EXIT charts are useful in the design of iterative receivers. It is well known that EXIT chart analysis can be used to show the existence of bottlenecks in a system [26]. In Fig. 4.16 there is an obvious bottleneck caused by the EXIT function of the TD, such that the SNR must be greater than approximately 1.75dB for the IC and TD EXIT functions to not intersect. EXIT charts can be used to design the receiver such that bottlenecks are avoided or at least minimized to improve the efficiency (power and number of iterations required to achieve target BER) of the system. As shown in Section 4.6.1 the distance from capacity is proportional to the area between the EXIT functions. The “S” shaped EXIT function of the TD combined with the almost linear IC EXIT function creates a large area between the EXIT curves, so there is a large capacity loss inherent in the system. Much work has been done on the design of receiver components such that the EXIT charts are matched [63, 134, 62, 135, 29]. The various methods proposed include mapping schemes and the use of irregular CCs. Since the work in this thesis is in general concerned with the 3GPP UMTS, these approaches are not suitable as they require changes in the transmitter. However, in Chapter 5 and Chapter 6 power and scheduling optimization algorithms will be proposed.
In this section EXIT charts will be used to investigate two methods which could po- tentially be used to improve the performance of the iterative receiver. The first involves feedback of a posteriori information, and the second requires the TD to store the old internal a priori information for initialization before the next activation.
4.11.1 APP Feedback
The bottleneck in the 3GPP EXIT chart of Fig. 4.16 occurs at lowa priori MI. In order to lower the convergence threshold, greater gains in MI on the first activations of the decoder are required. Since the MAP decoder is optimal, and the TD is close to optimal, it is not possible to achieve greater gains in extrinsic MI.
We propose feeding backa posteriori information from the TD to the IC after the first TD activation. The idea was suggested in [136] and EXIT chart analysis suggests it may aid convergence, as shown in Fig. 4.25, where the MI of the APP information is shown by a solid line and the extrinsic information with a dashed line. Note that the MI at the output of the decoder for IA < 0.15 is equal to the input as the TD does not add any extrinsic information.
The results of decoding with APP feedback on the first receiver iteration only are shown in Fig. 4.26 and compared with an identical system without APP feedback. The decoding schedule was for ir = 4 receiver iterations and id = 6 TD iterations on each activation of the TD. Observe that there are potential gains, in this case 0.05dB, however the correlations introduced can be detrimental to the receivers performance. This approach must therefore be validated for the desired system configuration to ensure its suitability. 4.11.2 A priori Initialization
In an IMUD receiver there are typically fewer TDs than users, the receiver shares the physical resources according to some higher-level protocol. On each activation of the TD
4.11 Receiver Optimization 75 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 IA IE APP Feedback Extrinsic Feedback
Figure 4.25: EXIT function of the 3GPP turbo code APP and extrinsic output.
0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 10−7 10−6 10−5 10−4 10−3 10−2 10−1 100 SNRγb(dB) BER No APP Feedback APP Feedback
Figure 4.26: BER performance of the IMUD receiver withKT = 10 users and spreading
factor N = 20, with and without APP feedback (on first receiver iteration only), running 4 receiver and 6 turbo decoder iterations.
for a given user the a priori inputs in the constituent MAP decoders IADec(1) and IADec(2)
are initialized to zero, that is, no prior information is known about the systematic bits. If IADec(1) is not reset there is potential to achieve small gains in receiver performance. We propose the use of a memory element in the receiver to hold thea priori information
IADec(1) for the next TD activation. The EXIT function for the TD running 1, 3 and 6 iterations with no a priori reset is shown in Fig. 4.27. Note that when id is low the initial value ofIADec(1) has a large influence on the EXIT characteristics of the turbo code. However, whenid is large the EXIT characteristics are almost independent ofIADec(1).
The BER performance of the IMUD with K = 20 users and a spreading factor N = 20, with and without resetting the a priori information IADec(1) in the TD, is shown in Fig. 4.28. This plot highlights the observations made from the EXIT chart analysis, that
0 0.2 0.4 0.6 0.8 1 0 0.5 1 0 0.2 0.4 0.6 0.8 1 ITD A Idec(1) A I T D E (a)id= 1 0 0.2 0.4 0.6 0.8 1 0 0.5 1 0 0.2 0.4 0.6 0.8 1 ITD A Idec(1) A I T D E (b) id= 3 0 0.2 0.4 0.6 0.8 1 0 0.5 1 0 0.2 0.4 0.6 0.8 1 ITDA Idec(1) A I T D E (c)id= 6
Figure 4.27: EXIT function for the 3GPP TD with non-zeroa priori input to constituent MAP decoder 1.
the benefits of using a memory to store past values ofIADec(1) decrease with increasingid. Due to the added complexity of storing the past values, the use of past values should only be considered if id / 3, as for an increased number of TD iterations the gains become insignificant.