RNCRNCRNCRNC

RNC takes data from

any cell in active set


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5.3.4 E-DCH Absolute Grant Channel, E-AGCHAn absolute grant gives the terminal information about the allowed resources it may use in uplink. The term

absolute is slightly misleading as the value is a relative value to the power used on the associated dedicated

channel. The value is a 5-bit field giving a value between 0 and 31. Figure 5-12 shows the main principle of

the channel.

Figure 5-12 Absolute grant channel

The absolute grant is a relative value in relation to the power controlled DCH. This means that the base

station can specify a power value that can be translated to a number of added kilobits/s on top of the DCH

transmission. Figure 5-13 shows an example of what an absolute power grant is. In this example is the base

station allocating more power (resources) to the terminal. Before and after the absolute grant, there is a fixed

relationship between the power used on the DCH and the power used on E-DCH when transmitting.

Figure 5-13 Meaning of an absolute grant

5.3.5 E-DCH Relative Grant Channel, E-RGCHThis new downlink channel orders the terminal to perform a single step-up or step-down of the relative

transmission power given in an absolute grant. If the base station finds that the resources given to one

terminal should be adjusted, the relative grant method can be used. It may also be used by any other cell thanthe cells included in the active set of the terminal. This would be the case when a cell discovers heavy

interference from one HSUPA user transmitting at high power. Cells included in the active set may send UP,

Absolute grant value (0-31)

UE identity

Serving

E-DCH cell

UE transmit

power

DPCCH

power level

E-DPDCH

power level

Node B has given UE more

power to use for E-DCH

Time


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DOWN or HOLD commands to the terminal while other cells may only send DOWN commands. Figure 5-

14 shows the main principle.

Figure 5-14 Relative grant channel

5.4 Transmission Time IntervalHSDPA changed the TTI value from R99 to 2 ms. This value makes it possible to have the fast scheduling

and link adaptation that gives so good characteristics to HSDPA. HSUPA has two different TTI values, the

10 ms and the new 2 ms. The reason for introducing the 2 ms TTI was to reduce the delay and latency in the

radio network. The highest bitrates in HSUPA require a 2 ms TTI. However, at lower bitrates than 2 Mbit/s,

the difference in capacity is not that big. The 2 ms TTI is therefore optional for many terminal categories

supporting lower bitrates. There is also another problem with the 2 ms TTI at operation near the cell edge (or

indoor usage with high attenuation). If a high number of users are located at the cell edge, then the downlink

transmission to them would consume too much power and it becomes impossible. This means that 10 ms TTI

has to be used at cell edge as can be seen in Figure 5-15.

Figure 5-15 Transmission time interval and coverage

5.5 RetransmissionThe principle of retransmission used in HSDPA is rather similar to the principles used in HSUPA. Both

chase combining and incremental redundancy are available for retransmission in HSUPA. The difference is

that the procedure in HSUPA is synchronous while the base station in HSDPA could schedule the

retransmission at any time. The number of HARQ processes is also limited to 4 within the 10 ms TTI. Figure

5-16 shows the retransmission principle used when 10 ms TTI is used.

DOWNUP, DOWN or HOLD

Cell in E-DCH

active set

Any other cel l

2 or 10 ms TTI

can be used

Only 10 ms TTI

Too much downlinkpower i f 2 ms TTI


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Figure 5-16 Retransmission for 10 ms transmission time interval

After reception of the data on the E-DCH channel, the base station has to send an acknowledgement (ACK)

or a negative acknowledgement (NACK) within 14-16 ms. The terminal will then schedule the

retransmission to 3 x 10 ms later than the initial transmission. This means that the base station does no need

to know which HARQ process the retransmission belongs to. The only indication given by the terminal is if

the data is new or not.

5.6 Terminal CategoriesThe principle of having terminal categories has the same function as in HSDPA, it enables manufacturers to

release terminals in a phased development with better and better characteristics. The defined terminal

categories can be seen in Figure 5-17.

Figure 5-17 HSUPA terminal categories

It can be seen that only 3 out of the 6 categories support the shorter 2 ms TTI. It is anticipated that initial

terminals only support 10 ms TTI as the highest bitrates shown in the table are not offered initially. It can

also be seen that there is no difference in bitrate between the 2 and 10 ms TTI for terminal category 2

offering 1.45 Mbit/s in both cases. To offer the highest possible bitrates, 5.76 Mbit/s, support for 2 ms TTI is

needed. The main reason for requiring 2 ms TTI for the highest bitrates is that the transport blocks would be

too large using 10 ms TTI (difficulties in compatibility for protocols and channel coding functions).

E-DCH with data

E-HICH: NACK

E-DCH Data

1:st retransmission

E-HICH NACK

Data

Time

14-16 ms

30 ms (3 TTI)

Category

1

2

3

4

5

6

Peak data rate

at 10 ms TTI

Channels and

spreading factorSuppor ted TTIs

1 x SF4

2 x SF4

2 x SF4

2 x SF2

2 x SF2

2 x SF2 + 2 x SF4

10

2 and 10

10

2 and 10

10

2 and 10

0.72 Mbit /s

1.45 Mbit /s

1.45 Mbit /s

2 Mbit/s

2 Mbit/s

2 Mbit/s

Peak data rate

at 2 ms TTI

-

1.45 Mbit /s

-

2.91 Mbit /s

-

5.76 Mbit /s


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Some headers will consume bits as will be shown in chapter 6 Protocols. The medium access protocol

(MAC) will in HSUPA add some information used for identification and also scheduling. Figure 5-18 shows

the available data rates for radio link control protocol (RLC) when the packed data unit size (PDU) is set to

320 bits.

Figure 5-18 RLC data rates with a 320-bit packet data unit size

Category

1

2

3

4

5

6

Maximum datarate with 10 ms TTI

0.672 Mbit /s

1.376 Mbit /s

1.376 Mbit /s

1.888 Mbit /s

1.888 Mbit /s

1.888 Mbit /s

Maximum datarate with 2 ms TTI

-

1.280 Mbit /s

-

2.720 Mbit /s

-

5.440 Mbit /s


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6 Protocols

6.1 User plane Protocols for HSDPAThe main change of the system if one compare R99 with R5 including HSDPA, is the introduction of the

MAC high-speed (MAC-hs) protocol in the base station. An R99 base station has no protocols that take any

decisions at all. All the intelligence is located in the radio network controller (RNC). MAC-hs will

implement most of the user plane functionality as well as some control functions related to HSDPA. Figure

6-1 shows the location of the protocols in the protocol stack.

Figure 6-1 User plane protocols for HSDPA with MAC-hs

It can be seen that the protocol (MAC-hs) is located in the base station as well as in the mobile terminal. Ontop of that is the old MAC-d protocol that takes care of dedicated channels. This protocol is located in the

RNC and is more or less unmodified. The reason for having this protocol is to enable channel type switching.

The channel type switching is the process when a dedicated channel is modified (e.g. from FACH to DCH or

from DCH to HSDPA in downlink).

The radio link control protocol (RLC) is also unmodified and located in the RNC. The RLC protocol can be

configured to operate in three different modes:

Unacknowledged mode (UM)

the protocol will only number RLC frames to identify lost frames. It is used for applications that do not

require retransmission like streaming and voice over IP.

Transparent mode (TM)

the RLC protocol is not used at all and will not add any headers to the data. This mode of RLC is only

used for circuit switched voice and will not be used in connection with HSDPA.

Acknowledged mode (AM)

the protocol will create numbered frames with checksums enabling retransmission. This mode is

typically used by applications requiring reliable data transfer with low requirements on delay and delay

variations. Examples are file transfer and streaming with large play-out buffers. When RLC operates in

AM mode, it is possible to have retransmission both at layer 1 (HSDPA) and layer 2 (RLC). This is

affecting service quality at handover or when HSDPA buffers need to be flushed for other reasons.

The MAC-hs protocol is therefore taking care of most of the functions related to HSDPA. The main parts of

the protocol can be seen in Figure 6-2.

RLC

RNCRNCRNCRNC

MAC-d

MAC-hs

WCDMA L1

UuIub

Frame

protocol

RLC

MAC-d

MAC-hs

WCDMA L1

Frame

protocol

HSDPA & HSUPA Overview

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