Adaptive Modulation, Coding, and Transmission Rates

In order to reach the highest possible transmission rate during favorable transmission condi- tions a new modulation scheme has been introduced with HSDPA. The new modulation scheme allows the system to transfer four bits per transmission step. As 16 values can be coded in four bits 24_{, the modulation scheme is called 16QAM. Under ideal conditions}

this increases the total capacity of a cell by a factor of two while maintaining the channel bandwidth of 5 MHz. Under less favorable radio conditions, HSDPA uses the existing QPSK modulation which transmits two bits per transmission step.

In addition to changing the modulation scheme the network can also alter the coding scheme and the number of simultaneously used HS-DSCH channels for a terminal on a per frame basis. This behavior is influenced by the channel quality index (CQI) which is

frequently reported by the terminal. The CQI has a range from 1 (very bad) to 31 (very good) and tells the network how many redundancy bits are required to keep the block error rate (BLER) below 10%. For a real network this means that under less favorable conditions more bits per frame are used for error detection and correction. This reduces the transmission speed but ensures that a stable connection between network and terminal is maintained. As modulation and coding is controlled on a per user basis, bad radio conditions for one user have no negative effects for other users in the cell to which the same HS-DSCHs are assigned for data transmission.

By adapting the modulation and coding schemes it is also possible to keep the power needed for the HSDPA channels at a constant level. The strategy of HSDPA to use a constant power level of for example 40% of the total transmission power available in a cell and instead adapt the transmission speeds of the users is quite different from the strategy of Release 99 dedicated channels. Here the bandwidth of a connection is stable while the transmission power is adapted depending on the user’s changing signal quality. Only if the power level can no longer be increased to ensure a stable connection, does the network take action and increase the spreading factor to reduce the bandwidth of the connection.

The capabilities of the terminal also influence the maximum data rate of a connection. The standard defines a number of different device categories which are listed in 3GPP TS 25.306 [15]. Table 3.7 shows some of these categories and their properties.

With a Category 6 terminal that supports both QPSK and 16QAM, the following maximum transmission speed can be reached:

7298 bits per TTI (which are distributed over five HS-PDSCH channels) every 2 milliseconds= 1/0002 × 7298 = 36 Mbit/s

This corresponds to a speed of 720 kbit/s per channel with a spreading factor of 16. Compared to a Release 99 dedicated channel with 384 kbit/s, which uses a spreading factor of eight the transmission is four times faster. This is achieved by using the 16QAM modulation instead of QPSK which doubles the maximum speed and by reducing the number of error detection and correction bits while signal conditions are favorable. Thus, an HSDPA cell is in theory about four times as fast as a Release 99 cell. In operational networks the speed increase will not be as high because not all terminals will be close to the cell and thus many will only be able to use QPSK and a higher number of redundancy bits in order to ensure a stable transmission.

Table 3.7 A selection of HSDPA terminal categories HS-DSCH category Maximum number of simultaneous HS-PDSCH Minimum TTI interval Maximum number of transport block bits per TTI

6 5 1 7298 (16QAM)

11 5 2 3630 (QPSK only)

A Category 11 terminal on the other hand, which is limited to QPSK modulation, can only receive data in every second frame. Thus for this category of terminals the maximum speed is:

3630 bits every 4 milliseconds= 1/0004 × 3630 = 900 kbit/s

In the future there may also be devices that are capable of receiving more than five HS-DSCH channels simultaneously and can thus reach a maximum speed of 14.4 Mbit/s. However, this would consume all resources of a cell and could of course only be reached under ideal signal conditions.

As has been shown, there are many factors that influence how fast data can be sent to a terminal. The following list summarizes once again the main factors:

• signal quality;

• number of active HSDPA users in a cell;

• number of established channels for voice and video telephony in the cell; • number of users that use a dedicated channel for data transmission in a cell; • terminal category;

• bandwidth of the connection of the Node-B to the RNC; • interference generated by neighboring cells;

• achievable throughput in other parts of the network, as high data rates cannot be sustained by all web servers or other end points.

It should be noted at this point that the transmission speeds that can be reached with HSDPA also have an impact on other parts of the terminal. Apart from increased processing power of the terminal in general, the interface to a remote terminal like a notebook needs to be capable of handling data at these speeds. The maximum transmission rate of Bluetooth up to version 1.2, for example, is just 700 kbit/s (see Chapter 6). This is not sufficient by far for HSDPA data rates and thus new devices should also support the Bluetooth enhanced data rate extension in order not to become the bottleneck of the end-to-end connection.