<PAGE>
                                                                   EXHIBIT 10.20


                                (CLEARWIRE LOGO)


February 1, 2005



ITFS Spectrum Consultants, LLC
ATTN: Rudolph J. Geist, Managing Member
1010 Wayne Ave., Ste. 950
Silver Spring, MD 20910

Dear Rudy:

This letter confirms our understanding and agreement with respect to the payment
of consideration for the assistance by ITFS Spectrum Consultants, LLC ("ISC")
with respect to the acquisition of those MMDS channels and, if applicable, the
lease of those ITFS channels listed on the attached Exhibit A.

Upon (i) the closing of each acquisition of the MMDS channel(s) pursuant to the
purchase agreements dated September 23, 2004, between Fixed Wireless Holdings,
LLC "FWH"), a wholly owned subsidiary of Clearwire Corporation ("Clearwire"),
and the licensees listed in Section 1 of the attached Exhibit A and for (ii) the
closing of an acquisition of the authorizations held by Communications Ventures,
Inc. which are listed on Exhibit A, Clearwire will pay to ISC an amount equal to
*** multiplied by the number of CPOPs for such MMDS channels as set forth in the
third column of the attached Exhibit A for such channels. Such amounts will be
paid *** in immediately available funds and *** in warrants to purchase shares
of Class A common stock of Clearwire, such warrants to have the same terms and
conditions as described in the Spectrum Acquisition Consulting Agreement between
ISC and Clearwire of date even herewith (the "SACA"). In addition, upon the
execution of an agreement to lease the excess capacity over and above what is
required to be retained by the ITFS license holder, Clearwire will pay to ISC an
amount equal to $0.02 multiplied by the number of CPOPs as listed in the third
column of part 2 on the attached Exhibit A for such channels; provided however,
if such lease agreement does not include an option to purchase the licenses,
then the amount paid shall equal to ONE CENT ($0.01) multiplied by the number of
CPOPs as set forth in the third column of the attached Exhibit A for such
channels.

The consideration paid pursuant to this letter shall be treated as Consulting
Consideration shall be included together with any other Consulting Consideration
paid to ISC under the SACA for the purpose of determining when ISC has been paid
the Consideration Maximum (as defined in the SACA).

[* * * Portions of this Exhibit have been omitted and filed separately
with the Securities and Exchange Commission as part of an application for
confidential treatment pursuant to the Securities Act of 1933, as amended]


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<PAGE>


Please indicate your agreement with this letter by signing in the space provided
below and return an signed original of this letter to Clearwire Corporation at
the address set forth above.

Sincerely,

CLEARWIRE CORPORATION




By /s/ Benjamin G. Wolff
  -----------------------------
Name: Benjamin G. Wolff
Title: Executive Vice President


 AGREED:

 ITFS SPECTRUM CONSULTANTS, LLC




By /s/ Rudolph J. Geist
  -----------------------------
Name: Rudolph J. Geist
Title: Managing Member



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<PAGE>


                                    EXHIBIT A


1.       MMDS Channels




<TABLE>
<CAPTION>
LICENSEE                   MARKET            CPOP              CALL SIGN

<S>                        <C>               <C>               <C>
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
***                        ***               ***               ***
</TABLE>



2.       ITFS Channels


<TABLE>
<CAPTION>
LICENSEE                   MARKET            CPOP              CALL SIGN

<S>                        <C>               <C>               <C>
***                        ***               ***               ***
***                        ***               ***               ***
</TABLE>




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