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I have used instructions given in same question but got this

make -C /lib/modules/4.15.0-38-generic/build M=/home/shubham/tmp/rtbth modules

make[1]: Entering directory '/usr/src/linux-headers-4.15.0-38-generic'

  CC [M]  /home/shubham/tmp/rtbth/rtbth_core_bluez.o

/home/shubham/tmp/rtbth/rtbth_core_bluez.c: In function ‘rtbt_hps_iface_init’:

/home/shubham/tmp/rtbth/rtbth_core_bluez.c:381:22: error: ‘HCI_BREDR’ undeclared (first use in this function); did you mean ‘HCI_RESET’?

     hdev->dev_type = HCI_BREDR;

                      ^~~~~~~~~

                      HCI_RESET

/home/shubham/tmp/rtbth/rtbth_core_bluez.c:381:22: note: each undeclared identifier is reported only once for each function it appears in

scripts/Makefile.build:332: recipe for target '/home/shubham/tmp/rtbth/rtbth_core_bluez.o' failed

make[2]: *** [/home/shubham/tmp/rtbth/rtbth_core_bluez.o] Error 1

Makefile:1551: recipe for target '_module_/home/shubham/tmp/rtbth' failed

make[1]: *** [_module_/home/shubham/tmp/rtbth] Error 2

make[1]: Leaving directory '/usr/src/linux-headers-4.15.0-38-generic'

Makefile:24: recipe for target 'all' failed

make: *** [all] Error 2

What to do with this?

This is easy,fast and worked greatly for me:
https://webwiks.com/techcorner/get-ralink-rt3290-bluetooth-work-in-linux/

Step 1: Download the Ralink rt3290 bluetooth Driver Package
Now the first step is to download the ralink rt3290 bluetooth Linux driver (rtbth-dkms) package. Download this driver file from below link:

for Ubuntu 18.04:

https://launchpadlibrarian.net/389907029/rtbth-dkms_3.9.6~git20180923-1~bionic~ppa1_amd64.deb (updated wrt link above)

Please Note: The current deb package of Ralink rt3290 bluetooth is only available for x64 based distribution for 32bit distors you need to manually compile the bluetooth package for more please follow this link.

Step 2: Install Ralink rt3290 bluetooth Driver
After you have downloaded driver file, the next step is to install the driver package, Although you can install driver package via command line but we recommend Gdebi as it can install any debian package within seconds. If you don’t have Gdebi installer then please install it via Software center or just open terminal by pressing Ctrl+Alt+T and type the below code:

sudo apt-get install Gdebi

Press Enter and when the installation succeed, Open the rtbth-dkms.deb file via Gdebi install and install the package.

Step 3: Run the Ralink rt3290 bluetooth module (rtbth)
As you have completed above steps, now just after the installation finish, the next step is to check the bluetooth module- by typing below cmd in terminal:

sudo modprobe rtbth

Soon as you launch this command you will get message “init complete” this indicates that your module have been successfully loaded. If the terminal doesn’t gives you any output then its advisable to please check your bluetooth area whether the ubuntu detects bluetooth or not. (If it doesn’t then read the below steps)

If the first command doesn’t works then toogle your WiFi switch on off and again type:

sudo modprobe rtbth

or:

sudo modprobe -r rtbth

And, Press Enter, You will see your Wireless or Bluetooth light which indicates your rt3290 bluetooth activated and you can now use your bluetooth adapter.

Step 4: Auto-Load Ralink rt3290 (rtbth) module at startup
Now, the final step is to auto load the rtbth module at startup which prevents you from activating module at every boot. For that you need to add the module in modules.conf file.

All you have to do is to find Kernel module file and its Located in /etc/modules-load.d/modules.conf

You can manually Browse this file in Nautilus or any other file manager. All you have to do is to run File Manager in Terminal by typing the command:

Sudo nautilus

or:

sudo su & nautilus

But if find this difficult then type in terminal:

sudo gedit /etc/modules-load.d/modules.conf

Then, Press Enter.

It will open the notepad file of modules.conf and after that enter rtbth at end of file.

Just click on save and restart your PC and see if the modules load automatically.

Good Luck!

It’s worked for me too; but when i restarted the computer, bluetooth driver has gone again. then i tried
“sudo modprobe rtbth” command again and then worked again.
i guess there is an issue about modules.conf file. pls help..