THIS DEBENTURE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED (THE "SECURITIES ACT") OR ANY APPLICABLE STATE SECURITIES LAW AND MAY
NOT BE TRANSFERRED UNLESS (i) THERE IS AN EFFECTIVE REGISTRATION STATEMENT UNDER
THE SECURITIES ACT OR (ii) IN THE OPINION OF COUNSEL ACCEPTABLE TO THE COMPANY,
REGISTRATION UNDER THE SECURITIES ACT OR SUCH APPLICABLE STATE SECURITIES LAWS
IS NOT REQUIRED IN CONNECTION WITH SUCH TRANSFER.

                       DIGITAL TRANSMISSION SYSTEMS, INC.
              11.50% Subordinated Debenture due September 25, 2002


No. ________                                                    Atlanta, Georgia
$4,000,000.00                                                 September 25, 1977


         FOR VALUE RECEIVED, Digital Transmission Systems, Inc., a Delaware
corporation (the "Company"), promises to pay to the order of Sirrom Capital
Corporation d/b/a Tandem Capital, a Tennessee corporation ("Purchaser"),
pursuant to the Debenture Purchase Agreement (as hereinafter defined) at such
place as Purchaser may from time to time designate in writing, in lawful money
of the United States of America and in immediately available funds, by automatic
debit, the principal sum of Four Million Dollars ($4,000,000.00) and any accrued
but unpaid interest thereon.

         This Debenture is referred to in and is execute and delivered pursuant
to, a Debenture Purchase Agreement dated of even date herewith between the
Company and Purchaser (the "Debenture Purchase Agreement"), to which reference
is hereby made for a statement of the terms and conditions under which this
Debenture may be repaid and accelerated. Capitalized terms not otherwise defined
herein shall have the meanings ascribed to them in the Debenture Purchase
Agreement.

         Interest shall accrue from the date of issue of this Debenture at the
rate of 11.50% per annum, payable quarterly by automatic debit on the first day
of each March, June, September and December, commencing December 1, 1997, and
ending at maturity, to mature on September 25, 2002.

         Interest shall be computed on the basis of a 360-day year and the
actual number of days elapsed.

         Any principal payment due under this Debenture not paid when due,
whether at stated maturity, by notice of repayment, by acceleration or
otherwise, and any accrued but unpaid interest shall, to the extent permitted by
applicable law, thereafter bear interest (compounded monthly and payable upon
demand) at an annual rate of 15% in respect of such principal and such unpaid
interest until such unpaid amounts have been paid in full (whether before or
after judgment).


<PAGE>


                                                                               2


         This Debenture is subject to (i) optional redemption by the Company
without penalty and as provided by Section 1.2 of the Debenture Purchase
Agreement, and (ii) mandatory redemption at the election of the holder and as
provided by Section 1.3 of the Debenture Purchase Agreement. All payments made
hereunder shall be applied first to interest and then to outstanding principal.

         If payment hereunder becomes due and payable on a Saturday, Sunday, or
legal holiday, under the laws of the State of Tennessee, the due date thereof
shall be extended to the next succeeding business day.

         Demand, presentment, protest, diligence, notice of dishonor, and any
other formality are hereby expressly waived by the Company and any endorser or
guarantor.

         If there is any default under this Debenture, and this Debenture is
placed in the hands of an attorney for collection, or is collected through any
court, including any bankruptcy court, the Company promises to pay to the order
of the holder hereof such holder's reasonable attorneys' fees and court costs
actually incurred in collecting or attempting to collect or securing or
attempting to secure this Debenture or enforcing the holder's rights with
respect to the Collateral, to the extent allowed by the laws of the State of
Tennessee or any state in which any Collateral is situated.

         THIS DEBENTURE HAS BEEN DELIVERED IN, AND SHALL BE CONSTRUED IN
ACCORDANCE WITH AND GOVERNED BY THE LAWS OF, THE STATE OF GEORGIA APPLICABLE TO
CONTRACTS MADE AND TO BE PERFORMED THEREIN WITHOUT REGARD TO THE CONFLICTS OF
LAW PROVISIONS THEREOF.

         The holder of this Debenture may, with or without notice to any party,
and without affecting the obligations of any maker, surety, guarantor, endorser,
accommodation party, or any other party to this Debenture (i) extend the time
for payment of either principal or interest from time to time, (ii) release or
discharge any one or more parties liable on the Debenture, (iii) suspend the
right to enforce this Debenture with respect to any persons, (iv) change,
exchange, or release any property in which the holder has any interest securing
this Debenture, (v) justifiably or otherwise, impair any of the Collateral or
suspend the right to enforce against any such Collateral, and (vi) at any time
it deems it necessary or proper, call for and, should it be made available,
accept, as additional security, the signature or signatures of additional
parties or a security interest in property of any kind or description or both.

         This Debenture is subordinated to certain other indebtedness of the
Company to the extent and with the effect set forth in the Debenture Purchase
Agreement.

         This Debenture is registered on the books of the Company and is
transferable only by surrendered thereof at the principal office of the Company
at


<PAGE>


                                                                               3


3000 Northwoods Parkway, Building 330, Norcross, Georgia 30071, or such other
address as the Company shall have advised the holder of the Debenture in
writing, duly endorsed or accompanied by a written instrument of transfer duly
executed by the registered holder of this Debenture or its attorney duly
authorized in writing. Payment of or on account of principal, premium, if any,
and interest on this Debenture shall be made only to or upon the order in
writing of the registered holder thereof.

         Any provision herein, or in the Debenture Purchase Agreement, or any
other document executed or delivered in connection herewith or therewith, or in
any other agreement or commitment, whether written or oral, expressed or
implied, to the contrary notwithstanding, neither the Purchaser nor any holder
hereof shall in any event be entitled to receive or collect, nor shall any
amounts received hereunder be credited, so that Purchaser or any holder hereof
shall be paid, as interest, a sum greater than the maximum amount permitted by
applicable law to be charged to the person primarily obligated to pay this
Debenture at the time in question. If any construction of this Debenture or the
Debenture Purchase Agreement, or any and all other papers, agreements or
commitments, indicate a different right given to Purchaser or any holder hereof
to ask for, demand, or receive any larger sum as interest, such is a mistake in
calculation or wording which this clause shall override and control, it being
the intention of the parties that this Debenture, the Debenture Purchase
Agreement, and all other documents executed or delivered in connection herewith
shall in all ways comply with applicable law and proper adjustments shall
automatically be made accordingly. If Purchaser or any holder hereof ever
receives, collects, or applies as interest, any sum in excess of the maximum
amount permitted by applicable law, if any, such excess amount shall be applied
to the reduction of the unpaid principal balance of this Debenture, and if this
Debenture is paid in full, any remaining excess shall be paid to the Company. In
determining whether or not the interest paid or payable, under any specific
contingency, exceeds the maximum amount permitted by applicable law, if any, the
Company and any holder hereof shall, to the maximum extent permitted under
applicable law: (i) characterize any non-principal payment as an expense or fee
rather than as interest, and (ii) "spread" the total amount of interest
throughout the entire term of the Debenture.


                     [rest of page intentionally left blank]


<PAGE>


                                                                               4


         IN WITNESS WHEREOF, the Company has caused this Debenture to be
executed in its corporate name by the undersigned officer, thereunto duly
authorized.


                                        DIGITAL TRANSMISSION SYSTEMS, INC.


                                        By:  /s/ Andres C. Salazar
                                             -----------------------------
                                        Name:    Andres C. Salazar
                                             -----------------------------
                                        Title:   President and CEO
                                             -----------------------------


Pay to the order of WI-LAN INC.

     without recourse

          January 7, 2000


                                        FINOVA MEZZANINE CAPITAL INC.
                                        (f/k/a Sirrom Capital Corporation)


                                        By: /s/ Donald F. Barrirkman
                                            -----------------------------
                                            Donald F. Barrirkman, Vice President


<PAGE>


                                                                               5


                    THIRD AMENDMENT TO SUBORDINATED DEBENTURE


         This Third Amendment to the Subordinated Debenture ("Amendment") is
made and entered into as of the 7th day of January, 2000, by and between DIGITAL
TRANSMISSION SYSTEMS, INC. (the "Company"), a Delaware corporation, and WI-LAN,
INC. (the "Transferee").

                              W I T N E S S E T H :

         WHEREAS, Finova Mezzanine Capital Inc. ("Finova") and the Company
previously executed that 11.5% Subordinated Debenture due September 25, 2002, in
the original principal amount of $4,000,000, as amended by that First Amendment
to Subordinated Debenture dated as of October 21, 1998 and the Second Amendment
to the Subordinated Debenture dated as of February 5, 1999 (the "Debenture");

         WHEREAS, effective the date hereof, Finova has transferred the
Debenture to Transferee and immediately upon such transfer Transferee exercised
its conversion rights with respect to $1,310,000 principal amount of the
Debenture;

         NOW, THEREFORE, for valuable consideration, the receipt and sufficiency
of which are acknowledged, it is agreed as follows:

         1. The Debenture is hereby amended so that the Principal Amount of the
Debenture shall be $690,000.

         2. The Debenture remains in full effect, as amended hereby.


            [The Remainder of this page is intentionally left blank]



<PAGE>


                                                                               6


Executed as of the dated stated above.


                                        DIGITAL TRANSMISSION SYSTEMS, INC.


                                        By:  /s/ Andres C. Salazar
                                             -----------------------------
                                        Title:   Chief Executive Officer
                                             -----------------------------


                                        WI-LAN, INC.


                                        By:       [Signature Not Legible]
                                             -----------------------------
                                        Title:    Chief Executive Officer
                                             -----------------------------