prism2_cs problem at "long" distance

Hello,
I'am using these cards on ARM-based embedded systems:
Socket 0:
  product info: "SMC", "SMC2531W-B EliteConnect Wireless Adapter", "", ""
  manfid: 0xd601, 0x0010
  function: 6 (network)

The wlan interfaces are used in Ad-Hoc mode with linux-wlan-ng-0.2.0 on
a 2.4.18 kernel.


Everything is ok as long as the distance between cards is small. We did
an outdoor experiment with two cards at 150 meters. The link was
unstable (ping stopped during a few seconds when a car or even a man was
moving between the cards), we expect this to be normal. The strange
observation is that after some time (5-15 minutes), the connection
between the two cards was not possible anymore while nothing between the
cards would prevent the link to be established. Without moving the
devices, one of the card was ejected and reinserted and... it worked
again. The same cards in the same machines appear to perfectly work at
short distances. Is there any good already-known reason for this to be
observed ?

The adapters are all working in Ad-Hoc mode, without encryption, at
2Mb/s:

#=======USER MIB SETTINGS=============================
# You can add the assignments for various MIB items
#  of your choosing to this variable, separated by 
#  whitespace.  The wlan-ng script will then set each one.
# Just uncomment the variable and set the assignments 
#  the way you want them.

#USER_MIBS="p2CnfRoamingMode=1 p2CnfShortPreamble=mixed"

#=======WEP===========================================
# [Dis/En]able WEP.  Settings only matter if PrivacyInvoked is true
lnxreq_hostWEPEncrypt=false     # true|false
lnxreq_hostWEPDecrypt=false     # true|false
dot11PrivacyInvoked=false       # true|false
dot11WEPDefaultKeyID=0          # 0|1|2|3
dot11ExcludeUnencrypted=false   # true|false, in AP this means WEP is required.

# If PRIV_GENSTR is not empty, use PRIV_GENTSTR to generate 
#  keys (just a convenience)
PRIV_GENERATOR=/sbin/nwepgen    # nwepgen, Neesus compatible
PRIV_KEY128=false               # keylength to generate
PRIV_GENSTR=""

# or set them explicitly.  Set genstr or keys, not both.
dot11WEPDefaultKey0=            # format: xx:xx:xx:xx:xx   or
dot11WEPDefaultKey1=            #         xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:x
dot11WEPDefaultKey2=            #  e.g.   01:20:03:40:05   or
dot11WEPDefaultKey3=            #         01:02:03:04:05:06:07:08:09:0a:0b:0c:0
#=======SELECT STATION MODE===================
IS_ADHOC=y                      # y|n, y - adhoc, n - infrastructure

#======= INFRASTRUCTURE STATION  ===================
# What kind of authentication?
AuthType="opensystem"           # opensystem | sharedkey (requires WEP)

#======= ADHOC STATION ============================
BCNINT=100                      # Beacon interval (in Kus)
#osiris: 11
CHANNEL=11                      # DS channel for BSS (1-14, depends 
                                #   on regulatory domain)
BASICRATES="2 4"                # Rates for mgmt&ctl frames (in 500Kb/s)
OPRATES="2 4"                   # Supported rates in BSS (in 500Kb/s)
#OPRATES="2 4 11 22"            # Supported rates in BSS (in 500Kb/s)


This is what the kernel says when the card is inserted:

init_module: prism2_cs.o: 0.2.0 Loaded
init_module: dev_info is: prism2_cs
prism2_cs: index 0x01, irq 5, io 0xfd000100-0xfd00017f
ident: nic h/w: id=0x801b 1.0.0
ident: pri f/w: id=0x15 1.1.1
ident: sta f/w: id=0x1f 1.8.2
MFI:SUP:role=0x00:id=0x01:var=0x01:b/t=1/1
CFI:SUP:role=0x00:id=0x02:var=0x02:b/t=1/1
PRI:SUP:role=0x00:id=0x03:var=0x01:b/t=4/4
STA:SUP:role=0x00:id=0x04:var=0x01:b/t=1/15
PRI-CFI:ACT:role=0x01:id=0x02:var=0x02:b/t=1/1
STA-CFI:ACT:role=0x01:id=0x02:var=0x02:b/t=1/1
STA-MFI:ACT:role=0x01:id=0x01:var=0x01:b/t=1/1
Prism2 card SN: 99SA01000000
linkstatus=CONNECTED

regards
sébastien