Литература / UMTS-Report

Table 1-9 Required values for Eb/N0 and C/I for the LCD 384 kbit/s service

Kc = number of codes per time slot per user, K = number of users per time slot, UL/DL = uplink/downlink, CC = at the output of the inner convolutional decoder, RS = at the output of the outer Reed Solomon decoder

Table 1-10 Required values for Eb/N0 and C/I for the LCD 2048 kbit/s service

Kc = number of codes per time slot per user, K = number of users per time slot, UL/DL = uplink/downlink, CC = at the output of the inner convolutional decoder, RS = at the output of the outer Reed Solomon decoder

Corresponding error rate curves are shown in Figure 1-7 to Figure 1-9 for the LCD 144 kbit/s service, in Figure 1-10 to Figure 1-13 for the LCD 384 kbit/s service and in Figure 1-14 to Figure 1-15 for the LCD 2048 kbit/s service. In these figures, the coded BER at the output of the inner convolutional decoder (ccBER) is depicted.

1.3.3 UDD services

In this section, link level simulation results for the UDD services are given. The UDD services are implemented by using a type II hybrid ARQ scheme. This ARQ scheme is explained in the following for improving the code rate from one transmission to the next from 1 to 2/3 to 1/2 to 1/3 and to 1/4. Some of these steps can also be omitted, for instance the 1/3 and 1/4 code rate can be omitted or for instance the 2/3 code rate can be omitted. In the used ARQ scheme, the user data is encoded with a 1/4 rate convolutional code and interleaved over 8 bursts. Rate compatible punctured convolutional (RCPC) codes are used [6]. The coding and interleaving are done in such a way that decoding is possible after two of eight bursts have been received. Thus, the effective code rate is 1 after the reception of these two bursts and the packets to be transmitted are divided into blocks of 2x136-8 bits =

264 bits each in the case of QPSK (528 bits in the case of 16QAM), which constitutes the user block including data, CRC, block number, and encoder tail. If the decoding is not successful, the third burst is sent and decoding is reattempted. After the third burst, the code rate is 2/3. If the decoding is still not successful, the fourth burst is sent and decoding is done again, now with the code rate of 1/2. Then, two more bursts are sent, leading to a code rate of 1/3 and two further bursts, leading to a code rate 1/4. If the decoding is not successful, the burst with the lowest signal to noise-and-interference value is resent and the original burst and the retransmitted burst are combined by maximum ratio combining. This repetition coding is repeated until the decoding is successful. The system parameters for implementing the UDD services are summarized in Table 1-11. Both code pooling and time slot pooling are considered.

Table 1-11 System parameters for the UDD services

In the link level simulations, an ideal CRC (cyclic redundancy check) is modelled. The effects of ARQ are included in the system level simulations. The aim of the link-level simulations is to find the required Eb/N0 values to achieve certain BERs and BLERs. The following alternatives have been simulated as

being extreme cases:

∙allocating 1 code in 1, 2, ... 8 time slots to a user in the uplink, with 1 user being active per time slot,

∙allocating 9 codes in 1, 2, ... 8 time slots to a user in the uplink, with 1 user being active per time slot,

∙allocating 1 code in 1, 2, ... 8 time slots to a user in the downlink, with 4 users being active per time slot,

∙allocating 9 codes in 1, 2, ... 8 time slots to a user in the downlink, with 1 user being active per time slot.

These cases are extreme cases with respect to code pooling. The performance when pooling other numbers of codes is inbetween the extreme cases given here. Since it is likely that these applications will be executed on a laptop, antenna diversity in the downlink is also included in the results.

Based on a throughput analysis, the bit rates achievable depending on the average C/I are determined. The achievable bit rates are determined by taking into account the necessary retransmissions due to block errors and the related decrease of the effective information bit rate. The considered ARQ scheme is improving the code rate from one transmission to the next from 1 to 2/3 to 1/2 to 1/3 and to 1/4. The results including the code rate 2/3 are better compared to the case when omitting the code rate 2/3.

The required C/I values in order to achieve the required nominal bit rate are summarized in Table 1-12 for UDD 144 kbit/s, in Table 1-13 for UDD 384 kbit/s and in Table 1-14 for UDD 2048 kbit/s.

Table 1-12 Required value for C/I for the UDD 144 kbit/s service, code rates 1, 2/3, 1/2, 1/3 and 1/4

Kc = number of codes per time slot per user, K = number of users per time slot, UL/DL = uplink/downlink, TS = number of time slots per user

Table 1-13 Required value for C/I for the UDD 384 kbit/s service, code rates 1, 2/3, 1/2, 1/3 and 1/4

Kc = number of codes per time slot per user, K = number of users per time slot, UL/DL = uplink/downlink, TS = number of time slots per user

Table 1-14 Required value for C/I for the UDD 2048 kbit/s service, code rates 1, 2/3, 1/2, 1/3 and 1/4

UDD 2048

Outdoor to Indoor and Pedestrian A, 3 km/h

Indoor Office A, 3 km/h

Kc = number of codes per time slot per user, K = number of users per time slot, UL/DL = uplink/downlink, TS = number of time slots per user

Corresponding bit rate curves are shown in Figure 1-16 to Figure 1-18 for the UDD 144 kbit/s service, in Figure 1-19 to Figure 1-21 for the UDD 384 kbit/s service and in Figure 1-22 to Figure 1-23 for the UDD 2048 kbit/s service. In these figures, the achievable bit rate is depicted versus the average C/I.

1.4 Possible improvements

The link level simulation results shown in this document can be improved for example in the following ways:

∙enhanced power control (e.g. frame-by-frame) for low velocities, for instance for the Indoor and Pedestrian test cases with 3 km/h; an exemplary test has shown a gain of 2.5 dB, cf. the section on the speech service, when using enhanced power control instead of frequency hopping,

∙shared slots in the downlink for increasing diversity,

∙modification of the FEC scheme in the case of 16 QAM so that the 4 bits forming a 16QAM symbol are equally protected,

∙improved CDMA codes,

∙use of better joint detection algorithms, e.g. decision feedback equalizers [3], which require essentially the same computational complexity as the minimum mean square error and zero forcing block linear equalizers used in the simulations,

∙joint detection or interference cancellation process including the strongest intercell interferers,

∙improved channel estimation, e.g., based not only on the midamble portions, but also on the datacarrying part of the burst or based on more than one burst, especially for low velocities,

∙improved channel coding, e.g., optimized puncturing pattern, better coding schemes as for instance Turbo codes, concatenated codes for low BER,

∙utilization of the reciprocal channel in the case of TDD operation for e.g. open loop control and preequalization.

References

[1]UMTS TR 30.03, "Universal Mobile Telecommunications System (UMTS); Selection procedures for the choice of radio transmission technologies of the UMTS," version 3.0.0.

[2]ETSI/SMG2 Tdoc 260/97, "Common Workplan of SMG2 UTRA Concept Groups".

[3]A. Klein, G. K. Kaleh, and P. W. Baier, "Zero forcing and minimum mean-square-error equalization for multiuser detection in code-division multiple-access channels," IEEE Transactions on Vehicular Technology, vol. 45, May 1996, pp. 276-287.

[4]B. Steiner and P. W. Baier, "Low cost channel estimation in the uplink receiver of CDMA mobile radio systems," Frequenz, vol. 47, Nov./Dez. 1993, pp. 292-298.

[5]ETSI/SMG2 Tdoc 329/97, "Next simulation test cases".

[6]P. Frenger, P. Orten, T. Ottosson, and A. Svensson, "Rate matching in multichannel systems using RCPC-codes," in Proc. IEEE Vehicular Technology Conference, Arizona, 1997, pp. 354-357.