Receivers and CQI Measures for MIMO-CDMA Systems in Frequency-Selective Channels
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Receivers and CQI Measures for MIMO-CDMA Systems in Frequency-Selective Channels Jianzhong (Charlie) Zhang Nokia Research Center, 6000 Connection Drive, Irving, TX 75039, USA Email: [email protected]
Balaji Raghothaman Nokia Research Center, 6000 Connection Drive, Irving, TX 75039, USA Email: [email protected]
Yan Wang Nokia Research Center, 6000 Connection Drive, Irving, TX 75039, USA Email: [email protected]
Giridhar Mandyam Nokia Research Center, 6000 Connection Drive, Irving, TX 75039, USA Email: [email protected] Received 1 March 2004; Revised 9 November 2004 We investigate receiver designs and CQI (channel quality indicator) measures for the jointly encoded (JE) and separately encoded (SE) types of MIMO transmission. For the JE transmission, we develop a per-Walsh code joint detection structure consisting of a front-end linear filter followed by joint symbol detection among all the streams. We derive a class of filters that maximize the socalled constrained mutual information, and show that the conventional LMMSE and MVDR equalizers belong to this class. This constrained mutual information also provides us with a CQI measure describing the MIMO link quality, similar to the notion of generalized SNR. Such a measure is essential for both link adaptation and also to provide a means of link-to-system mapping. For the case of SE transmission, we extend the successive decoding algorithm of per-antenna rate control (PARC) to multipath channels, and show that in this case successive decoding achieves the constrained mutual information. Meanwhile, similar to the case of JE schemes, we also derive proper CQI measures for the SE schemes. Keywords and phrases: CDMA, MIMO, PARC, CQI, link-to-system mapping, constrained optimization.
1.
INTRODUCTION
Information-theoretic studies in [4, 5] showed that multipletransmit, multiple-receive-antenna MIMO systems offer potential for realizing high spectral efficiency in a wireless communications system. In [6, 7], a practical MIMO configuration, a Bell Labs layered space-time (BLAST) system, is deployed to realize this high spectral efficiency for a narrowband TDMA system. MIMO schemes are also being considered for standardization in WCDMA/HSDPA, and may be considered for CDMA2000 as well in the near future. From the point of view of packet transmission with forward error-correction coding, MIMO schemes can be classified into two categories, namely, jointly encoded (JE) and separately encoded (SE). In a JE scheme, a single encoded packet is transmitted over multiple spatial streams, whereas in SE
each spatial stream consists of a separately encoded packet. Coded-VBLAST and its variants [8], as well as space-time codes [9], fall under the JE category, while schemes such as per-antenna rate control (PARC) and its variants belong to the SE category [2, 10, 11]. For both JE and SE schemes, one key aspect of the MIMO-CDMA system study is to design receivers that can reliably decode the transmitted signals in a frequencyselective channel, where the signal is
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