![\"M-QAM](\"http:\/\/www.raymaps.com\/wp-content\/uploads\/2012\/05\/MQAM_AWGN.jpg\")
Now let\u2019s consider the case of multiple antennas. Multiple antennas at the transmitter or receiver means multiple RF chains which are power hungry. In particular, simultaneous transmission from multiple transmit antennas means more power consumption [1]. In addition to power consumption in the RF front-end there might also be higher power consumption at the baseband at the receiver side since more complex signal processing is required to separate the signals. Specifically, brute force methods such as Maximum Likelihood (ML) are quite power hungry.<\/p>\n\n\n\n
A relatively new technique to improve the spectral efficiency and the energy efficiency of wireless communication system is called Index Modulation (IM) [2]. There are two main types of IM, one that uses multiple transmit antennas (Spatial Modulation) and one that uses multiple carriers (OFDM-IM). We will focus here on Spatial Modulation also known as SM.<\/p>\n\n\n\n
As previously discussed, having multiple RF\nchains at the transmitter increases the energy consumption. What if we can use\nsingle RF chain with multiple transmit antennas i.e., we use multiplexing. This\nhas two advantages. An obvious one is that energy consumption is reduced at the\ntransmitter end. A not so obvious one is that, by selecting which antenna to\ntransmit from, there is implicit exchange of information between transmitter\nand receiver. <\/p>\n\n\n\n
Let\u2019s assume that we have four antennas and we transmit from only one during a symbol period then we can transmit two bits by this simple selection scheme, assuming that receiver can detect that which transmit antenna was used. The total number of bits transmitted is then given as log2(nT) + log2(M). Where nT is the number of transmit antennas and M is the constellation size. A slightly more complex variant of this scheme is where we select a subset of transmit antennas for each symbol duration. This has the potential to further improve the spectral efficiency of the system at the cost of energy efficiency. <\/p>\n\n\n\n
An Illustrative Example<\/h2>\n\n\n\n
Let\u2019s assume that we have BPSK modulation and there are two transmit antennas. For each symbol period we either transmit a +1 or -1 from antenna Tx-1 or Tx-2. The channel gain from Tx-1 to the receiver is 0.50 and channel gain from Tx-2 to the receiver is 0.25 (we assume these gains are static and known at the receiver). So, during any given symbol period the received signal could be -0.50, -0.25, 0.25 or 0.50. That is, we have expanded the signal constellation from 2 to 4. Let\u2019s assume that we receive -0.50 at the receiver. This means that a -1 was transmitted from Tx-1. Similarly, if we receive -0.25 it means that a -1 was transmitted from Tx-2 and so on. Tx-1 or Tx-2 or for that matter any number of antennas could be selected based upon a lookup table at the transmitter side. Tx-1 could be selected when the input is -1 (or binary 0) and Tx-2 could be selected when input is +1 (or binary 1).<\/p>\n\n\n\n
Applying the formula, we discussed above,\nnumber of transmitted bits can be calculated as:<\/p>\n\n\n\n
log2(nT) + log2(M)= log2(2) + log2(2)=1+1=2 bits\/time slot<\/p>\n\n\n\n
A last point about antenna correlation. Assume that the transmit antennas are closely spaced and the channel coefficients are 0.25 and 0.30 respectively. Then the expanded constellation points would be much closer together and the probability of error increases!<\/p>\n\n\n\n
References<\/h2>\n\n\n\n
[1] Jing Jiang, Mehrdad Dianati, Muhammad\nAli Imran, Rahim Tafazolli, and Yan Chen, \u201cOn the Relation Between Energy Efficiency\nand Spectral Efficiency of Multiple-Antenna Systems,\u201d IEEE Transactions on\nVehicular Technology, 2013.<\/p>\n\n\n\n
[2] Ertugrul Basar, \u201cIndex Modulation\nTechniques for 5G Wireless Networks,\u201d IEEE Communications Magazine, 2016.<\/p>\n","protected":false},"excerpt":{"rendered":"
A relatively new technique to improve the spectral efficiency and the energy efficiency of wireless communication system is called Index Modulation (IM). There are two main types of IM, one that uses multiple transmit antennas (Spatial Modulation) and one that uses multiple carriers (OFDM-IM). We will focus here on Spatial Modulation also known as SM.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[219],"tags":[216,42,105],"class_list":["post-3739","post","type-post","status-publish","format-standard","hentry","category-modulation","tag-5g-6g","tag-mimo","tag-modulation"],"_links":{"self":[{"href":"https:\/\/www.raymaps.com\/index.php\/wp-json\/wp\/v2\/posts\/3739","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.raymaps.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.raymaps.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.raymaps.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.raymaps.com\/index.php\/wp-json\/wp\/v2\/comments?post=3739"}],"version-history":[{"count":7,"href":"https:\/\/www.raymaps.com\/index.php\/wp-json\/wp\/v2\/posts\/3739\/revisions"}],"predecessor-version":[{"id":4276,"href":"https:\/\/www.raymaps.com\/index.php\/wp-json\/wp\/v2\/posts\/3739\/revisions\/4276"}],"wp:attachment":[{"href":"https:\/\/www.raymaps.com\/index.php\/wp-json\/wp\/v2\/media?parent=3739"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.raymaps.com\/index.php\/wp-json\/wp\/v2\/categories?post=3739"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.raymaps.com\/index.php\/wp-json\/wp\/v2\/tags?post=3739"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}